Performance
Engine Performance
Note applicable to Chapters 4 and 5: Although the 135 exam focuses on the BE-1900 and the 121 exam focuses on the B-727 and DC-9, all aircraft are candidates for all tests. Study the questions applicable to your test by noting the test category above each question number.
There are three types of engines in use on modern airplanes: reciprocating engine, turboprop engine and turbojet engine. The type of engine selected for a particular airplane design depends primarily on the speed range of the aircraft. The reciprocating engine is most efficient for aircraft with cruising speeds below 250 MPH, the turboprop works best in the 250 MPH to 450 MPH range and the turbojet engine is most efficient above 450 MPH.
Manifold pressure (MAP) is a measurement of the power output of a reciprocating engine. It is basically the pressure in the engine's air inlet system. In a normally-aspirated (unsupercharged) engine, the MAP will drop as the aircraft climbs to altitude. This severely limits a piston-powered airplane's altitude capability.
Most piston-powered airplanes flown by air carriers are turbocharged. On this type of engine, exhaust gas from the engine is used as a power source for a compressor that in turn raises the MAP at any given altitude. The flow of exhaust gas to the turbocharger is controlled by a device called a waste gate.
Turbocharging allows an aircraft to fly at much higher altitudes than it would be able to with normally-aspirated engines. The term critical altitude is used to describe the effect of turbocharging on the aircraft's performance. The critical altitude of a turbocharged reciprocating engine is the highest altitude at which a desired manifold pressure can be maintained.
The pilots of reciprocating-engine-powered aircraft must be very careful to observe the published limits on manifold pressure and engine RPM. In particular, high RPM and low MAP can produce severe wear, fatigue and damage.
Both turboprops and turbojet engines are types of gas turbine engines. All gas turbine engines consist of an air inlet section, a compressor section, the combustion section, the turbine section and the exhaust. Air enters the inlet at roughly ambient temperature and pressure. As it passes through the compressor the pressure increases and so does the temperature due to the heat of compression. Bleed air is tapped off the compressor for such accessories as air conditioning and thermal anti-icing.
The section connecting the compressor and the combustion sections is called the diffuser. In the diffuser, the cross sectional area of the engine increases. This allows the air stream from the compressor to slow and its pressure to increase. In fact, the highest pressure in the engine is attained at this point.
Next, the air enters the combustion section where it is mixed with fuel and the mixture is ignited. Note that there is no need for an ignition system that operates continuously (such as the spark plugs in a piston engine) because the uninterrupted flow of fuel and air will sustain combustion after an initial "light off." The combustion of the fuel-air mixture causes a great increase in volume and because there is higher pressure at the diffuser, the gas exits through the turbine section. The temperature of the gas rises rapidly as it passes from the front to the rear of the combustion section. It reaches its highest point in the engine at the turbine inlet. The maximum turbine inlet temperature is a major limitation on turbojet performance, and without cooling, it could easily reach up to 4,000°F, far beyond the limits of the materials used in the turbine section. To keep the temperature down to an acceptable 1,100° to 1,500°F, surplus cooling air from the compressor is mixed aft of the burners.
The purpose of the turbine(s) is to drive the compressor(s) and they are connected by a drive shaft. Since the turbines take energy from the gas, both the temperature and pressure drop.
The gases exit the turbine section at very high velocity into the tailpipe. The tailpipe is shaped so that the gas is accelerated even more, reaching maximum velocity as it exits into the atmosphere.
Combinations of slow airspeed and high engine RPM can cause a phenomenon in turbine engines called compressor stall. This occurs when the angle of attack of the engine's compressor blades becomes excessive and they stall. If a transient stall condition exists, the pilot will hear an intermittent "bang" as backfires and flow reversals in the compressor take place. If the transient condition develops into a steady state stall, the pilot will hear a loud roar and experience severe engine vibrations. The steady state compressor stall has the most potential for severe engine damage, which can occur literally within seconds of the onset of the stall.
If a compressor stall occurs in flight, the pilot should reduce fuel flow, reduce the aircraft's angle of attack and increase airspeed.
The turboprop is a turbine engine that drives a conventional propeller. It can develop much more power per pound than can a piston engine and is more fuel efficient than the turbojet engine. Compared to a turbojet engine, it is limited to slower speeds and lower altitudes (25,000 feet to the tropopause). The term equivalent shaft horsepower (ESHP) is used to describe the total engine output. This term combines its output in shaft horsepower (used to drive the propeller) and the jet thrust it develops.
As the density altitude is increased, engine performance will decrease. When the air becomes less dense, there is not as much oxygen available for combustion and the potential thrust output is decreased accordingly. Density altitude is increased by increasing the pressure altitude or by increasing the ambient temperature. Relative humidity will also affect engine performance. Reciprocating engines in particular will experience a significant loss of BHP (Brake Horsepower). Turbine engines are not affected as much by high humidity and will experience very little loss of thrust.
Takeoff Performance Terminology
Clearway -- a plane beyond the end of a runway which does not contain obstructions and can be considered when calculating takeoff performance of turbine-powered transport category airplanes. The first segment of the takeoff of a turbine-powered airplane is considered complete when it reaches a height of 35 feet above the runway and has achieved V2 speed. Clearway may be used for the climb to 35 feet.
Stopway -- an area designated for use in decelerating an aborted takeoff. It cannot be used as a part of the takeoff distance but can be considered as part of the accelerate-stop distance.
Regulation requires that a transport category airplane's takeoff weight be such that, if at any time during the takeoff run the critical engine fails, the airplane can either be stopped on the runway and stopway remaining, or that it can safely continue the takeoff. This means that a maximum takeoff weight must be computed for each takeoff. Factors which determine the maximum takeoff weight for an airplane include runway length, wind, flap position, runway braking action, pressure altitude and temperature.
In addition to the runway-limited takeoff weight, each takeoff requires a computation of a climb-limited takeoff weight that will guarantee acceptable climb performance after takeoff with an engine inoperative. The climb-limited takeoff weight is determined by flap position, pressure altitude and temperature.
When the runway-limited and climb-limited takeoff weights are determined, they are compared to the maximum structural takeoff weight. The lowest of the three weights is the limit that must be observed for that takeoff. If the airplane's actual weight is at or below the lowest of the three limits, adequate takeoff performance is ensured. If the actual weight is above any of the limits a takeoff cannot be made until the weight is reduced or one or more limiting factors (runway, flap setting, etc.) is changed to raise the limiting weight.
After the maximum takeoff weight is computed and it is determined that the airplane's actual weight is within limits, then V1, VR and V2 are computed. These takeoff speed limits are contained in performance charts and tables of the airplane flight manual, and are observed on the captain's airspeed indicator. By definition they are indicated airspeeds.
V1 (Takeoff Decision Speed) is the speed during the takeoff at which the airplane can experience a failure of the critical engine and the pilot can abort the takeoff and come to a full safe stop on the runway and stopway remaining, or the pilot can continue the takeoff safely. If an engine fails at a speed less than V1, the pilot must abort; if the failure occurs at a speed above V1 he/she must continue the takeoff.
VR (Rotation Speed) is the IAS at which the aircraft is rotated to its takeoff attitude with or without an engine failure. VR is at or just above V1.
V2 (Takeoff Safety Speed) ensures that the airplane can maintain an acceptable climb gradient with the critical engine inoperative.
VMU (Minimum Unstick Speed) is the minimum speed at which the airplane may be flown off the runway without a tail strike. This speed is determined by manufacturer's tests and establishes minimum V1 and VR speeds. The flight crew does not normally compute the VMU speed separately.
V1 is computed using the actual airplane gross weight, flap setting, pressure altitude and temperature. Raising the pressure altitude, temperature or gross weight will all increase the computed V1 speed. Lowering any of those variables will lower the V1 speed.
A wind will change the takeoff distance. A headwind will decrease it and a tailwind will increase it. While a headwind or tailwind component does affect the runway limited takeoff weight, it usually has no direct effect on the computed V1 speed. The performance tables for a few airplanes include a small correction to V1 for very strong winds. For those airplanes, a headwind will increase V1 and a tailwind will decrease it.
A runway slope has the same effect on takeoff performance as a wind. A runway which slopes uphill will increase the takeoff distance for an airplane and a downslope will decrease it. A significant slope may require an adjustment in the V1 speed. An upslope will require an increase in V1 and a downslope will require a decrease.
If there is slush on the runway or if the antiskid system is inoperative, the stopping performance of the airplane is degraded. This requires that any aborted takeoff be started at a lower speed and with more runway and stopway remaining. This means that both the runway-limited takeoff weight and the V1 used for takeoff be lower than normal.
Calculating "V" Speeds
The table in FAA Figure 82 is used in several problems to determine the pressure altitude from the indicated altitude and the local altimeter setting. The table uses the local altimeter setting to indicate the proper correction to field elevation. For example, assume the local altimeter setting is 29.36" Hg. Enter the table in the left-hand column labeled "QNH IN. HG." and then find the range of altimeter settings that contains 29.36" Hg. Read the correction to elevation in the center column. In this case, add 500 feet to the field elevation to determine the pressure altitude. If the altimeter setting is given in millibars, enter the table in the right-hand column.
B-727 "V" Speeds
V1, VR, and V2 for the B-727 are determined by using the table at the right side of FAA Figure 83. Using Operating Conditions G-1 (FAA Figure 81), follow these steps for determining the "V" speeds. Enter the table at the top left in the row appropriate for the pressure altitude and go across until you come to a column containing a temperature range which includes the given value. In this case, enter in the row labeled 1 to 3 (pressure altitude = 1,550 feet) and go to the first column which contains the temperature of +23°F (be sure to use the Fahrenheit or Centigrade ranges as appropriate). Go down the first column until in the row appropriate for a flap setting of 15° and a gross weight of 140,000 pounds. The V1 and VR speeds are 122 knots and the V2 speed is 137 knots. No further adjustments or corrections are required. Notice that on the B-727, V1 and VR are always the same speed.
B-737 "V" Speeds
Using Operating Conditions R-1 (FAA Figure 53), follow the steps for determining the "V" speeds (See FAA Figure 55). Enter the table at the top left in the row appropriate for the pressure altitude and go across until in a column containing a temperature range which includes the given value. In this case, enter in the row labeled -1 to 1 (pressure altitude = 500 feet, refer to FAA Figure 54) and go to the first column which contains the temperature of +50°F (be sure to use the Fahrenheit or Centigrade ranges as appropriate). Go down the first column until in the row appropriate for a flap setting of 15° and a gross weight of 90,000 pounds. The V1 speed is 120 knots, the VR speed is 121 knots and the V2 speed is 128 knots. There are two possible adjustments to make to the V1 speed only. They are noted at the bottom of the table.
DC-9 "V" Speeds
The first step for calculating V1 and VR is to find the basic speeds in the table at the top of FAA Figure 47. Using the conditions from Operating Conditions A-1 from FAA Figure 45, the "V" speeds for a weight of 75,000 pounds are: V1 = 120.5, and VR = 123.5 knots. Next, a series of corrections must be applied for pressure altitude, ambient temperature, runway slope, wind component, and engine and wing ice protection. There are table values for all these corrections at the bottom of FAA Figure 47 except for pressure altitude and ambient temperature.
The first step in the altitude correction is to use the table in FAA Figure 46 to determine the pressure altitude. Using the altimeter setting from Operating Conditions A-1 (29.40" Hg), the table shows a correction of +500 feet. The pressure altitude then is 3,000 feet (2,500 + 500).
Next, enter the graphs as shown in FAA Figure 47. Draw two vertical lines representing the pressure altitude of 3,000 feet. Next, draw a horizontal line through the ambient temperature (+10°F) to intersect each of the vertical lines. In this case, the lines meet in the "zero" correction area for both V1 and VR. Notice that the correction is marked by bands. For example, if the lines crossed anywhere in the highest shaded band, the correction would be +1 knot.
Next, set up a table similar to the one below to apply any necessary corrections:
V1 VR
Table Value 120.5 123.5
Pressure Alt & Temp 0 0
Slope (+1%) + 1.5 + .9
10 HW + .3 0
Ice Protection + .8 + .8
Corrected Speeds 123.1 125.2
Calculating Takeoff Power
B-727 Takeoff EPR
EPR (Engine Pressure Ratio) is the thrust indication used on many turbojet aircraft. Basically, it is the ratio of the engine exhaust pressure to the intake pressure. For example, if the exhaust pressure is exactly twice the intake pressure, the EPR is 2.00.
The EPR setting for maximum takeoff thrust will vary with altitude and temperature. In addition, reductions in EPR have to be made when bleed air from the compressor section is used for air conditioning, engine anti-ice and internal regulation of the engine.
The table at the top of FAA Figure 83 is used to determine Takeoff EPR. Enter the table with temperature and pressure altitude. To determine pressure altitude, use FAA Figure 82.
Using the data from Operating Conditions G-1 (FAA Figure 81), the field elevation is 1,050 feet and the altitude correction is +500 feet, which results in a pressure altitude of 1,550 feet. Since there is no listing for this altitude in the table in FAA Figure 83, it is necessary to interpolate. At 1,000 feet and an OAT of 23°F, the EPR for Engines 1 and 3 is 2.15 and for Engine 2 it is 2.16. At 2,000 feet the corresponding values are 2.21 for Engines 1 and 3, and 2.22 for Engine 2. Since the pressure altitude in Condition G-1 is roughly halfway between 1,000 and 2,000, the EPR setting should be halfway between as well. The EPR for Engines 1 and 3 is 2.18 and for Engine 2 it is 2.19.
The EPR values in the table are for a given bleed air configuration only. This configuration is noted at the upper right-hand corner of the table: "EPR Bleed Corrections, Eng 1 & 3 on, Eng 2 off." Normally, 1 & 3 are on, and 2 is off -- anything different will require a correction. If this is not the configuration in use, corrections are listed at the bottom left-hand corner of the table. The "air conditioning" correction applies only to Engines 1 and 3 and only if the air conditioning is off. The "engine anti-ice on" correction applies only to Engine 2.
The last of the possible corrections is for 6th stage bleed air. This applies only if both of two conditions are met. The 6th stage bleed must be on and the OAT must be 10°C (50°F) or warmer. If these conditions are met, a value of .05 is subtracted from the engine number 2 EPR.
The best method of solving this type of problem is to set up a table which accounts for all the possible corrections. Using the data from Operating Conditions G-1:
Eng. 1 & 3 Eng. 2
Table Value 2.18 2.19
A/C Off + .04 --
Eng. AI On -- - .03
6th Stage Bleed -- --
Takeoff EPR 2.22 2.16
B-737 Takeoff EPR
The Takeoff EPR table at the top of FAA Figure 55 is similar to the B-727 takeoff EPR. In the table of FAA Figure 55, two EPR values are found: one for temperature and one for altitude (be sure to use the table in FAA Figure 54 to determine the pressure altitude). The lower of the two is the takeoff EPR. For example if the temperature is 50°F at a pressure altitude of 500 feet, the temperature-limited EPR is 2.04 and the altitude-limited EPR is 2.035. (The altitude-limited EPR is 2.01 from sea level up to 1,000 feet.) The only possible correction would be for if the air conditioning bleeds are off.
Climb Performance
The best rate-of-climb speed for any airplane is the speed at which there is the greatest difference between the power required for level flight and the power available from the engines. The L/Dmax speed for any airplane is the one that requires the least power for level flight since it is the lowest drag speed. Because the power output of prop-driven airplanes is relatively constant at all speeds, L/Dmax is the best rate-of-climb speed for them.
Turbojet engines produce more power as the aircraft speed increases. Even though drag increases at speeds above L/Dmax, the engine's power output increases even more so that the maximum difference between power required and power available is achieved at a higher airspeed. For a turbojet, the best rate-of-climb speed is faster than L/Dmax.
DC-9 Performance Tables
The tables in FAA Figures 49 and 50 allow you to determine the time, fuel and distance required for a climb to cruising altitude after takeoff. The table in FAA Figure 49 is used if a "high speed" climb is planned, and the table in FAA Figure 50 is used for a "long range" climb. Each of the tables is broken into four "sub-tables" representing different initial weights. Assume a climb on the long range schedule from sea level to 34,000 feet with an initial weight of 84,000 pounds. The climb will require 17.1 minutes, 2,570 pounds of fuel, and cover a distance of 109.7 NM.
A headwind or tailwind component in the climb will change the distance flown. Assume that there is an average 20-knot headwind in the climb described above. The first step is to compute the average "no wind" ground speed (GS). A distance of 109.7 NM flown in 17.1 minutes works out to a GS of 384.9 knots. A headwind component of 20 knots will reduce this GS to 364.9 knots. The distance flown in 17.1 minutes at 364.9 knots is 104 NM.
Note: Using a CX-2 computer, select "Dist Flown" from the menu and enter TIME and GS. No other adjustments or corrections are required for these tables.
B-737 Climb Performance Tables
The tables in FAA Figures 57 and 58 allow you to determine the time and fuel required for a climb to cruising altitude after takeoff. The table in FAA Figure 57 is for ISA temperatures, and the table in FAA Figure 58 is for ISA +10°C. Each intersection of Brake Release Weight and Cruise Altitude has a box with four numbers. These are the time, the fuel, the distance and the TAS required to climb from a sea level airport to cruise altitude in calm wind conditions. For example, with a brake release weight of 110,000 pounds, a climb to 33,000 feet in ISA +10°C conditions will require 26 minutes, 4,100 pounds of fuel and cover a distance of 154 NM.
A headwind or tailwind component in the climb will change the distance flown. Assume that there is an average 20-knot headwind in the climb described above. The first step is to compute the average "no wind" GS. A distance of 154 NM flown in 26 minutes works out to a GS of 355.4 knots. A headwind component of 20 knots will reduce this GS to 335.4 knots. The distance flown in 26 minutes at 335.4 knots is 145.3 NM.
Note: Using a CX-2 computer, select "Dist Flown" from the menu and enter time and GS. Do not use the TAS from the table as that will result in an inaccurate answer.
Departure from an airport that is significantly above sea level will reduce the fuel required for the climb. Notice that departure from a 2,000-foot airport will reduce the climb fuel by 100 pounds, however the effect on time and distance flown is negligible.
B-737 Climb and Cruise Power Tables
The Max Climb & Max Continuous EPR Table at the top of FAA Figure 60 is similar to the one discussed in Takeoff EPR. In this table two EPR values are found -- one for temperature and one for altitude. The lower of the two is the maximum climb/continuous EPR. For example, if the temperature is +10°C at a pressure altitude of 10,000 feet, the temperature-limited EPR is 2.04 and the altitude-limited EPR is 2.30. (The altitude-limited EPR is 2.30 from 5,660 feet and up.) The max EPR is 2.04.
The Max Cruise EPR Table supplies one EPR value for a given TAT (Total Air Temperature) in one of two altitude ranges. The correction tables are similar to ones used previously and apply to both tables.
Cruise Performance
The maximum range speed for an aircraft is determined by its L/D curve. Propeller-driven airplanes will achieve best range performance if they are flown at the speed that yields L/Dmax. In turbojet aircraft, a somewhat more complex relationship between lift and drag determines best range. Turbojets always have a best range speed higher than L/Dmax.
A headwind or tailwind will affect the miles per unit of fuel burned. If an airplane is operating at its best-range airspeed and encounters a headwind, it should speed up to minimize the time in the adverse wind. By the same token, an airplane with a tailwind can slow down and let the wind maintain its ground speed with a lower fuel flow. The exact amount of airspeed change that is useful varies with airplane type.
Turbojet engines have a strong preference for operations at high altitudes and airspeeds. Both lower temperatures and higher altitudes increase engine efficiency by requiring a lower fuel flow for a given thrust. Besides increased engine efficiency, lift and drag both decrease at higher altitudes, so less thrust is required.
Turbine engines are much more efficient when operated at the upper end of their RPM range. Generally, the optimum cruise altitude for a turbojet airplane is the highest at which it is possible to maintain the optimum aerodynamic conditions (best angle of attack) at maximum continuous power. The optimum altitude is determined mainly by the aircraft's gross weight at the beginning of cruise.
As an aircraft burns fuel and becomes lighter, the optimum cruise altitude slowly increases and the speed that yields the optimum cruise performance slowly decreases. Since it is seldom practical to change speed and altitude constantly, it is common procedure to maintain a constant Mach cruise at a flight level close to optimum. As fuel is burned, thrust is reduced to maintain the constant Mach number.
Landing Considerations
VS -- stalling speed or the minimum steady flight speed at which the airplane is controllable.
VSO -- stalling speed or the minimum steady flight speed in the landing configuration.
VREF -- reference speed. It is normally 1.3 x VSO.
Even with all the aircraft's high lift devices extended, a typical air carrier airplane has a high approach speed and a long landing roll. An airplane is normally flown at 1.3 times the VSO speed for the aircraft's weight. Of course, 1.3 times VSO is an indicated airspeed and the ground speed will vary depending on wind, altitude and temperature. A high temperature or high altitude approach will increase an aircraft's ground speed for any given approach speed.
Once an airplane has touched down on a runway there are 3 ways of slowing it to a stop: aerodynamic braking, use of the wheel brakes, and reverse thrust. Aerodynamic braking is not very effective in slowing large jet aircraft and so it is generally not used to a great extent. Reverse thrust is effective mainly at higher airspeeds. The wheel brakes are effective at all speeds and are the primary means of stopping the aircraft.
The typical technique for stopping an aircraft on a normal landing is to apply reverse thrust (or prop reverse) immediately upon touchdown. This takes maximum advantage of reverse thrust when it is most effective and it saves wear on the wheel brakes, which heat up very rapidly at high ground speeds. Shortly after touchdown, the spoilers are deployed. This reduces lift and increases drag. As the aircraft slows, the main wheel brakes are applied to bring it down to taxiing speed. The brakes are most effective when lift has been reduced (by spoilers and low airspeed) and more of the aircraft's weight is carried by the landing gear.
Water on a runway will increase the landing rollout because the reduced coefficient of friction makes the wheel brakes less effective. This is particularly true at high ground speeds.
A very dangerous possibility when landing on a wet runway is hydroplaning. When hydroplaning occurs, the wheel brakes are almost totally ineffective. This not only greatly increases the landing rollout, but also introduces the possibility of losing directional control on sliding off the side of the runway. There are three types of hydroplaning.
Dynamic hydroplaning occurs when a tire rolls through standing water, forms a bow wave, and then rolls up on top of the wave, losing all contact with the runway. The minimum speed at which dynamic hydroplaning can start is related to tire pressure. As a rule of thumb, dynamic hydroplaning will start at speeds of greater than nine times the square root of the tire pressure in pounds per square inch. The practical application is that your nose wheel can hydroplane at a lower speed than the mains because of its lower pressure. Once dynamic hydroplaning has started, it can continue to much lower speeds.
Viscous hydroplaning occurs when there is a thin film of water covering a smooth surface such as a painted or rubber-coated portion of the runway. Viscous hydroplaning can occur at much lower speeds than dynamic hydroplaning.
Reverted rubber hydroplaning occurs during a locked wheel skid. Water trapped between the tire and the runway is heated by friction, and the tire rides along a pocket of steam.
When landing on a water-covered runway, fly the approach as close to "on speed" as possible. Landing at a higher than recommended speed will greatly increase the potential for hydroplaning. After touchdown, use aerodynamic braking and reverse thrust to maximum possible extent, saving the use of wheel brakes until the speed is low enough to minimize the possibility of hydroplaning.
Regulations (14 CFR §121.195) require that when a turbojet aircraft is dispatched to an airport where the runways are forecast to be wet or slippery, the effective length of the landing runway must be 115% of what is required under dry conditions. Since runways cannot be lengthened, the effect of this rule is to lower the maximum allowable landing weight of aircraft on wet runways for dispatch purposes.
Landing Performance Tables and Graphs
The graphs in FAA Figures 88 through 92 are used to compare landing distances under various conditions. All the graphs are used in the same manner; enter at the bottom with the gross weight, draw a vertical line to the appropriate diagonal line, and from there draw a horizontal line to the landing distance.
Question 8746 is typical of the questions that use these graphs. It asks for a comparison of a 40°-flap landing on a wet runway using three different weights and stopping techniques. Line 1 is the distance using brakes and spoilers at 122,500 pounds. Line 2 is the distance using brakes and reversers at 124,000 pounds. Line 3 is the distance using brakes, spoilers, and reversers at a weight of 131,000 pounds. The graph shows that Answer C will have the shortest stopping distance (about 3,200 feet).
The graphs in FAA Figures 92 and 93 are used to determine the thrust required to maintain a given airspeed with various flap configurations with the landing gear both up and down. The graph in Figure 93 is for a 110,000-pound aircraft, while the graph in Figure 92 is for a 140,000-pound aircraft. Notice that the solid curved lines represent level flight and that the two dashed lines represent a 3° glide slope.
The graph works in a manner similar to those in the previous questions. Enter the graph at the bottom with the airspeed, draw a vertical line to the appropriate curved line and then draw horizontally to the required thrust.
The two dashed lines represent an approach condition, and the VREF speed is marked with a line and circle on each of the curves. For example, VREF for a 140,000-pound airplane with 40° flaps is 123 knots, and with 30° flaps it is 127 knots.
Some problems require the computation of headwind, tailwind and crosswind components. The wind direction, wind speed and course must be known. Most often these problems are part of a takeoff or landing computation. If this is the case, the course corresponds to the runway number (i.e., Rwy 35 = 350° magnetic), and the winds are references to magnetic north and in knots. This type of computation can be accomplished on the CX-2 computer or by use of the Wind Component Graph in FAA Figure 74. This computation cannot be done on an E6-B computer and usually the scales on a CR-type computer do not give an answer of sufficient accuracy.
Determine the headwind and crosswind components for an aircraft landing on runway 35 with the winds from 300° at 20 knots. Using the CX-2 computer: Select "X/H-Wind" from the menu, then enter the wind direction, wind speed and runway number at the prompts. The crosswind is -15 knots (a negative crosswind is from the left, a positive is from the right). The headwind component is -13 knots (a negative number indicates a headwind, a positive indicates a tailwind).
The Landing Speed Table in FAA Figure 75 is used to determine the aircraft's approach speed (VREF) at various weights and flap settings. For example, the 100,000-pound airplane using a 30° flap setting has a VREF of 135 knots. This speed is adjusted by adding one half of any headwind component as noted at the bottom of the table. Gust corrections are not required on this test. If the headwind component were 13 knots, the correction would be 7 knots (always round up) and the corrected VREF would be 143 knots.
The Go-Around EPR Table at the top of FAA Figure 75 is similar to the one discussed for takeoff EPR. In this table there are two EPR values; one for temperature and one for altitude. The lower of the two is the go-around EPR. For example, if the temperature is 15°C TAT at a pressure altitude of 500 feet, the temperature-limited EPR is 2.00. The altitude-limited EPR is 2.01(you need to interpolate between 2.04 and 1.98). There are possible corrections for air conditioning OFF and for engine and wing anti-ice ON. Notice that there are three different temperature scales.
Miscellaneous Performance
VC -- design cruising speed.
VmO/MmO -- maximum operating limit speed.
The Boeing 727 Holding Table in FAA Figure 85 and the Boeing 737 Holding Table in FAA Figure 69 show the holding EPR, indicated airspeed and fuel flow per engine for various weights and altitudes. You will need to interpolate for conditions between listed weights and altitudes.
Assume a Boeing 727 is holding at 24,000 feet at a weight of 195,000 pounds. What is the EPR, IAS and fuel flow required? This problem will require interpolation both between altitudes and weights. Let's do the steps for EPR first:
1. Since the weight of 195,000 pounds is exactly halfway between two table listed weights, it is easiest to start by determining the required EPR at each of the two nearest altitudes. At 25,000 feet, the EPR for a 200,000-pound airplane is 1.85, and for a 190,000-pound airplane it is 1.81. Add those two values and divide by 2. The EPR is 1.83. The same calculation for 20,000 feet yields an EPR of 1.675.
2. Since the altitude given (24,000) is not halfway between the two table values it is necessary to calculate the amount the EPR changes per 1,000 feet of altitude. Determine the difference between the EPR values at each altitude (1.83 - 1.675 = .155), then divide that number by 5 (.155 ¸ 5 = .031).
3. The EPR variation per 1,000 feet is .031, and the EPR decreases as altitude decreases. To determine the EPR for 24,000 feet, subtract .031 from the EPR for 25,000 feet (1.83 - .031 = 1.799).
Similar interpolations give a holding speed of 264 knots and a fuel flow per engine of 3,508 pounds per hour.
Some questions ask "what is the fuel burned in a particular time?" This is a fuel burn problem identical to those covered in the flight logs section. Remember that the fuel flow rates are given per engine. The Boeing 727 (FAA Figure 85) is a three-engine airplane and the Boeing 737 (FAA Figure 69) is a twin-engine airplane.
An encounter with strong turbulence can result in structural damage to an aircraft, or inadvertent stall. The sudden changes in wind direction and speed can result in very rapid changes in an aircraft's angle of attack. A sudden increase in angle of attack will cause the airplane to accelerate upward, increasing both the load factor and the stalling speed.
For any combination of weight and altitude there will be a recommended "rough air" speed that provides the best protection from stalls and from the possibility of overstressing the aircraft. When clear air turbulence has been reported in the area, a pilot should slow to the rough air speed upon encountering the first ripple of turbulence.
In severe turbulence, it may be impossible to maintain a constant airspeed or altitude. If this happens, the pilot should set the power to that which would maintain the desired airspeed and maintain a level flight attitude, accepting large variations in airspeed and altitude.
Due to the inaccuracy of the EPR gauges in turbulent air, some aircraft use an N1 power setting to maintain thrust. The Turbulent Air Penetration Table in FAA Figure 64 shows the N1 power setting for various weights and altitudes. For example, the power setting for a 110,000-pound airplane at 30,000 feet is 82.4%.
This RPM may have to be adjusted for temperature. For example, at 30,000 feet, the RPM must be changed 1.6% for every 10°C deviation from ISA (add for temperatures above ISA and subtract for temperatures below ISA). The ISA TAT (Total Air Temperature) is listed for each altitude. Assume an actual TAT of -8°C. This is 15° warmer than standard and would require adding 2.4% to the table value.
The tables in FAA Figure 87 are used to compute the time, fuel and distance required to descend from cruise altitude. There are four different tables representing different speed schedules in the descent. The table at the top left is used if planning to descend at Mach .80 until intercepting an IAS of 250 knots. The table at the bottom right is for a descent at Mach .80 until intercepting 350 knots and holding that until 10,000 feet and then slowing to 250 knots. Be sure to use the correct table for the planned schedule. For example: Using a descent schedule of .80 M/250 KIAS, if descending from FL370 at a weight of 130,000 pounds, the required time is 26 minutes, the fuel burn is 1,570 pounds and the required distance in air miles (NAM) is 125.5.
Engine-Out Procedures
VmC -- minimum control speed with the critical engine inoperative.
VXSE -- best single engine angle-of-climb speed.
VYSE -- best single engine rate-of-climb speed.
When an engine fails in flight, the effect on aircraft performance is drastic. For example, the loss of one engine on a two-engine aircraft will result in a loss of climb performance in excess of 50%. Climb performance is determined by the amount of power available in excess of that required for level flight. The one remaining engine must provide all of the power required for level flight. It may be able to develop little or no excess power that would allow for a climb.
When an engine fails in cruise flight, the pilot should slow the aircraft to its best single-engine rate-of-climb speed (VYSE) and apply maximum continuous power on the remaining engine. The airplane may or may not be able to climb. If it cannot climb at the present altitude, at least it will descend at the minimum possible rate of sink and level off at its maximum engine-out altitude. It may be necessary to dump fuel to improve the altitude capability of the aircraft.
A multi-engine airplane should never be flown below its minimum control speed (VmC). If it is below VmC and an engine failure occurs, it may be impossible to maintain directional control with the other engine operating at full power. VmC will vary with the aircraft's center of gravity location. VmC will be highest with the CG at its most rearward-allowed position.
A three- or four-engine turbine-powered airplane, used by an air carrier, may be ferried to a maintenance base with one engine inoperative if certain requirements are met. These requirements include:
· The airplane model must have been test flown to show that such an operation is safe.
· The operator's approved flight manual must contain performance data for such an operation.
· The operating weight of the aircraft must be limited to the minimum required for flight plus any required reserve fuel.
· Takeoffs are usually limited to dry runways.
· The computed takeoff performance must be within acceptable limits (this will vary depending on the type of aircraft).
· The initial climb cannot be over thickly-populated areas.
· Only required flight crewmembers may be on the aircraft.
· Weather conditions at the takeoff and destination airports must be VFR.
The table in FAA Figure 70 shows the time required to dump to a given fuel weight. The column at the left edge of the table is the initial fuel weight and the row at the top of the table is the ending fuel weight. The intersection of any of those weights is the time required to dump from the beginning weight to the ending weight.
Question 8678 asks for the dump time required to reach an aircraft weight of 144,500 pounds given an initial aircraft weight of 180,500 pounds and a Zero Fuel Weight of 125,500 pounds.
1. Determine the initial fuel weight by subtracting the Zero Fuel Weight from the initial aircraft weight:
180,500 - 125,500 = 55,000 pounds.
2. Determine the ending fuel load by subtracting the Zero Fuel Weight from the ending aircraft weight:
144,500 - 125,500 = 19,000 pounds.
3. From the table, determine the fuel dump time to go from 55,000 pounds to 19,000 pounds. Interpolate as necessary:
Dump time = 15.25 minutes
The tables in FAA Figure 72 are used to determine the maximum altitude a Boeing 737 can maintain with one of its engines inoperative. The engine bleed configuration determines which of the three tables is to be used. The upper table is for all anti-ice off, the middle is for engine anti-ice only, and the lower is for when both engine and wing anti-ice are in use. Once the correct table has been selected, find the aircraft weight at the left-hand side and follow that row across to the appropriate ISA temperature. The only possible adjustment to the table value for the level-off altitude is covered by the note at the bottom of the page. The note allows an increase of 800 feet in the level-off altitude if the air conditioning is off and the aircraft is below 17,000 feet.
Assume the aircraft weighs 100,000 pounds at the time of its engine failure, and the engine anti-ice is on. If the temperature is ISA, the level-off altitude is 19,400 feet. Even if the air conditioning were off, it would have no effect since the level-off is above 17,000 feet.
Beech 1900 Cruise Tables
The tables in FAA Figures 21 through 25 are used to determine cruise, fuel flow and TAS for various temperatures, weights and altitudes. Use FAA Figure 23 in ISA +10°C conditions, FAA Figure 24 in ISA, and FAA Figure 25 in ISA -10°C.
For example, Operating Conditions BE-31 (FAA Figure 21) specify a weight of 15,000 pounds at a pressure altitude of 22,000 feet and an OAT of -19°C. The first step is to decide which of the three tables to use. Altitudes are listed in the left-hand column of each table, beside each altitude IOAT (Indicated OAT) and the OAT which corresponds to the ISA temperature for the table. All temperatures in these problems are OAT rather than IOAT. Figure 23 (ISA +10°C) shows an OAT at 22,000 feet of -19°C which is the given value. There is no column of data for a 15,000-pound aircraft, but interpolation of the data for 16,000 and 14,000 pounds yields a TAS of 228 knots and a total fuel flow of 633 pounds per hour.
Questions 8489 through 8493 ask for the flight time at cruise, using the data from these tables. The first step is to compute the ground speed using the table TAS, the given winds and true course.
Since the true course is given, it is not necessary to convert the wind direction to magnetic. Given a TAS of 228 knots and the winds and course in Operating Conditions BE-31, the GS is 216.0 knots. The next step is to solve for leg time. At a GS of 216.0 knots, it will take 1 hour 17 minutes 47 seconds to fly 280 NM.
Questions 8494 through 8498 ask for the total fuel burn using the data from these tables. The first step is to compute the leg time as described above. Next, using the total fuel flow from the table, compute the fuel burn. With a fuel flow of 633 pounds per hour and a cruise time of 1 hour 17 minutes, the fuel burned is 812.3 pounds.
Flight Planning Graphs and Tables
Aircraft manufacturers publish flight planning graphs or tables that enable the flight crews to quickly estimate the time and fuel required to fly certain trips. These tables or graphs allow adjustments for aircraft weight, wind, altitude, cruise speed and other variables.
The graph in FAA Figure 61 is used to determine the time and fuel required for a planned flight. It allows for trip distance, wind, flight altitude, landing weight, and temperature. This example shows the solution for Operating Condition X-1.
Start at the bottom of the graph in FAA Figure 62 with the trip distance (2,000 NM) and draw a vertical line to the reference line representing a zero knot wind component. Condition X-1 states that there is a 50-knot tailwind component. From the reference line follow the curved diagonal line back to the -50-knot line. This will result in an equivalent trip distance of 1,800 NM. From there, draw a vertical line through both sets of diagonal lines representing cruise altitude.
To determine the trip time, find the point of intersection between the vertical line and the diagonal in the upper set which represents the cruise altitude (27,000 feet). From there draw a horizontal line to the reference line representing 0° deviation from ISA. Since the temperature in Condition X-1 is ISA +10°, from the reference line parallel the diagonals down and to the left to the ISA +10° line. From there draw a horizontal line to the trip time (3 hours, 55 minutes).
To determine the trip fuel, find the point of intersection (on the lower set of altitude diagonals) between the vertical line drawn earlier and the diagonal line used for the cruise altitude. From there, draw a horizontal line, the reference line representing a landing weight of 65,000 pounds. Since the landing weight in Condition X-1 is 70,000 pounds, parallel the diagonal lines up and to the right until meeting the vertical 70,000-pound line. From there, draw a horizontal line to the trip fuel (26,000 pounds).
The table in FAA Figure 67 shows the time and fuel required for trips of various distances. For example, a trip of 340 NM requires 55 minutes and consumes 5,550 pounds of fuel.
A wind will change both the time and fuel required for a given flight. The notes at the bottom of the table explain the correction factors. The formulas are:
Change in time = Time x Wind Component ¸ TAS
and
Change in Fuel = Fuel x Wind Component ¸ TAS
Assume a 25-knot tailwind for the conditions above. The formulas are:
Change in Time = 55 min. x (-25) ¸ 438 = -3.1 minutes
and
Change in Fuel = 5,550 lb. x (-25) ¸ 438 = -316.8 pounds
The corrected time is 51.9 minutes (55 - 3.1), and the corrected fuel is 5,233.2 pounds (5,550 - 316.8). Notice that a tailwind is entered as a negative number because it reduces the required time and fuel. A headwind should be entered as a positive number since it will increase the required time and fuel.
The table in FAA Figure 52 is used to plan for possible diversion to an alternate airport. For example, in Condition L-1 (FAA Figure 51) the alternate is 110 NAM away. It will take 29 minutes to fly there and make an approach. It will also require 3,400 pounds of fuel, which includes an additional 15 minutes of holding fuel.
The conditions specify that there is 15 minutes holding at the alternate. Since Note 2 in FAA Figure 52 specifically states that 15 minutes holding is not included in the time, then we must add the 15 minutes to the time listed in the alternate planning chart. Wind is not a factor since both figures deal in Nautical Air Miles (NAM).
29 minutes + 15 minutes = 44 minutes
Landing weight way be calculated by subtracting fuel burn from gross weight as follows:
85,000 lbs - 3,400 lbs = 81,600 lbs landing weight
Typical Flight Logs
Flight logs are used to accurately plan the time and fuel required for a flight. In the following paragraphs we describe all the steps required to complete a flight log.
1. Determine the magnetic courses for each leg and determine the leg distances.
2. Apply variations to the winds aloft.
3. Determine the temperature in relation to ISA.
4. Determine Mach number and convert to TAS.
5. Compute ground speed.
6. Calculate and record the time for each leg.
7. Compute fuel flow.
8. Compute total fuel.
9. Determine the reserve fuel.
10. Compute fuel burn to alternate.
11. Add the en route, reserve, alternate, and missed approach fuel to find the total fuel required for the flight.
Computation of Temperature at Cruise Altitude
Temperature is often expressed as a deviation from ISA which is the standard day temperature (i.e., ISA -2°). This temperature can be computed by the following procedure:
1. Compute ISA by multiplying the altitude in thousands of feet times -2° and then adding 15°. For example: ISA at 27,000 feet = 27 x (-2°) +15 = -39°.
2. Apply the deviation from ISA. ISA -2° at 27,000 feet = (-39°) + (-2°) = -41°
Computation of True Airspeed Using Mach Number
True Airspeed (TAS) can be computed from Mach number and Outside Air Temperature (OAT).
Using the CX-2 computer, select "Plan Mach#" from the menu, then enter the OAT and the Mach number at the appropriate prompts.
Using an E6-B computer, follow these steps:
1. In the small window labeled "Airspeed Correction" or "True Airspeed," align the arrow labeled "Mach Number" with the OAT on the scale adjacent the window.
2. Find the Mach number on the inner of the two main scales and then read the TAS opposite it on the outer scale.
Note: Some "CR"-type mechanical computers have a window in which a Mach Index is aligned with a Mach number inside the window. Don't use this scale. It is designed to use Indicated Temperature and will give an inaccurate TAS when OAT is used.
See the instruction manual of your individual computer for more detailed instructions.
Specific Range
Specific range is the term used to describe the rate of fuel burn per nautical air mile flown. It is calculated by using TAS and fuel flow only. Wind has no effect on specific range. To calculate specific range in nautical air miles per 1,000 pounds, use the formula:
NAM/1,000 = TAS x 1,000 ¸ PPH.
TAS should be calculated from the Mach number as in the paragraph above. PPH can be taken directly from the flight log.
Saturday, March 22, 2008
Friday, March 21, 2008
Airline Transport Pilot Preparation : Aerodynamics
Aerodynamics
Lift and Drag
The four forces acting on an aircraft in flight are lift, weight, thrust and drag. Weight always acts vertically toward the center of the earth. Lift acts perpendicular to the relative wind (not always vertically). Thrust and drag act opposite each other and parallel to the relative wind.
Lift is produced by air flowing over the curved wing surfaces. The air flowing over the upper surface of the wing is deflected further than that flowing across the lower surface and therefore is accelerated. Bernoulli's Principle states that when a gas is accelerated, its pressure decreases. Thus the pressure on the upper wing surface is lower than that on the lower surface and lift is produced.
Angle of attack is the angle between the relative wind and chord line of wing. At zero angle of attack, the pressure on the upper surface of the wing is still less than atmospheric, but the wing is producing minimum lift. As the angle of attack is increased, the lift developed by the wing increases proportionately. This is true until the angle of attack exceeds a critical value, when the air flowing over the top of the wing breaks up into a turbulent flow and the wing stalls.
Angle of attack and indicated airspeed determine the total lift. An increase in either indicated airspeed or angle of attack increases total lift (up to the stalling angle of attack) and a decrease in either decreases total lift. To maintain the same total lift (i.e., maintain level flight), a pilot has to change the angle of attack anytime indicated airspeed is changed. For example, as indicated airspeed is decreased, the angle of attack must be increased to compensate for the loss of lift. The relationship between indicated airspeed and lift for a given angle of attack involves the law of squares. If the angle of attack does not change, total lift varies with the square of the indicated airspeed. For example, if the airspeed doubles, the lift will increase by four times.
Indicated airspeed can be thought of as having two elements -- the actual speed of the airplane through the air (true airspeed) and the density of the air. As altitude increases, air density decreases. To maintain the same indicated airspeed at altitude an aircraft must fly at a higher true airspeed. To produce the same amount of lift at altitude, a higher true airspeed is required for a given angle of attack.
A wing will always stall at the same angle of attack. The load factor, weight and density altitude will cause the stalling true airspeed to vary, but the stall angle of attack will always be the same.
A curve comparing total drag to parasite and induced drag reveals an airspeed at which drag is at a minimum value. At higher airspeeds, total drag increases because of increasing parasite drag. At lower airspeeds, induced drag increases which increases the total drag. Since the lift stays constant (equal to weight), the low point on the curve is the airspeed that produces the best lift to drag (L/D) ratio. This point is referred to as L/Dmax.
A change in weight changes the L/D curve. The amount of parasite drag is mainly a function of indicated airspeed. The amount of induced drag is a function of angle of attack. When an aircraft's weight is increased, any indicated airspeed will require a higher angle of attack to produce the required lift. This means that induced drag will increase with increases in weight while there will be little change in parasite drag.
When an airplane is within about one wingspan of the ground, the flow of air around the wingtips is inhibited by the close proximity of the ground. This ground effect reduces induced drag (and therefore total drag) and increases lift. As an airplane flies out of ground effect on takeoff, the increased induced drag will require a higher angle of attack.
Critical Engine and VmC
Because of "P-Factor" on most propeller-driven airplanes, the loss of one particular engine at high angles of attack would be more detrimental to performance than the loss of the other. One of the engines has its thrust line closer to the aircraft centerline. The loss of this engine would more adversely affect the performance and handling of the aircraft; therefore this is the "critical engine."
For unsupercharged engines, VmC decreases as altitude is increased. Stalls should never be practiced with one engine inoperative because of the potential for loss of control. Engine out approaches and landings should be made the same as normal approaches and landings.
Banking at least 5° into the good engine ensures the airplane will be controllable at any speed above the certificated VmC, that the airplane will be in a minimum drag configuration for best climb performance, and that the stall characteristics will not be degraded. Engine out flight with the ball centered is never correct.
Maneuvering Flight
In a turn, centrifugal force is counterbalanced by a portion of the lift of the wing. The horizontal component of lift turns the airplane and the vertical component of lift opposes gravity. When the pilot rolls the airplane into a turn he must increase the total lift of the wing so that the vertical component is equal to the airplane's weight. This is done by increasing the angle of attack. If no compensation is made for the loss of vertical component of lift in a turn, the aircraft will develop a sink rate.
Load factor is the ratio of the weight supported by the wings to the actual weight of the aircraft. For example, if an aircraft with a gross weight of 2,000 pounds were subjected to a total load of 6,000 pounds in flight, the load factor would be 3 Gs. On the ground or in unaccelerated flight the load factor is one. Conditions which can increase the load factor are vertical gusts (turbulence) and level turns. In a level turn, the load factor is dependent only on the angle of bank. Airspeed, turn rate or aircraft weight have no effect on load factor.
Rate of turn is the number of degrees per second at which the aircraft turns. The rate of turn is dependent on both the aircraft's airspeed and its angle of bank. To increase the rate of turn, the pilot must increase the angle of bank or decrease the airspeed or both. The rate of turn will decrease if the bank angle is decreased or if the airspeed is increased. The radius of turn is also dependent on both the bank angle and the airspeed. If angle of bank is increased or airspeed is decreased, the radius of turn will decrease. If bank angle is shallowed or if airspeed is increased, the radius of turn will increase.
Stability
Static stability describes the initial reaction of an aircraft after it has been disturbed from equilibrium in one or more of its axes of rotation. If the aircraft has an initial tendency to return to its original attitude of equilibrium, it has positive static stability. When it continues to diverge, it exhibits negative static stability. If an aircraft tends to remain in its new, disturbed state, it has neutral static stability. Most airplanes have positive static stability in pitch and yaw, and are close to neutrally stable in roll.
When an aircraft exhibits positive static stability in one of its axes, the term "dynamic stability" describes the long term tendency of the aircraft. When an aircraft is disturbed from equilibrium and then tries to return, it will invariably overshoot the original attitude and then pitch back. This results in a series of oscillations. If the oscillations become smaller with time, the aircraft has positive dynamic stability. If the aircraft diverges further away from its original attitude with each oscillation, it has negative dynamic stability.
The entire design of an aircraft contributes to its stability (or lack of it) in each of its axes of rotation. However, the vertical tail is the primary source of direction stability (yaw), and the horizontal tail is the primary source of pitch stability. The center of gravity (CG) location also affects stability. If the CG is toward its rearward limit, the aircraft will be less stable in both roll and pitch. As the CG is moved forward, the stability improves. Even though an airplane will be less stable with a rearward CG, it will have some desirable aerodynamic characteristics due to reduced aerodynamic loading of horizontal tail surface. This type of an airplane will have a slightly lower stall speed and will cruise faster for a given power setting.
High Speed Flight
Mach number is the ratio of the true airspeed to the speed of sound (TAS ¸ Speed of Sound). For example, an aircraft cruising at Mach .80 is flying at 80% of the speed of sound. The speed of sound is Mach 1.0.
A large increase in drag occurs when the air flow around the aircraft exceeds the speed of sound (Mach 1.0). Because lift is generated by accelerating air across the upper surface of the wing, local air flow velocities will reach sonic speeds while the aircraft Mach number is still considerably below the speed of sound. With respect to Mach cruise control, flight speeds can be divided into three regimes -- subsonic, transonic and supersonic. The subsonic regime can be considered to occur at aircraft Mach numbers where all the local air flow is less than the speed of sound. The transonic range is where some but not all the local air flow velocities are Mach 1.0 or above. In supersonic flight, all the air flow around the aircraft exceeds Mach 1.0. The exact Mach numbers will vary with each aircraft type but as a very rough rule of thumb the subsonic regime occurs below Mach .75, the transonic regime between Mach .75 and Mach 1.20, and the supersonic regime over Mach 1.20.
A limiting speed for a subsonic transport aircraft is its critical Mach number (Mcrit). That is the speed at which airflow over the wing first reaches, but does not exceed, the speed of sound. At Mcrit there may be sonic but no supersonic flow.
When an airplane exceeds its critical Mach number, a shock wave forms on the wing surface that can cause a phenomenon known as shock stall. If this shock stall occurs symmetrically at the wing roots, the loss of lift and loss of downwash on the tail will cause the aircraft to pitch down or "tuck under." This tendency is further aggravated in sweptwing aircraft because the center of pressure moves aft as the wing roots shock stall. If the wing tips of a sweptwing airplane shock stall first, the wing's center of pressure would move inward and forward causing a pitch up motion.
The less airflow is accelerated across the wing, the higher the critical Mach number (i.e., the maximum flow velocity is closer to the aircraft's Mach number). Two ways of increasing MCRIT in jet transport designs are to give the wing a lower camber and increase wing sweep. A thin airfoil section (lower camber) causes less airflow acceleration. The sweptwing design has the effect of creating a thin airfoil section by inducing a spanwise flow, thus increasing the effective chord length.
Although a sweptwing design gives an airplane a higher critical Mach number (and therefore a higher maximum cruise speed), it results in some undesirable flight characteristics. One of these is a reduced maximum coefficient of lift. This requires that sweptwing airplanes extensively employ high lift devices, such as slats and slotted flaps, to get acceptably low takeoff and landing speeds. The purpose of high lift devices such as flaps, slats and slots is to increase lift at low airspeeds and to delay stall to a higher angle of attack.
Another disadvantage of the sweptwing design is the tendency, at low airspeeds, for the wing tips to stall first. This results in loss of aileron control early in the stall, and in very little aerodynamic buffet on the tail surfaces.
Dutch roll tendency is typical of sweptwing designs. If such an airplane yaws, the advancing wing is at a higher angle of attack and presents a greater span to the airstream than the retreating wing. This causes the aircraft to roll in the direction of the initial yaw and simultaneously to reverse its direction of yaw. When the yaw reverses, the airplane then reverses its direction of roll and yaw again. This roll-yaw coupling is usually damped out by the vertical stabilizer. But, at high speeds and in turbulence, this may not be adequate, so most aircraft are also equipped with a yaw damper to help counteract any Dutch roll tendency.
Primary Flight Controls
Because of the high air loads, it is very difficult to move the flight control surfaces of jet aircraft with just mechanical and aerodynamic forces. So flight controls are usually moved by hydraulic actuators. Flight controls are divided into primary flight controls and secondary or auxiliary flight controls. The primary flight controls are those that maneuver the aircraft in roll, pitch and yaw. These include the ailerons, elevator and rudder. Secondary (or auxiliary) flight controls include tabs, trailing-edge flaps, leading-edge flaps, spoilers and slats.
Roll control of most jet aircraft is accomplished by ailerons and flight spoilers. The exact mix of controls is determined by the aircraft's flight regime. In low speed flight all control surfaces operate to provide the desired roll control. As the aircraft moves into higher speed operations, control surface movement is reduced to provide approximately the same roll response to a given input through a wide range of speeds.
Many aircraft have two sets of ailerons -- inboard and outboard. The inboard ailerons operate in all flight regimes. The outboard ailerons work only when the wing flaps are extended and are automatically locked out when flaps are retracted. This allows good roll response in low speed flight with the flaps extended and prevents excessive roll and wing bending at high speeds when the flaps are retracted.
Spoilers increase drag and reduce lift on the wing. If raised on only one wing, they aid roll control by causing that wing to drop. If the spoilers raise symmetrically in flight, the aircraft can either be slowed in level flight or can descend rapidly without an increase in airspeed. When the spoilers rise on the ground at high speeds, they destroy the wing's lift which puts more of the aircraft's weight on its wheels which in turn makes the brakes more effective.
Often aircraft have both flight and ground spoilers. The flight spoilers are available both in flight and on the ground. However, the ground spoilers can only be raised when the weight of the aircraft is on the landing gear. When the spoilers deploy on the ground, they decrease lift and make the brakes more effective. In flight, a ground-sensing switch on the landing gear prevents deployment of the ground spoilers.
Vortex generators are small (an inch or so high) aerodynamic surfaces located in different places on different airplanes. They prevent undesirable airflow separation from the surface by mixing the boundary airflow with the high energy airflow just above the surface. When located on the upper surface of a wing, the vortex generators prevent shock-induced separation from the wing as the aircraft approaches its critical Mach number. This increases aileron effectiveness at high speeds.
Tabs
Flight control surfaces are sometimes equipped with servo tabs. These tabs are on the trailing edge of the control surface and are mechanically linked to move opposite the direction of the surface. If the tab moves up, the surface moves down. This "servo" movement moves the control surface.
One method of modifying the downward tail load through changes in airspeed and configuration is by using trim tabs. Trim tabs are moved by a separate trim control from the cockpit. Movement of the trim tab (like the servo tab) is opposite that of the primary control surface.
Anti-servo tabs move in the same direction as the primary control surface. This means that as the control surface deflects, the aerodynamic load is increased by movement of the anti-servo tab. This helps to prevent the control surface from moving to a full deflection. It also makes a hydraulically-boosted flight control more aerodynamically effective than it would otherwise be.
Some jet aircraft have control tabs for use in the event of loss of all hydraulic pressure. Movement of the control wheel moves the control tab which causes the aerodynamic movement of the control surface. The control tab is used only during manual reversion; that is, with the loss of hydraulic pressure. They work the same as a servo tab but only in the manual mode.
High-Lift Devices
Sweptwing jet aircraft are equipped with a number of high-lift devices. These include leading edge flaps, slots or slats, and trailing edge flaps. The purpose of all high-lift devices is to increase lift at low airspeeds and to delay stall until a higher angle of attack.
The two most common types of leading-edge devices are slats and Krueger flaps. The Krueger flap extends from the leading edge of the wing, increasing its camber. The slat also extends from the wing's leading edge but it creates a gap or slot. This slot allows high energy from under the wing to flow over the top of the wing that delays stall to a higher angle of attack than would otherwise occur. It is common to find Krueger flaps and slats on the same wing.
Lift and Drag
The four forces acting on an aircraft in flight are lift, weight, thrust and drag. Weight always acts vertically toward the center of the earth. Lift acts perpendicular to the relative wind (not always vertically). Thrust and drag act opposite each other and parallel to the relative wind.
Lift is produced by air flowing over the curved wing surfaces. The air flowing over the upper surface of the wing is deflected further than that flowing across the lower surface and therefore is accelerated. Bernoulli's Principle states that when a gas is accelerated, its pressure decreases. Thus the pressure on the upper wing surface is lower than that on the lower surface and lift is produced.
Angle of attack is the angle between the relative wind and chord line of wing. At zero angle of attack, the pressure on the upper surface of the wing is still less than atmospheric, but the wing is producing minimum lift. As the angle of attack is increased, the lift developed by the wing increases proportionately. This is true until the angle of attack exceeds a critical value, when the air flowing over the top of the wing breaks up into a turbulent flow and the wing stalls.
Angle of attack and indicated airspeed determine the total lift. An increase in either indicated airspeed or angle of attack increases total lift (up to the stalling angle of attack) and a decrease in either decreases total lift. To maintain the same total lift (i.e., maintain level flight), a pilot has to change the angle of attack anytime indicated airspeed is changed. For example, as indicated airspeed is decreased, the angle of attack must be increased to compensate for the loss of lift. The relationship between indicated airspeed and lift for a given angle of attack involves the law of squares. If the angle of attack does not change, total lift varies with the square of the indicated airspeed. For example, if the airspeed doubles, the lift will increase by four times.
Indicated airspeed can be thought of as having two elements -- the actual speed of the airplane through the air (true airspeed) and the density of the air. As altitude increases, air density decreases. To maintain the same indicated airspeed at altitude an aircraft must fly at a higher true airspeed. To produce the same amount of lift at altitude, a higher true airspeed is required for a given angle of attack.
A wing will always stall at the same angle of attack. The load factor, weight and density altitude will cause the stalling true airspeed to vary, but the stall angle of attack will always be the same.
A curve comparing total drag to parasite and induced drag reveals an airspeed at which drag is at a minimum value. At higher airspeeds, total drag increases because of increasing parasite drag. At lower airspeeds, induced drag increases which increases the total drag. Since the lift stays constant (equal to weight), the low point on the curve is the airspeed that produces the best lift to drag (L/D) ratio. This point is referred to as L/Dmax.
A change in weight changes the L/D curve. The amount of parasite drag is mainly a function of indicated airspeed. The amount of induced drag is a function of angle of attack. When an aircraft's weight is increased, any indicated airspeed will require a higher angle of attack to produce the required lift. This means that induced drag will increase with increases in weight while there will be little change in parasite drag.
When an airplane is within about one wingspan of the ground, the flow of air around the wingtips is inhibited by the close proximity of the ground. This ground effect reduces induced drag (and therefore total drag) and increases lift. As an airplane flies out of ground effect on takeoff, the increased induced drag will require a higher angle of attack.
Critical Engine and VmC
Because of "P-Factor" on most propeller-driven airplanes, the loss of one particular engine at high angles of attack would be more detrimental to performance than the loss of the other. One of the engines has its thrust line closer to the aircraft centerline. The loss of this engine would more adversely affect the performance and handling of the aircraft; therefore this is the "critical engine."
For unsupercharged engines, VmC decreases as altitude is increased. Stalls should never be practiced with one engine inoperative because of the potential for loss of control. Engine out approaches and landings should be made the same as normal approaches and landings.
Banking at least 5° into the good engine ensures the airplane will be controllable at any speed above the certificated VmC, that the airplane will be in a minimum drag configuration for best climb performance, and that the stall characteristics will not be degraded. Engine out flight with the ball centered is never correct.
Maneuvering Flight
In a turn, centrifugal force is counterbalanced by a portion of the lift of the wing. The horizontal component of lift turns the airplane and the vertical component of lift opposes gravity. When the pilot rolls the airplane into a turn he must increase the total lift of the wing so that the vertical component is equal to the airplane's weight. This is done by increasing the angle of attack. If no compensation is made for the loss of vertical component of lift in a turn, the aircraft will develop a sink rate.
Load factor is the ratio of the weight supported by the wings to the actual weight of the aircraft. For example, if an aircraft with a gross weight of 2,000 pounds were subjected to a total load of 6,000 pounds in flight, the load factor would be 3 Gs. On the ground or in unaccelerated flight the load factor is one. Conditions which can increase the load factor are vertical gusts (turbulence) and level turns. In a level turn, the load factor is dependent only on the angle of bank. Airspeed, turn rate or aircraft weight have no effect on load factor.
Rate of turn is the number of degrees per second at which the aircraft turns. The rate of turn is dependent on both the aircraft's airspeed and its angle of bank. To increase the rate of turn, the pilot must increase the angle of bank or decrease the airspeed or both. The rate of turn will decrease if the bank angle is decreased or if the airspeed is increased. The radius of turn is also dependent on both the bank angle and the airspeed. If angle of bank is increased or airspeed is decreased, the radius of turn will decrease. If bank angle is shallowed or if airspeed is increased, the radius of turn will increase.
Stability
Static stability describes the initial reaction of an aircraft after it has been disturbed from equilibrium in one or more of its axes of rotation. If the aircraft has an initial tendency to return to its original attitude of equilibrium, it has positive static stability. When it continues to diverge, it exhibits negative static stability. If an aircraft tends to remain in its new, disturbed state, it has neutral static stability. Most airplanes have positive static stability in pitch and yaw, and are close to neutrally stable in roll.
When an aircraft exhibits positive static stability in one of its axes, the term "dynamic stability" describes the long term tendency of the aircraft. When an aircraft is disturbed from equilibrium and then tries to return, it will invariably overshoot the original attitude and then pitch back. This results in a series of oscillations. If the oscillations become smaller with time, the aircraft has positive dynamic stability. If the aircraft diverges further away from its original attitude with each oscillation, it has negative dynamic stability.
The entire design of an aircraft contributes to its stability (or lack of it) in each of its axes of rotation. However, the vertical tail is the primary source of direction stability (yaw), and the horizontal tail is the primary source of pitch stability. The center of gravity (CG) location also affects stability. If the CG is toward its rearward limit, the aircraft will be less stable in both roll and pitch. As the CG is moved forward, the stability improves. Even though an airplane will be less stable with a rearward CG, it will have some desirable aerodynamic characteristics due to reduced aerodynamic loading of horizontal tail surface. This type of an airplane will have a slightly lower stall speed and will cruise faster for a given power setting.
High Speed Flight
Mach number is the ratio of the true airspeed to the speed of sound (TAS ¸ Speed of Sound). For example, an aircraft cruising at Mach .80 is flying at 80% of the speed of sound. The speed of sound is Mach 1.0.
A large increase in drag occurs when the air flow around the aircraft exceeds the speed of sound (Mach 1.0). Because lift is generated by accelerating air across the upper surface of the wing, local air flow velocities will reach sonic speeds while the aircraft Mach number is still considerably below the speed of sound. With respect to Mach cruise control, flight speeds can be divided into three regimes -- subsonic, transonic and supersonic. The subsonic regime can be considered to occur at aircraft Mach numbers where all the local air flow is less than the speed of sound. The transonic range is where some but not all the local air flow velocities are Mach 1.0 or above. In supersonic flight, all the air flow around the aircraft exceeds Mach 1.0. The exact Mach numbers will vary with each aircraft type but as a very rough rule of thumb the subsonic regime occurs below Mach .75, the transonic regime between Mach .75 and Mach 1.20, and the supersonic regime over Mach 1.20.
A limiting speed for a subsonic transport aircraft is its critical Mach number (Mcrit). That is the speed at which airflow over the wing first reaches, but does not exceed, the speed of sound. At Mcrit there may be sonic but no supersonic flow.
When an airplane exceeds its critical Mach number, a shock wave forms on the wing surface that can cause a phenomenon known as shock stall. If this shock stall occurs symmetrically at the wing roots, the loss of lift and loss of downwash on the tail will cause the aircraft to pitch down or "tuck under." This tendency is further aggravated in sweptwing aircraft because the center of pressure moves aft as the wing roots shock stall. If the wing tips of a sweptwing airplane shock stall first, the wing's center of pressure would move inward and forward causing a pitch up motion.
The less airflow is accelerated across the wing, the higher the critical Mach number (i.e., the maximum flow velocity is closer to the aircraft's Mach number). Two ways of increasing MCRIT in jet transport designs are to give the wing a lower camber and increase wing sweep. A thin airfoil section (lower camber) causes less airflow acceleration. The sweptwing design has the effect of creating a thin airfoil section by inducing a spanwise flow, thus increasing the effective chord length.
Although a sweptwing design gives an airplane a higher critical Mach number (and therefore a higher maximum cruise speed), it results in some undesirable flight characteristics. One of these is a reduced maximum coefficient of lift. This requires that sweptwing airplanes extensively employ high lift devices, such as slats and slotted flaps, to get acceptably low takeoff and landing speeds. The purpose of high lift devices such as flaps, slats and slots is to increase lift at low airspeeds and to delay stall to a higher angle of attack.
Another disadvantage of the sweptwing design is the tendency, at low airspeeds, for the wing tips to stall first. This results in loss of aileron control early in the stall, and in very little aerodynamic buffet on the tail surfaces.
Dutch roll tendency is typical of sweptwing designs. If such an airplane yaws, the advancing wing is at a higher angle of attack and presents a greater span to the airstream than the retreating wing. This causes the aircraft to roll in the direction of the initial yaw and simultaneously to reverse its direction of yaw. When the yaw reverses, the airplane then reverses its direction of roll and yaw again. This roll-yaw coupling is usually damped out by the vertical stabilizer. But, at high speeds and in turbulence, this may not be adequate, so most aircraft are also equipped with a yaw damper to help counteract any Dutch roll tendency.
Primary Flight Controls
Because of the high air loads, it is very difficult to move the flight control surfaces of jet aircraft with just mechanical and aerodynamic forces. So flight controls are usually moved by hydraulic actuators. Flight controls are divided into primary flight controls and secondary or auxiliary flight controls. The primary flight controls are those that maneuver the aircraft in roll, pitch and yaw. These include the ailerons, elevator and rudder. Secondary (or auxiliary) flight controls include tabs, trailing-edge flaps, leading-edge flaps, spoilers and slats.
Roll control of most jet aircraft is accomplished by ailerons and flight spoilers. The exact mix of controls is determined by the aircraft's flight regime. In low speed flight all control surfaces operate to provide the desired roll control. As the aircraft moves into higher speed operations, control surface movement is reduced to provide approximately the same roll response to a given input through a wide range of speeds.
Many aircraft have two sets of ailerons -- inboard and outboard. The inboard ailerons operate in all flight regimes. The outboard ailerons work only when the wing flaps are extended and are automatically locked out when flaps are retracted. This allows good roll response in low speed flight with the flaps extended and prevents excessive roll and wing bending at high speeds when the flaps are retracted.
Spoilers increase drag and reduce lift on the wing. If raised on only one wing, they aid roll control by causing that wing to drop. If the spoilers raise symmetrically in flight, the aircraft can either be slowed in level flight or can descend rapidly without an increase in airspeed. When the spoilers rise on the ground at high speeds, they destroy the wing's lift which puts more of the aircraft's weight on its wheels which in turn makes the brakes more effective.
Often aircraft have both flight and ground spoilers. The flight spoilers are available both in flight and on the ground. However, the ground spoilers can only be raised when the weight of the aircraft is on the landing gear. When the spoilers deploy on the ground, they decrease lift and make the brakes more effective. In flight, a ground-sensing switch on the landing gear prevents deployment of the ground spoilers.
Vortex generators are small (an inch or so high) aerodynamic surfaces located in different places on different airplanes. They prevent undesirable airflow separation from the surface by mixing the boundary airflow with the high energy airflow just above the surface. When located on the upper surface of a wing, the vortex generators prevent shock-induced separation from the wing as the aircraft approaches its critical Mach number. This increases aileron effectiveness at high speeds.
Tabs
Flight control surfaces are sometimes equipped with servo tabs. These tabs are on the trailing edge of the control surface and are mechanically linked to move opposite the direction of the surface. If the tab moves up, the surface moves down. This "servo" movement moves the control surface.
One method of modifying the downward tail load through changes in airspeed and configuration is by using trim tabs. Trim tabs are moved by a separate trim control from the cockpit. Movement of the trim tab (like the servo tab) is opposite that of the primary control surface.
Anti-servo tabs move in the same direction as the primary control surface. This means that as the control surface deflects, the aerodynamic load is increased by movement of the anti-servo tab. This helps to prevent the control surface from moving to a full deflection. It also makes a hydraulically-boosted flight control more aerodynamically effective than it would otherwise be.
Some jet aircraft have control tabs for use in the event of loss of all hydraulic pressure. Movement of the control wheel moves the control tab which causes the aerodynamic movement of the control surface. The control tab is used only during manual reversion; that is, with the loss of hydraulic pressure. They work the same as a servo tab but only in the manual mode.
High-Lift Devices
Sweptwing jet aircraft are equipped with a number of high-lift devices. These include leading edge flaps, slots or slats, and trailing edge flaps. The purpose of all high-lift devices is to increase lift at low airspeeds and to delay stall until a higher angle of attack.
The two most common types of leading-edge devices are slats and Krueger flaps. The Krueger flap extends from the leading edge of the wing, increasing its camber. The slat also extends from the wing's leading edge but it creates a gap or slot. This slot allows high energy from under the wing to flow over the top of the wing that delays stall to a higher angle of attack than would otherwise occur. It is common to find Krueger flaps and slats on the same wing.
Airline Transport Pilot Preparation : Equipment, Navigation and Facilities
Equipment, Navigation and Facilities
Inoperative Equipment
A certificate holder's manual must contain enroute flight, navigation and communication procedures, including procedures for the dispatch, release or continuance of a flight if a required piece of equipment becomes inoperative.
When any required instrument or equipment in an aircraft is inoperative, the airplane cannot be flown unless that aircraft's Minimum Equipment List (MEL) allows such a flight.
The pilot-in-command of an aircraft operating IFR in controlled airspace shall report to ATC immediately any malfunction of navigational, approach or communications equipment that occurs in flight. The report must include:
· Aircraft identification;
· Equipment affected;
· Degree to which the capability of the aircraft to operate IFR in the ATC system is impaired; and
· Nature and extent of assistance desired from ATC.
Pitot-Static Instruments
Modern jet transports usually have three pitot-static systems. There are separate systems for the captain's and co-pilot's instruments plus an auxiliary system that provides a backup for either of the two primary systems. The instruments that require static pressure input are airspeed, Mach, altitude and vertical speed indicators. In addition, the airspeed and Mach indicators need a source of pitot pressure. Besides the flight instruments, static pressure input is required for the Mach warning, autopilot, flight director, flight recorder and cabin differential pressure. Pitot input is required for all those systems except for cabin differential pressure. The usual source for these non-flight instruments is the auxiliary pitot-static system.
Altimeters compare the sea level pressure setting in their window with the outside air pressure sensed through the static system. The difference is displayed as the altitude above sea level. Part of the preflight check is to verify the accuracy of the altimeters. An altimeter should be considered questionable if the indicated altitude varies by more the 75 feet from a known field elevation.
The altimeter setting used by pilots is always the station pressure of the reporting station corrected to sea level. Station pressure is the actual pressure at field elevation.
True altitude is the actual height of the aircraft above sea level. This is the same as indicated altitude when standard temperatures exist. When the temperature is warmer than standard, true altitude is higher than indicated altitude. When the temperature is colder than standard day conditions, just the opposite is true. Corrected altitude (approximately true altitude) can be calculated but it is neither practical nor useful to do so in most situations. When setting an altimeter, a pilot should just use the appropriate altimeter setting and disregard the effects of nonstandard atmospheric pressures and temperatures.
Pressure altitude is the altitude indicated when the altimeter is set to standard sea level pressure of 29.92" Hg. Density altitude is used in aircraft performance computations. It is pressure altitude corrected for nonstandard temperatures. If the temperature is warmer than standard, density altitude will be higher than pressure altitude.
The local altimeter setting is used when flying below FL180 and the altimeter is 31.00" Hg or less. Special procedures apply when the local pressure is more than 31.00" Hg because most altimeters cannot be set higher than that. In the United States, all altimeters are set to 29.92" Hg when climbing through FL180. Caution: outside the United States the transition altitude is often something other than FL180.
A common reason for altimeter errors is incorrect setting of the altimeter. If the setting in the altimeter is higher than the actual sea level pressure, the altimeter will read higher than the actual altitude. If the setting is too low, the altimeter will read lower than it really is. As a rough rule of thumb, the magnitude of the error is about 1,000 feet for each 1" Hg that the altimeter is off. For example, if the altimeter is set to 29.92" Hg, but the real sea level pressure is 30.57" Hg, the altimeter will read about 650 feet lower than the actual airplane's altitude (30.57 - 29.92 = .65" Hg = 650 feet). In this example, the airplane would be 650 feet higher than the indicated altitude.
The airspeed indicators compare pitot pressure with static pressure and display the difference as indicated airspeed. This indicated airspeed equals the aircraft's actual speed through the air (True Airspeed) only under standard day conditions at sea level. Under almost all flight conditions, true airspeed will be higher than indicated airspeed because of the lower ambient pressures at altitude.
The Machmeter displays aircraft speed as a percentage of the speed of sound. For example, an aircraft cruising at a Mach number of .82 is flying at 82% of the speed of sound. The Machmeter works in a manner similar to the airspeed indicator in that it compares pitot and static pressure, but these inputs are corrected by an altimeter mechanism.
If a pitot tube becomes blocked, the airspeed and Mach indicators will read inaccurately. If pressure is trapped in the pitot line, the airspeed will read inaccurately high as the aircraft climbs, low as it descends, and will be unresponsive to changes in airspeed. The airspeed indicator acts as an altimeter because only the static pressure changes. This situation occurs in icing conditions if both the ram air inlet and the drain hole of the pitot tube become completely blocked by ice.
If the pitot tube is blocked but the static port and the pitot drain hole remain open, the indicated airspeed will drop to zero. The drain pitot tube drain hole allows the pressure in the pitot line to drop to atmospheric and therefore there is no differential between the static and pitot pressures.
Pitot tubes and static ports are electrically heated to prevent ice formations that could interfere with proper operation of the systems. They are required to have "power on" indicator lights to show proper operation. In addition, many aircraft have an ammeter that shows the actual current flow to the pitot and static ports.
Since the magnetic compass is the only direction-seeking instrument in most airplanes, the pilot must be able to turn the airplane to a magnetic compass heading and maintain this heading. It is influenced by magnetic dip which causes northerly turning error and acceleration/deceleration error. When northerly turning error occurs, the compass will lag behind the actual aircraft heading while turning through headings in the northern half of the compass rose, and lead the aircraft's actual heading in the southern half. The error is most pronounced when turning through north or south, and is approximately equal in degrees to the latitude.
The acceleration/deceleration error is most pronounced on headings of east and west. When accelerating, the compass indicates a turn toward the north, and when decelerating it indicates a turn toward the south. The acronym ANDS is a good memory aid:
A accelerate
N north
D decelerate
S south
No errors are apparent while on east or west headings, when turning either north or south.
Safety of Flight Equipment
Airborne weather radar is used to detect and avoid areas of heavy precipitation such as thunderstorms. With few exceptions, all air carrier aircraft must be equipped with an approved airborne weather radar unit. The radar must be in satisfactory operating condition prior to dispatch on an IFR or night VFR flight if thunderstorms (or other hazardous weather) that could be detected by the radar are forecast along the intended route of flight. An aircraft may be dispatched with an inoperative radar unit if one of two conditions is met:
· The flight will be able to remain in day VFR flight conditions, or
· Hazardous weather is not forecast.
An air carrier's operations manual must contain procedures for the flight crew to follow if the weather radar fails in flight.
A ground proximity warning system (GPWS) must be installed on all large turbine powered airplanes. The GPWS gives aural and visual warnings when an aircraft too close to the terrain is in an improper configuration for landing, or when it deviates below glide slope on an ILS approach.
TCAS I (Traffic Alert and Collision Avoidance System) provides proximity warning only, to assist the pilot in the visual acquisition of intruder aircraft. No recommended avoidance maneuvers are provided nor authorized as a result of a TCAS I warning. TCAS II provides traffic advisories (TAs) and resolution advisories (RAs). Resolution advisories provide recommended maneuvers in a vertical direction to avoid conflicting traffic. TCAS does not alter or diminish the pilot's basic authority and responsibility to ensure safe flight. After the conflict, return to the ATC clearance in effect. If a deviation occurs, contact ATC as soon as practical.
Cockpit voice recorders are required on large turbine engine powered airplanes and large four engine reciprocating powered airplanes. The recorder must operate from before the start of the before starting checklist to the completion of the secure cockpit checklist. Although the recorder runs for the entire flight, only the most recent 30 minutes of information need be retained on the recorder tape.
An approved flight recorder must be installed on all airplanes certified for operations above 25,000 feet and on all turbine powered airplanes. What the flight recorder must record varies from airplane to airplane, but at a minimum it must record:
· Time
· Altitude
· Airspeed
· Vertical acceleration
· Heading
· Time of each radio transmission to or from ATC.
An air carrier must keep the flight recorder data until an aircraft has been operated at least 25 hours after the data was removed. However, 1 hour of the oldest recorded data may be erased to test the flight recorder.
The cockpit voice and flight recorder data can be used to identify malfunctions and irregularities with the aircraft and in carrying out investigations under NTSB Part 830. It cannot be used by the FAA for enforcement purposes. If an incident occurs which would require the immediate notification of the NTSB, the data must be kept by the operator for at least 60 days.
Communications
Each flag and domestic operator must have a two-way radio system that, under normal conditions, allows reliable and rapid communications between its aircraft and the appropriate dispatch office. For operations within the 48 contiguous states, this system must be independent of any operated by the U.S. government.
One source of current weather information in flight is the Enroute Flight Advisory Service (Flight Watch). This service is available nationwide from Flight Service Stations on frequency 122.0 MHz. Flight Watch is limited to weather information only. It is not to be used to open or close flight plans or for any ATC function.
Navigation Equipment
When an aircraft is flown IFR or VFR Over-the-Top it must have a dual installation of the navigation radios required to fly that route. This means that an aircraft flying Victor airways or jet routes must have two operable VOR systems. Only one ILS system and one marker beacon system is required under Part 121.
When an aircraft is navigating over routes using low frequency, ADF or Radio Range, it only needs one receiver for those NAVAIDs, if it is also equipped with two VOR receivers. If that is the case, the VOR stations must be located such that the aircraft could complete the flight to a suitable airport and make an instrument approach if the low frequency system fails. The airplane must also be fueled to allow for such a failure.
Whenever a different VOR station is tuned, the pilot must listen to the Morse code identification. This will ensure that the correct frequency has been tuned and that a usable signal is available. Occasionally, when a VOR station is undergoing routine maintenance, it will broadcast a signal that is not reliable enough to use for navigation. This condition is indicated in one of two ways. Either the coded ident will be turned off or the ident will be changed to the letters T - E - S - T. Other than the identifier, the station may appear to be broadcasting a normal signal.
To be flown IFR, an aircraft must have had its VORs checked within the past 30 days. The pilots may check the accuracy of the VORs in one of several ways.
The VORs may be checked using a VOT facility on an airport. The VOT broadcasts the 360° radial and so the CDI needle should center either on a setting of 360° with a FROM indication or on 180° with a TO indication. A deviation of +/-4° is acceptable for a VOT check.
If a VOT is not available, a VOR checkpoint may be used instead. The aircraft must be moved to the checkpoint and the designated radial set in the CDI course. The acceptable variation for a ground check is +/-4°. For an airborne check the allowable variation is +/-6°.
If no VOT or VOR check point is available, the VORs may be checked against each other. This is called a "dual VOR check." Tune the VORs to the same station and check the difference in indicated bearing. If they are within 4° of each other, the check is satisfactory. This check can be performed on the ground or in the air.
The person making a VOR check must make an entry in the aircraft log or other record. A proper entry includes the date, place and bearing error. The checker must sign the entry. Besides the VOR check, the altimeter system and the transponder must have been checked within the last 24 calendar months (14 CFR §91.411 and §91.413).
Whenever VOR receivers are required on board an aircraft operating within the United States, it must also have at least one DME receiver on board as well. Note: 14 CFR §91.205 requires a DME only if the aircraft is operated above FL240. 14 CFR §121.349 makes the DME required equipment for all air carrier aircraft operating in the U.S. If the DME fails in flight, the pilot must inform ATC as soon as possible.
DME indicates the actual distance from the station to the receiving aircraft in nautical miles. That is different from the horizontal distance because the aircraft is always higher than the DME ground station and altitude is included in the slant range. As a practical matter, the difference between the horizontal distance and the "slant range" is insignificant at distances of more than 10 miles from the station. There is a considerable error close to the station when the aircraft is at high altitudes. In such a situation, almost all of the slant range distance is vertical. When an aircraft passes over a DME station, the distance indicated at station passage is the altitude of the aircraft above the station in nautical miles. For example, if an airplane flew over a VORTAC site 12,000 feet above the station, the DME would indicate 2.0 NM.
Horizontal Situation Indicator (HSI)
The Horizontal Situation Indicator (HSI) is a combination of two instruments: the heading indicator and the VOR.
The aircraft heading is displayed on the rotating azimuth card under the upper lubber line. The arrowhead indicates the course that is selected. The tail of the arrow indicates the reciprocal of the course selected.
The course deviation bar operates with a VOR/LOC navigation receiver to indicate either left or right deviations from the course that is selected with the course-indicating arrow. It moves left or right to indicate deviation from the centerline in the same manner that the angular movement of a conventional VOR/LOC needle indicates deviation from course.
The desired course is selected by rotating the course-indicating arrow in relation to the azimuth card by means of the course set knob. This gives the pilot a pictorial presentation. The fixed aircraft symbol and the course deviation bar display the aircraft relative to the selected course as though the pilot was above the aircraft looking down.
The TO/FROM indicator is a triangular-shaped pointer. When this indicator points to the head of the course arrow, it indicates that the course selected, and if properly intercepted and flown, will take the aircraft TO the selected facility, and vice versa.
The glide slope deviation pointer indicates the relationship of the aircraft to the glide slope. When the pointer is below the center position, the aircraft is above the glide slope and an increased rate of descent is required.
To orient where the aircraft is in relation to the facility, first determine which radial is selected (look at the arrowhead). Next, determine whether the aircraft is flying to or away from the station (look at the TO/FROM indicator) to find which hemisphere the aircraft is in. Next, determine how far from the selected course the aircraft is (look at the deviation bar) to find which quadrant the aircraft is in. Last, consider the aircraft heading (under the lubber line) to determine the aircraft's position within the quadrant.
Radio Magnetic Indicator (RMI)
The compass card shows the aircraft heading at all times under the lubber line. The two needles show the bearings TO and FROM the number 1 and number 2 VORs. The thin needle is usually the number 1 VOR and the double bar needle shows number 2. Often, one or both needles can be selected to display ADF bearing information. The head of each needle shows the magnetic bearing to the station and the tail shows the bearing from (radial).
To orient where the aircraft is in relation to the facility, first determine which radial is selected to find which quadrant you are in (look at the tail of the needle; if you are trying to orient yourself relative to the VOR, make sure you are using the VOR needle). Next, consider the aircraft heading (under the lubber line) to determine the aircraft's position within the quadrant.
Long Range Navigation Systems
When an air carrier operates on routes outside of the 48 contiguous states where the aircraft's position cannot be reliably fixed for more than one hour, special rules apply. The aircraft must either be equipped with a "specialized means of navigation" (INS or Doppler Radar), or one of the flight crewmembers must have a current flight navigator certificate. The FAA may also require either a navigator or the specialized navigation on routes which meet the one hour rule if they feel it's necessary. All routes that require either the navigator or specialized means of navigation must be listed in the air carrier's operations specifications.
Certain routes over the North Atlantic Ocean between North America and Europe require better than normal standards of navigation. Appendix C of 14 CFR Part 91 defines these routes and the required navigation standards. The Administrator (the FAA) has the authority to grant a deviation from the navigation standards of Appendix C if an operator requests one.
Inertial Navigation System (INS) is the primary system used by air carriers for over-water navigation. Prior to flight, the pilots enter the present latitude and longitude of the aircraft and the fixes that make up the desired route. The INS constantly updates its position by signals from self contained gyros and accelerometers. The unit then computes the direction and distance to the next fix and displays this information on the aircraft's navigational instruments. The system is completely self-contained and neither needs nor uses signals from any outside navigational source. If the INS gets input of the aircraft's heading and airspeed, it can compute and display the wind and any drift angle. When INS is used as the navigation system, the aircraft must have either two INS units or one INS and Doppler Radar unit (14 CFR §121.355).
LORAN-C is a pulsed, hyperbolic system operating in the 90 to 110 kHz frequency band. The system is based on measurement of the difference in time of arrival of pulses of RF energy radiated by a "chain" of transmitters located hundreds of miles apart. Within a chain, one station is designated as the Master (M) and the others are called secondaries, Whiskey (W), X-Ray (X), Yankee (Y) and Zulu (Z). Each chain is identified by its unique Group Repetition Interval (GRI).
NOTAM information on the status of any LORAN-C chain or station outages in the United States can be found in NOTAM (D)s under the identifier "LRN."
The LORAN-C receiver in the aircraft converts the time difference (TD) information into geographic coordinates. Using this information, it generates a course and distance to a designated waypoint fix. LORAN-C installation is approved on an individual basis. If a particular aircraft installation is approved for IFR operations, there will be an entry in the airplane's Flight Manual Supplement or the aircraft will have an FAA Form 337 (Major Repair or Alteration) approving the installation.
LORAN-C, originally developed as a marine navigational aid, has gained wide acceptance in the aviation community in recent years. The chains were set up in the U. S. coastal areas. Originally there was a gap in suitable coverage in the midwest and southwestern U.S. This gap has been filled with the commissioning of an additional chain. LORAN is approved for IFR in the 48 contiguous states. During the approach phase, the receiver must detect a lost signal, or signal blink, within 10 seconds of the occurrence and warn the pilot of the event.
Approach Systems
The primary instrument approach system in the United States is the Instrument Landing System (ILS). The system can be divided operationally into three parts: guidance, range and visual information. If any of the elements is unusable, the approach minimums may be raised or the approach may not be authorized at all.
The guidance information consists of the localizer for horizontal guidance and the glide slope for vertical guidance. The localizer operates on one of 40 frequencies from 108.10 MHz to 111.95 MHz. The glide slope operates on one of 40 paired UHF frequencies. The Morse code identifier of the localizer is the letter "I" (· ·) followed by three other letters unique to that facility. The portion of the localizer used for the ILS approach is called the front course. The portion of the localizer extending from the far end of the runway is called the back course. The back course may be used for missed approach procedures or for a back course approach if one is published.
Range information is usually provided by 75 MHz marker beacons or, occasionally, by DME. There are four types of marker beacons associated with ILS approaches -- the outer marker, the middle marker, the inner marker and the back course marker. Flying over any marker beacon will result in both visual and aural indications. The outer marker is identified by a blue light and continuous dashes in Morse code at a rate of 2 per second. The middle marker is indicated by a flashing amber light and alternating dots and dashes at a rate of 2 per second. The inner marker flashes the white light and sounds continuous dots at 6 per second. The back course marker will also flash the white light and sound a series of 2-dot combinations.
Often, an ADF facility (called a compass locator) is associated with an ILS approach. Usually it is located at the outer marker, but occasionally it is co-located with the middle marker. An outer compass locator is identified with the first 2 letters of the localizer identification group. A middle compass locator is identified by the last 2 letters of the localizer.
If a middle marker is out of service, the middle compass locator or PAR radar can be substituted. The middle marker being inoperative does not affect minimums during a Category I ILS approach.
The visual information portion of the ILS consists of approach lights, touchdown and centerline lights and runway lights.
The localizer is very narrow. In fact a full scale deflection (CDI moving from the center to full scale left or right) is only about 700 feet at the runway threshold.
Different aircraft will require different rates of descent to stay on glide slope. A good rule of thumb is that the vertical speed in feet per minute will be equal to about five times the ground speed in knots. For example, an aircraft with an approach speed of 140 knots will require a descent rate of about 700 feet per minute (140 x 5 = 700).
The lowest approach minimums that can be used for a normal (Category I) ILS approach are a DH of 200 feet and 1,800 feet RVR. A Category II ILS approach will have minimums as low as a DH of 100 feet and a visibility requirement of 1,200 feet RVR. The approach has to be approved for Category II minimums. In addition to suitable localizer, glide slope and marker beacons, the approach must have certain additional equipment working on the landing runway. This equipment includes an approach light system, High Intensity Runway Lights (HIRL), Touchdown Zone Lights (TDZL), Runway Centerline Lights (CL) and Runway Visual Range (RVR). Radar, VASI and Runway End Identifier Lights (REIL) are not required components of a Category II approach system. To descend below the DH from a Category II approach the pilot must be able to see one of the following:
· The runway threshold;
· The threshold markings;
· The threshold lights;
· The touchdown zone or the touchdown zone markings;
· The touchdown zone lights; or
· The approach light system, except that a pilot may not descend below 100 feet above the touchdown zone unless the red terminating bars or the red side row bars are distinctly visible and identifiable.
Some airports have Category IIIA approaches. This type of approach has a required visibility of as little as 700 feet RVR, and no DH.
The Simplified Directional Facility (SDF) and the Localizer-type Directional Air (LDA) are approach systems that give a localizer-type indication to the pilot, but with some significant differences. The LDA is essentially a localizer, but it is not aligned within 3° of the runway as a localizer must be. The localizer can be any width from 3° to 6° wide. If the LDA is within 30°, straight-in minimums will be published for it; if not, only circling minimums will be published. The SDF may or may not be aligned with the runway. The main difference between it and a localizer is that its width is fixed at either 6° or 12°.
GPS
The Global Positioning System (GPS) is a satellite-based radio navigational, positioning, and time transfer system. The GPS receiver verifies the integrity (usability) of the signals received from the GPS satellites through receiver autonomous integrity monitoring (RAIM) to determine if a satellite is providing corrupted information. Without RAIM capability, the pilot has no assurance of the accuracy of the GPS position. If RAIM is not available, another type of navigation and approach system must be used, another destination selected, or the trip delayed until RAIM is predicted to be available on arrival. The authorization to use GPS to fly instrument approaches is limited to U.S. airspace. The use of GPS in any other airspace must be expressly authorized by the FAA Administrator.
If a visual descent point (VDP) is published, it will not be included in the sequence of waypoints. Pilots are expected to use normal piloting techniques for beginning the visual descent. The database may not contain all of the transitions or departures from all runways and some GPS receivers do not contain DPs in the database. The GPS receiver must be set to terminal (+/-1 NM) course deviation indicator (CDI) sensitivity and the navigation routes contained in the database in order to fly published IFR charted departures and DPs. Terminal RAIM should be automatically provided by the receiver. Terminal RAIM for departure may not be available unless the waypoints are part of the active flight plan rather than proceeding direct to the first destination. Overriding an automatically selected sensitivity during an approach will cancel the approach mode annunciation. The RAIM and CDI sensitivity will not ramp down, and the pilot should not descend to MDA, but fly to the MAWP and execute a missed approach.
It is necessary that helicopter procedures be flown at 70 knots or less since helicopter departure procedures and missed approaches use a 20:1 obstacle clearance surface (OCS), which is double the fixed-wing OCS, and turning areas are based on this speed as well.
The pilot must be thoroughly familiar with the activation procedure for the particular GPS receiver installed in the aircraft and must initiate appropriate action after the missed approach waypoint (MAWP). Activating the missed approach prior to the MAWP will cause CDI sensitivity to immediately change to terminal (+/-1 NM) sensitivity and the receiver will continue to navigate to the MAWP. The receiver will not sequence past the MAWP. Turns should not begin prior to the MAWP. A GPS missed approach requires pilot action to sequence the receiver past the MAWP to the missed approach portion of the procedure. If the missed approach is not activated, the GPS receiver will display an extension of the inbound final approach course and the ATD will increase from the MAWP until it is manually sequenced after crossing the MAWP.
Any required alternate airport must have an approved instrument approach procedure other than GPS, which is anticipated to be operational and available at the estimated time of arrival and which the aircraft is equipped to fly. Missed approach routings in which the first track is via a course rather than direct to the next waypoint require additional action by the pilot to set the course. Being familiar with all of the inputs required is especially critical during this phase of flight.
Airport Lighting and Marking
A rotating beacon not only aids in locating an airport at night or in low visibility, it can also help to identify which airport is seen. Civilian airports have a beacon that alternately flashes green and white. A military airport has the same green and white beacon but the white beam is split to give a dual flash of white. A lighted heliport has a green, yellow and white beacon.
FAA Figure 129 shows the basic marking and lighting for a runway with a nonprecision approach. The threshold is marked with 4 stripes on either side of the centerline. 1,000 feet from the threshold, a broad "fixed distance" marker is painted on either side of the centerline (A). The runway lights are white for the entire length of the runway (as are the centerline lights if installed). The threshold is lit with red lights.
FAA Figure 130 shows the somewhat more elaborate ICAO markings for a nonprecision runway. In addition to the fixed distance marker, there are stripes painted on the runway every 500 feet to a distance of 3,000 feet from the threshold. This runway has either High Intensity Runway Lights (HIRL) or Medium Intensity Runway Lights (MIRL) installed. These lights are amber rather than white in the areas within 2,000 feet of the threshold. This gives the pilot a "caution zone" on landing rollout.
FAA Figure 131 shows the lighting and marking for a precision instrument runway. The runway striping has been modified to make it easier to tell exactly how much runway remains. The stripes are still at 500 foot intervals for the 3,000 feet from the threshold. The HIRL or MIRL turns amber for the 2,000 feet closest to the threshold. The centerline lighting has alternating red and white lights from 3,000 feet to 1,000 feet to go, and is all red lights in the 1,000 feet closest to the threshold.
In addition to the markings discussed above, some runways have distance remaining markers. These are simply signs showing the remaining runway in thousands of feet.
Taxi leadoff lights associated with runway centerline lights are green and yellow alternating lights, curving from the centerline of the runway to a point on the exit.
Some runways have Runway End Identifier Lights (REIL) installed at the threshold. These are synchronized flashing lights (usually strobes) placed laterally at either side of the runway threshold. Their purpose is to facilitate identification of a runway surrounded by numerous other lighting systems.
LAHSO is an acronym for "Land And Hold Short Operations." These operations include landing and holding short of an intersecting runway, an intersecting taxiway, or some other designated point on a runway other than an intersecting runway or taxiway. At controlled airports, ATC may clear a pilot to land and hold short. The pilot-in-command has the final authority to accept or decline any land and hold short clearance. The safety and operation of the aircraft remain the responsibility of the pilot. To conduct LAHSO, pilots should become familiar with all available information concerning LAHSO at their destination airport. Pilots should have, readily available, the published Available Landing Distance (ALD) and runway slope information for all LAHSO runway combinations at each airport of intended landing. Additionally, knowledge about landing performance data permits the pilot to readily determine that the ALD for the assigned runway is sufficient for safe LAHSO. If, for any reason, such as difficulty in discerning the location of a LAHSO intersection, wind conditions, aircraft condition, etc., the pilot elects to request to land on the full length of the runway, to land on another runway, or to decline LAHSO, a pilot is expected to promptly inform ATC, ideally even before the clearance is issued. A LAHSO clearance, once accepted, must be adhered to, just as any other ATC clearance, unless an amended clearance is obtained or an emergency occurs. However, a LAHSO clearance does not preclude a rejected landing. The airport markings, signage, and lighting associated with LAHSO consist of a three-part system of yellow hold-short markings, red and white signage and, in certain cases, in-pavement lighting.
Approach Lighting
An airplane approaching to land on a runway served by a Visual Approach Slope Indicator (VASI) must remain on or above the glide slope (except for normal bracketing) until a lower altitude is necessary for a safe landing.
A VASI gives the pilot a visual glide slope to follow when landing on certain runways. A VASI glide slope is normally about 3° (the same as an ILS) and the aim point is about 1,000 feet down the runway from the threshold. The angle and aim point of the VASI can be adjusted as necessary to accommodate the runway conditions. If a pilot of a high performance airplane is flying a VASI with a glide slope steeper than 3.5°, he/she should be aware that a longer than normal roll-out may result from the flare maneuver required by the steep angle.
Many runways used by air carrier aircraft have a three-bar VASI system to accommodate aircraft with a high cockpit such as Boeing 747 or DC-10. These aircraft need a glide slope that has an aim point further down the runway to ensure adequate clearance for the landing gear at the runway threshold. The pilot of such an airplane must use the two upwind lights (middle and far bars) for glide slope information.
The Precision Approach Path Indicator (PAPI) approach light system consists of a row of four lights perpendicular to the runway. Each light can be either red or white depending on the aircraft's position relative to the glide slope. The glide slope indications of a PAPI are as follows:
· High -- 4 white lights
· Slightly high -- 1 red, 3 white lights
· On glidepath -- 2 red, 2 white lights
· Slightly low -- 1 white, 3 red lights
· Low -- 4 red lights
A tri-color VASI consists of one light projector with three colors; red, green and amber. A "high" indication is amber, an "on glide slope" is green and a "low" is red. A tri-color VASI can be seen at a distance of 1/2 to 1 mile in daylight and up to 5 miles at night.
Pulsating visual approach slope indicators normally consist of a single light unit projecting a two-color visual approach path. The below glidepath indication is normally pulsating red and the above glidepath indication is normally pulsating white. The "on glide slope" indication for one type of system is a steady white light, while for another type it is an alternating red and white.
Inoperative Equipment
A certificate holder's manual must contain enroute flight, navigation and communication procedures, including procedures for the dispatch, release or continuance of a flight if a required piece of equipment becomes inoperative.
When any required instrument or equipment in an aircraft is inoperative, the airplane cannot be flown unless that aircraft's Minimum Equipment List (MEL) allows such a flight.
The pilot-in-command of an aircraft operating IFR in controlled airspace shall report to ATC immediately any malfunction of navigational, approach or communications equipment that occurs in flight. The report must include:
· Aircraft identification;
· Equipment affected;
· Degree to which the capability of the aircraft to operate IFR in the ATC system is impaired; and
· Nature and extent of assistance desired from ATC.
Pitot-Static Instruments
Modern jet transports usually have three pitot-static systems. There are separate systems for the captain's and co-pilot's instruments plus an auxiliary system that provides a backup for either of the two primary systems. The instruments that require static pressure input are airspeed, Mach, altitude and vertical speed indicators. In addition, the airspeed and Mach indicators need a source of pitot pressure. Besides the flight instruments, static pressure input is required for the Mach warning, autopilot, flight director, flight recorder and cabin differential pressure. Pitot input is required for all those systems except for cabin differential pressure. The usual source for these non-flight instruments is the auxiliary pitot-static system.
Altimeters compare the sea level pressure setting in their window with the outside air pressure sensed through the static system. The difference is displayed as the altitude above sea level. Part of the preflight check is to verify the accuracy of the altimeters. An altimeter should be considered questionable if the indicated altitude varies by more the 75 feet from a known field elevation.
The altimeter setting used by pilots is always the station pressure of the reporting station corrected to sea level. Station pressure is the actual pressure at field elevation.
True altitude is the actual height of the aircraft above sea level. This is the same as indicated altitude when standard temperatures exist. When the temperature is warmer than standard, true altitude is higher than indicated altitude. When the temperature is colder than standard day conditions, just the opposite is true. Corrected altitude (approximately true altitude) can be calculated but it is neither practical nor useful to do so in most situations. When setting an altimeter, a pilot should just use the appropriate altimeter setting and disregard the effects of nonstandard atmospheric pressures and temperatures.
Pressure altitude is the altitude indicated when the altimeter is set to standard sea level pressure of 29.92" Hg. Density altitude is used in aircraft performance computations. It is pressure altitude corrected for nonstandard temperatures. If the temperature is warmer than standard, density altitude will be higher than pressure altitude.
The local altimeter setting is used when flying below FL180 and the altimeter is 31.00" Hg or less. Special procedures apply when the local pressure is more than 31.00" Hg because most altimeters cannot be set higher than that. In the United States, all altimeters are set to 29.92" Hg when climbing through FL180. Caution: outside the United States the transition altitude is often something other than FL180.
A common reason for altimeter errors is incorrect setting of the altimeter. If the setting in the altimeter is higher than the actual sea level pressure, the altimeter will read higher than the actual altitude. If the setting is too low, the altimeter will read lower than it really is. As a rough rule of thumb, the magnitude of the error is about 1,000 feet for each 1" Hg that the altimeter is off. For example, if the altimeter is set to 29.92" Hg, but the real sea level pressure is 30.57" Hg, the altimeter will read about 650 feet lower than the actual airplane's altitude (30.57 - 29.92 = .65" Hg = 650 feet). In this example, the airplane would be 650 feet higher than the indicated altitude.
The airspeed indicators compare pitot pressure with static pressure and display the difference as indicated airspeed. This indicated airspeed equals the aircraft's actual speed through the air (True Airspeed) only under standard day conditions at sea level. Under almost all flight conditions, true airspeed will be higher than indicated airspeed because of the lower ambient pressures at altitude.
The Machmeter displays aircraft speed as a percentage of the speed of sound. For example, an aircraft cruising at a Mach number of .82 is flying at 82% of the speed of sound. The Machmeter works in a manner similar to the airspeed indicator in that it compares pitot and static pressure, but these inputs are corrected by an altimeter mechanism.
If a pitot tube becomes blocked, the airspeed and Mach indicators will read inaccurately. If pressure is trapped in the pitot line, the airspeed will read inaccurately high as the aircraft climbs, low as it descends, and will be unresponsive to changes in airspeed. The airspeed indicator acts as an altimeter because only the static pressure changes. This situation occurs in icing conditions if both the ram air inlet and the drain hole of the pitot tube become completely blocked by ice.
If the pitot tube is blocked but the static port and the pitot drain hole remain open, the indicated airspeed will drop to zero. The drain pitot tube drain hole allows the pressure in the pitot line to drop to atmospheric and therefore there is no differential between the static and pitot pressures.
Pitot tubes and static ports are electrically heated to prevent ice formations that could interfere with proper operation of the systems. They are required to have "power on" indicator lights to show proper operation. In addition, many aircraft have an ammeter that shows the actual current flow to the pitot and static ports.
Since the magnetic compass is the only direction-seeking instrument in most airplanes, the pilot must be able to turn the airplane to a magnetic compass heading and maintain this heading. It is influenced by magnetic dip which causes northerly turning error and acceleration/deceleration error. When northerly turning error occurs, the compass will lag behind the actual aircraft heading while turning through headings in the northern half of the compass rose, and lead the aircraft's actual heading in the southern half. The error is most pronounced when turning through north or south, and is approximately equal in degrees to the latitude.
The acceleration/deceleration error is most pronounced on headings of east and west. When accelerating, the compass indicates a turn toward the north, and when decelerating it indicates a turn toward the south. The acronym ANDS is a good memory aid:
A accelerate
N north
D decelerate
S south
No errors are apparent while on east or west headings, when turning either north or south.
Safety of Flight Equipment
Airborne weather radar is used to detect and avoid areas of heavy precipitation such as thunderstorms. With few exceptions, all air carrier aircraft must be equipped with an approved airborne weather radar unit. The radar must be in satisfactory operating condition prior to dispatch on an IFR or night VFR flight if thunderstorms (or other hazardous weather) that could be detected by the radar are forecast along the intended route of flight. An aircraft may be dispatched with an inoperative radar unit if one of two conditions is met:
· The flight will be able to remain in day VFR flight conditions, or
· Hazardous weather is not forecast.
An air carrier's operations manual must contain procedures for the flight crew to follow if the weather radar fails in flight.
A ground proximity warning system (GPWS) must be installed on all large turbine powered airplanes. The GPWS gives aural and visual warnings when an aircraft too close to the terrain is in an improper configuration for landing, or when it deviates below glide slope on an ILS approach.
TCAS I (Traffic Alert and Collision Avoidance System) provides proximity warning only, to assist the pilot in the visual acquisition of intruder aircraft. No recommended avoidance maneuvers are provided nor authorized as a result of a TCAS I warning. TCAS II provides traffic advisories (TAs) and resolution advisories (RAs). Resolution advisories provide recommended maneuvers in a vertical direction to avoid conflicting traffic. TCAS does not alter or diminish the pilot's basic authority and responsibility to ensure safe flight. After the conflict, return to the ATC clearance in effect. If a deviation occurs, contact ATC as soon as practical.
Cockpit voice recorders are required on large turbine engine powered airplanes and large four engine reciprocating powered airplanes. The recorder must operate from before the start of the before starting checklist to the completion of the secure cockpit checklist. Although the recorder runs for the entire flight, only the most recent 30 minutes of information need be retained on the recorder tape.
An approved flight recorder must be installed on all airplanes certified for operations above 25,000 feet and on all turbine powered airplanes. What the flight recorder must record varies from airplane to airplane, but at a minimum it must record:
· Time
· Altitude
· Airspeed
· Vertical acceleration
· Heading
· Time of each radio transmission to or from ATC.
An air carrier must keep the flight recorder data until an aircraft has been operated at least 25 hours after the data was removed. However, 1 hour of the oldest recorded data may be erased to test the flight recorder.
The cockpit voice and flight recorder data can be used to identify malfunctions and irregularities with the aircraft and in carrying out investigations under NTSB Part 830. It cannot be used by the FAA for enforcement purposes. If an incident occurs which would require the immediate notification of the NTSB, the data must be kept by the operator for at least 60 days.
Communications
Each flag and domestic operator must have a two-way radio system that, under normal conditions, allows reliable and rapid communications between its aircraft and the appropriate dispatch office. For operations within the 48 contiguous states, this system must be independent of any operated by the U.S. government.
One source of current weather information in flight is the Enroute Flight Advisory Service (Flight Watch). This service is available nationwide from Flight Service Stations on frequency 122.0 MHz. Flight Watch is limited to weather information only. It is not to be used to open or close flight plans or for any ATC function.
Navigation Equipment
When an aircraft is flown IFR or VFR Over-the-Top it must have a dual installation of the navigation radios required to fly that route. This means that an aircraft flying Victor airways or jet routes must have two operable VOR systems. Only one ILS system and one marker beacon system is required under Part 121.
When an aircraft is navigating over routes using low frequency, ADF or Radio Range, it only needs one receiver for those NAVAIDs, if it is also equipped with two VOR receivers. If that is the case, the VOR stations must be located such that the aircraft could complete the flight to a suitable airport and make an instrument approach if the low frequency system fails. The airplane must also be fueled to allow for such a failure.
Whenever a different VOR station is tuned, the pilot must listen to the Morse code identification. This will ensure that the correct frequency has been tuned and that a usable signal is available. Occasionally, when a VOR station is undergoing routine maintenance, it will broadcast a signal that is not reliable enough to use for navigation. This condition is indicated in one of two ways. Either the coded ident will be turned off or the ident will be changed to the letters T - E - S - T. Other than the identifier, the station may appear to be broadcasting a normal signal.
To be flown IFR, an aircraft must have had its VORs checked within the past 30 days. The pilots may check the accuracy of the VORs in one of several ways.
The VORs may be checked using a VOT facility on an airport. The VOT broadcasts the 360° radial and so the CDI needle should center either on a setting of 360° with a FROM indication or on 180° with a TO indication. A deviation of +/-4° is acceptable for a VOT check.
If a VOT is not available, a VOR checkpoint may be used instead. The aircraft must be moved to the checkpoint and the designated radial set in the CDI course. The acceptable variation for a ground check is +/-4°. For an airborne check the allowable variation is +/-6°.
If no VOT or VOR check point is available, the VORs may be checked against each other. This is called a "dual VOR check." Tune the VORs to the same station and check the difference in indicated bearing. If they are within 4° of each other, the check is satisfactory. This check can be performed on the ground or in the air.
The person making a VOR check must make an entry in the aircraft log or other record. A proper entry includes the date, place and bearing error. The checker must sign the entry. Besides the VOR check, the altimeter system and the transponder must have been checked within the last 24 calendar months (14 CFR §91.411 and §91.413).
Whenever VOR receivers are required on board an aircraft operating within the United States, it must also have at least one DME receiver on board as well. Note: 14 CFR §91.205 requires a DME only if the aircraft is operated above FL240. 14 CFR §121.349 makes the DME required equipment for all air carrier aircraft operating in the U.S. If the DME fails in flight, the pilot must inform ATC as soon as possible.
DME indicates the actual distance from the station to the receiving aircraft in nautical miles. That is different from the horizontal distance because the aircraft is always higher than the DME ground station and altitude is included in the slant range. As a practical matter, the difference between the horizontal distance and the "slant range" is insignificant at distances of more than 10 miles from the station. There is a considerable error close to the station when the aircraft is at high altitudes. In such a situation, almost all of the slant range distance is vertical. When an aircraft passes over a DME station, the distance indicated at station passage is the altitude of the aircraft above the station in nautical miles. For example, if an airplane flew over a VORTAC site 12,000 feet above the station, the DME would indicate 2.0 NM.
Horizontal Situation Indicator (HSI)
The Horizontal Situation Indicator (HSI) is a combination of two instruments: the heading indicator and the VOR.
The aircraft heading is displayed on the rotating azimuth card under the upper lubber line. The arrowhead indicates the course that is selected. The tail of the arrow indicates the reciprocal of the course selected.
The course deviation bar operates with a VOR/LOC navigation receiver to indicate either left or right deviations from the course that is selected with the course-indicating arrow. It moves left or right to indicate deviation from the centerline in the same manner that the angular movement of a conventional VOR/LOC needle indicates deviation from course.
The desired course is selected by rotating the course-indicating arrow in relation to the azimuth card by means of the course set knob. This gives the pilot a pictorial presentation. The fixed aircraft symbol and the course deviation bar display the aircraft relative to the selected course as though the pilot was above the aircraft looking down.
The TO/FROM indicator is a triangular-shaped pointer. When this indicator points to the head of the course arrow, it indicates that the course selected, and if properly intercepted and flown, will take the aircraft TO the selected facility, and vice versa.
The glide slope deviation pointer indicates the relationship of the aircraft to the glide slope. When the pointer is below the center position, the aircraft is above the glide slope and an increased rate of descent is required.
To orient where the aircraft is in relation to the facility, first determine which radial is selected (look at the arrowhead). Next, determine whether the aircraft is flying to or away from the station (look at the TO/FROM indicator) to find which hemisphere the aircraft is in. Next, determine how far from the selected course the aircraft is (look at the deviation bar) to find which quadrant the aircraft is in. Last, consider the aircraft heading (under the lubber line) to determine the aircraft's position within the quadrant.
Radio Magnetic Indicator (RMI)
The compass card shows the aircraft heading at all times under the lubber line. The two needles show the bearings TO and FROM the number 1 and number 2 VORs. The thin needle is usually the number 1 VOR and the double bar needle shows number 2. Often, one or both needles can be selected to display ADF bearing information. The head of each needle shows the magnetic bearing to the station and the tail shows the bearing from (radial).
To orient where the aircraft is in relation to the facility, first determine which radial is selected to find which quadrant you are in (look at the tail of the needle; if you are trying to orient yourself relative to the VOR, make sure you are using the VOR needle). Next, consider the aircraft heading (under the lubber line) to determine the aircraft's position within the quadrant.
Long Range Navigation Systems
When an air carrier operates on routes outside of the 48 contiguous states where the aircraft's position cannot be reliably fixed for more than one hour, special rules apply. The aircraft must either be equipped with a "specialized means of navigation" (INS or Doppler Radar), or one of the flight crewmembers must have a current flight navigator certificate. The FAA may also require either a navigator or the specialized navigation on routes which meet the one hour rule if they feel it's necessary. All routes that require either the navigator or specialized means of navigation must be listed in the air carrier's operations specifications.
Certain routes over the North Atlantic Ocean between North America and Europe require better than normal standards of navigation. Appendix C of 14 CFR Part 91 defines these routes and the required navigation standards. The Administrator (the FAA) has the authority to grant a deviation from the navigation standards of Appendix C if an operator requests one.
Inertial Navigation System (INS) is the primary system used by air carriers for over-water navigation. Prior to flight, the pilots enter the present latitude and longitude of the aircraft and the fixes that make up the desired route. The INS constantly updates its position by signals from self contained gyros and accelerometers. The unit then computes the direction and distance to the next fix and displays this information on the aircraft's navigational instruments. The system is completely self-contained and neither needs nor uses signals from any outside navigational source. If the INS gets input of the aircraft's heading and airspeed, it can compute and display the wind and any drift angle. When INS is used as the navigation system, the aircraft must have either two INS units or one INS and Doppler Radar unit (14 CFR §121.355).
LORAN-C is a pulsed, hyperbolic system operating in the 90 to 110 kHz frequency band. The system is based on measurement of the difference in time of arrival of pulses of RF energy radiated by a "chain" of transmitters located hundreds of miles apart. Within a chain, one station is designated as the Master (M) and the others are called secondaries, Whiskey (W), X-Ray (X), Yankee (Y) and Zulu (Z). Each chain is identified by its unique Group Repetition Interval (GRI).
NOTAM information on the status of any LORAN-C chain or station outages in the United States can be found in NOTAM (D)s under the identifier "LRN."
The LORAN-C receiver in the aircraft converts the time difference (TD) information into geographic coordinates. Using this information, it generates a course and distance to a designated waypoint fix. LORAN-C installation is approved on an individual basis. If a particular aircraft installation is approved for IFR operations, there will be an entry in the airplane's Flight Manual Supplement or the aircraft will have an FAA Form 337 (Major Repair or Alteration) approving the installation.
LORAN-C, originally developed as a marine navigational aid, has gained wide acceptance in the aviation community in recent years. The chains were set up in the U. S. coastal areas. Originally there was a gap in suitable coverage in the midwest and southwestern U.S. This gap has been filled with the commissioning of an additional chain. LORAN is approved for IFR in the 48 contiguous states. During the approach phase, the receiver must detect a lost signal, or signal blink, within 10 seconds of the occurrence and warn the pilot of the event.
Approach Systems
The primary instrument approach system in the United States is the Instrument Landing System (ILS). The system can be divided operationally into three parts: guidance, range and visual information. If any of the elements is unusable, the approach minimums may be raised or the approach may not be authorized at all.
The guidance information consists of the localizer for horizontal guidance and the glide slope for vertical guidance. The localizer operates on one of 40 frequencies from 108.10 MHz to 111.95 MHz. The glide slope operates on one of 40 paired UHF frequencies. The Morse code identifier of the localizer is the letter "I" (· ·) followed by three other letters unique to that facility. The portion of the localizer used for the ILS approach is called the front course. The portion of the localizer extending from the far end of the runway is called the back course. The back course may be used for missed approach procedures or for a back course approach if one is published.
Range information is usually provided by 75 MHz marker beacons or, occasionally, by DME. There are four types of marker beacons associated with ILS approaches -- the outer marker, the middle marker, the inner marker and the back course marker. Flying over any marker beacon will result in both visual and aural indications. The outer marker is identified by a blue light and continuous dashes in Morse code at a rate of 2 per second. The middle marker is indicated by a flashing amber light and alternating dots and dashes at a rate of 2 per second. The inner marker flashes the white light and sounds continuous dots at 6 per second. The back course marker will also flash the white light and sound a series of 2-dot combinations.
Often, an ADF facility (called a compass locator) is associated with an ILS approach. Usually it is located at the outer marker, but occasionally it is co-located with the middle marker. An outer compass locator is identified with the first 2 letters of the localizer identification group. A middle compass locator is identified by the last 2 letters of the localizer.
If a middle marker is out of service, the middle compass locator or PAR radar can be substituted. The middle marker being inoperative does not affect minimums during a Category I ILS approach.
The visual information portion of the ILS consists of approach lights, touchdown and centerline lights and runway lights.
The localizer is very narrow. In fact a full scale deflection (CDI moving from the center to full scale left or right) is only about 700 feet at the runway threshold.
Different aircraft will require different rates of descent to stay on glide slope. A good rule of thumb is that the vertical speed in feet per minute will be equal to about five times the ground speed in knots. For example, an aircraft with an approach speed of 140 knots will require a descent rate of about 700 feet per minute (140 x 5 = 700).
The lowest approach minimums that can be used for a normal (Category I) ILS approach are a DH of 200 feet and 1,800 feet RVR. A Category II ILS approach will have minimums as low as a DH of 100 feet and a visibility requirement of 1,200 feet RVR. The approach has to be approved for Category II minimums. In addition to suitable localizer, glide slope and marker beacons, the approach must have certain additional equipment working on the landing runway. This equipment includes an approach light system, High Intensity Runway Lights (HIRL), Touchdown Zone Lights (TDZL), Runway Centerline Lights (CL) and Runway Visual Range (RVR). Radar, VASI and Runway End Identifier Lights (REIL) are not required components of a Category II approach system. To descend below the DH from a Category II approach the pilot must be able to see one of the following:
· The runway threshold;
· The threshold markings;
· The threshold lights;
· The touchdown zone or the touchdown zone markings;
· The touchdown zone lights; or
· The approach light system, except that a pilot may not descend below 100 feet above the touchdown zone unless the red terminating bars or the red side row bars are distinctly visible and identifiable.
Some airports have Category IIIA approaches. This type of approach has a required visibility of as little as 700 feet RVR, and no DH.
The Simplified Directional Facility (SDF) and the Localizer-type Directional Air (LDA) are approach systems that give a localizer-type indication to the pilot, but with some significant differences. The LDA is essentially a localizer, but it is not aligned within 3° of the runway as a localizer must be. The localizer can be any width from 3° to 6° wide. If the LDA is within 30°, straight-in minimums will be published for it; if not, only circling minimums will be published. The SDF may or may not be aligned with the runway. The main difference between it and a localizer is that its width is fixed at either 6° or 12°.
GPS
The Global Positioning System (GPS) is a satellite-based radio navigational, positioning, and time transfer system. The GPS receiver verifies the integrity (usability) of the signals received from the GPS satellites through receiver autonomous integrity monitoring (RAIM) to determine if a satellite is providing corrupted information. Without RAIM capability, the pilot has no assurance of the accuracy of the GPS position. If RAIM is not available, another type of navigation and approach system must be used, another destination selected, or the trip delayed until RAIM is predicted to be available on arrival. The authorization to use GPS to fly instrument approaches is limited to U.S. airspace. The use of GPS in any other airspace must be expressly authorized by the FAA Administrator.
If a visual descent point (VDP) is published, it will not be included in the sequence of waypoints. Pilots are expected to use normal piloting techniques for beginning the visual descent. The database may not contain all of the transitions or departures from all runways and some GPS receivers do not contain DPs in the database. The GPS receiver must be set to terminal (+/-1 NM) course deviation indicator (CDI) sensitivity and the navigation routes contained in the database in order to fly published IFR charted departures and DPs. Terminal RAIM should be automatically provided by the receiver. Terminal RAIM for departure may not be available unless the waypoints are part of the active flight plan rather than proceeding direct to the first destination. Overriding an automatically selected sensitivity during an approach will cancel the approach mode annunciation. The RAIM and CDI sensitivity will not ramp down, and the pilot should not descend to MDA, but fly to the MAWP and execute a missed approach.
It is necessary that helicopter procedures be flown at 70 knots or less since helicopter departure procedures and missed approaches use a 20:1 obstacle clearance surface (OCS), which is double the fixed-wing OCS, and turning areas are based on this speed as well.
The pilot must be thoroughly familiar with the activation procedure for the particular GPS receiver installed in the aircraft and must initiate appropriate action after the missed approach waypoint (MAWP). Activating the missed approach prior to the MAWP will cause CDI sensitivity to immediately change to terminal (+/-1 NM) sensitivity and the receiver will continue to navigate to the MAWP. The receiver will not sequence past the MAWP. Turns should not begin prior to the MAWP. A GPS missed approach requires pilot action to sequence the receiver past the MAWP to the missed approach portion of the procedure. If the missed approach is not activated, the GPS receiver will display an extension of the inbound final approach course and the ATD will increase from the MAWP until it is manually sequenced after crossing the MAWP.
Any required alternate airport must have an approved instrument approach procedure other than GPS, which is anticipated to be operational and available at the estimated time of arrival and which the aircraft is equipped to fly. Missed approach routings in which the first track is via a course rather than direct to the next waypoint require additional action by the pilot to set the course. Being familiar with all of the inputs required is especially critical during this phase of flight.
Airport Lighting and Marking
A rotating beacon not only aids in locating an airport at night or in low visibility, it can also help to identify which airport is seen. Civilian airports have a beacon that alternately flashes green and white. A military airport has the same green and white beacon but the white beam is split to give a dual flash of white. A lighted heliport has a green, yellow and white beacon.
FAA Figure 129 shows the basic marking and lighting for a runway with a nonprecision approach. The threshold is marked with 4 stripes on either side of the centerline. 1,000 feet from the threshold, a broad "fixed distance" marker is painted on either side of the centerline (A). The runway lights are white for the entire length of the runway (as are the centerline lights if installed). The threshold is lit with red lights.
FAA Figure 130 shows the somewhat more elaborate ICAO markings for a nonprecision runway. In addition to the fixed distance marker, there are stripes painted on the runway every 500 feet to a distance of 3,000 feet from the threshold. This runway has either High Intensity Runway Lights (HIRL) or Medium Intensity Runway Lights (MIRL) installed. These lights are amber rather than white in the areas within 2,000 feet of the threshold. This gives the pilot a "caution zone" on landing rollout.
FAA Figure 131 shows the lighting and marking for a precision instrument runway. The runway striping has been modified to make it easier to tell exactly how much runway remains. The stripes are still at 500 foot intervals for the 3,000 feet from the threshold. The HIRL or MIRL turns amber for the 2,000 feet closest to the threshold. The centerline lighting has alternating red and white lights from 3,000 feet to 1,000 feet to go, and is all red lights in the 1,000 feet closest to the threshold.
In addition to the markings discussed above, some runways have distance remaining markers. These are simply signs showing the remaining runway in thousands of feet.
Taxi leadoff lights associated with runway centerline lights are green and yellow alternating lights, curving from the centerline of the runway to a point on the exit.
Some runways have Runway End Identifier Lights (REIL) installed at the threshold. These are synchronized flashing lights (usually strobes) placed laterally at either side of the runway threshold. Their purpose is to facilitate identification of a runway surrounded by numerous other lighting systems.
LAHSO is an acronym for "Land And Hold Short Operations." These operations include landing and holding short of an intersecting runway, an intersecting taxiway, or some other designated point on a runway other than an intersecting runway or taxiway. At controlled airports, ATC may clear a pilot to land and hold short. The pilot-in-command has the final authority to accept or decline any land and hold short clearance. The safety and operation of the aircraft remain the responsibility of the pilot. To conduct LAHSO, pilots should become familiar with all available information concerning LAHSO at their destination airport. Pilots should have, readily available, the published Available Landing Distance (ALD) and runway slope information for all LAHSO runway combinations at each airport of intended landing. Additionally, knowledge about landing performance data permits the pilot to readily determine that the ALD for the assigned runway is sufficient for safe LAHSO. If, for any reason, such as difficulty in discerning the location of a LAHSO intersection, wind conditions, aircraft condition, etc., the pilot elects to request to land on the full length of the runway, to land on another runway, or to decline LAHSO, a pilot is expected to promptly inform ATC, ideally even before the clearance is issued. A LAHSO clearance, once accepted, must be adhered to, just as any other ATC clearance, unless an amended clearance is obtained or an emergency occurs. However, a LAHSO clearance does not preclude a rejected landing. The airport markings, signage, and lighting associated with LAHSO consist of a three-part system of yellow hold-short markings, red and white signage and, in certain cases, in-pavement lighting.
Approach Lighting
An airplane approaching to land on a runway served by a Visual Approach Slope Indicator (VASI) must remain on or above the glide slope (except for normal bracketing) until a lower altitude is necessary for a safe landing.
A VASI gives the pilot a visual glide slope to follow when landing on certain runways. A VASI glide slope is normally about 3° (the same as an ILS) and the aim point is about 1,000 feet down the runway from the threshold. The angle and aim point of the VASI can be adjusted as necessary to accommodate the runway conditions. If a pilot of a high performance airplane is flying a VASI with a glide slope steeper than 3.5°, he/she should be aware that a longer than normal roll-out may result from the flare maneuver required by the steep angle.
Many runways used by air carrier aircraft have a three-bar VASI system to accommodate aircraft with a high cockpit such as Boeing 747 or DC-10. These aircraft need a glide slope that has an aim point further down the runway to ensure adequate clearance for the landing gear at the runway threshold. The pilot of such an airplane must use the two upwind lights (middle and far bars) for glide slope information.
The Precision Approach Path Indicator (PAPI) approach light system consists of a row of four lights perpendicular to the runway. Each light can be either red or white depending on the aircraft's position relative to the glide slope. The glide slope indications of a PAPI are as follows:
· High -- 4 white lights
· Slightly high -- 1 red, 3 white lights
· On glidepath -- 2 red, 2 white lights
· Slightly low -- 1 white, 3 red lights
· Low -- 4 red lights
A tri-color VASI consists of one light projector with three colors; red, green and amber. A "high" indication is amber, an "on glide slope" is green and a "low" is red. A tri-color VASI can be seen at a distance of 1/2 to 1 mile in daylight and up to 5 miles at night.
Pulsating visual approach slope indicators normally consist of a single light unit projecting a two-color visual approach path. The below glidepath indication is normally pulsating red and the above glidepath indication is normally pulsating white. The "on glide slope" indication for one type of system is a steady white light, while for another type it is an alternating red and white.
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