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2. Compressor discharge air controls the density-reference waste gate system.
A density controller holds a given density of air by automatically adjusting manifold pressure as airspeed, ambient pressure, temperature, altitude, and other variables change.
b. Oxygen and Oxygen Equipment. Most high-altitude airplanes come equipped with some type of fixed oxygen installation. If the airplane does not have a fixed installation, portable oxygen equipment must be readily accessible during flight. The portable equipment usually consists of a container, regulator, mask outlet, and pressure gauge. A typical 22-cubic ft portable container will contain enough oxygen for approximately 1.5 hours at 18,000 ft MSL for four people. Aircraft oxygen is usually stored in high-pressure system containers of 1,800 to 2,200 pounds per square inch (psi). The container should be fastened securely in the aircraft before flight. When the ambient temperature surrounding an oxygen cylinder decreases, pressure within that cylinder will decrease because pressure varies directly with temperature if the volume of a gas remains constant. A drop in indicated pressure on a supplemental oxygen cylinder can be an indication that the oxygen supply has merely compacted (not necessarily a leak). High-pressure oxygen containers should be marked with the psi tolerance (i.e., 1,800 psi) before filling the container to that pressure. Only oxygen that meets or exceeds the Society of Automotive Engineers International (SAE) Aerospace Standard AS8010 (as revised), Aviator’s Breathing Oxygen Purity Standard, should be used. To assure safety, periodic inspection and servicing should take place by a certified maintenance provide found at some fixed base operations and terminal complexes.
(1) Regulator and Mask Systems. Regulators and masks work on continuous flow, diluter demand, or pressure demand systems.
(a) Continuous Flow System. The continuous flow system supplies oxygen at a rate that may be controlled automatically or by the user. The mask is designed so the oxygen can be diluted with ambient air by allowing the user to exhale around the facepiece, and comes with a rebreather bag, which allows the individual to reuse part of the exhaled oxygen. Pilot masks sometimes allow greater oxygen flow than passengers’ masks, so it is important that pilots use the masks indicated for them. Although certificated up to 41,000 ft MSL, system capabilities require very careful attention when using continuous flow oxygen systems above 25,000 ft MSL.
(b) Diluter Demand and Pressure Demand Systems. Diluter demand and pressure demand systems supply oxygen only when the user inhales through the mask. An automix lever allows the regulators to automatically mix cabin air and oxygen or supply 100 percent oxygen, depending on the altitude. The demand mask provides a tight seal over the face to prevent dilution with outside air and can be used safely up to 40,000 ft MSL. Pilots who fly at those altitudes should not have beards and mustaches because air can easily seep in through the border of the mask. Pressure demand regulators also create airtight and oxygen-tight seals, but they also provide a positive pressure application of oxygen to the mask facepiece, which allows the user’s lungs to be pressurized with oxygen. This feature makes pressure demand regulators safe at altitudes above 40,000 ft MSL.
(2) Increased Fire Risk. Pilots should be aware of the increased danger of fire when using oxygen. Materials that are nearly fireproof in ordinary air may be susceptible to burning in
Page 34 Par 2-9 oxygen. Oils and greases may catch fire if exposed to oxygen and, therefore, cannot be used for sealing the valves and fittings of oxygen equipment. Smoking during the use of any kind of oxygen equipment must also be strictly forbidden.
(3) Equipment Inspection. A certified maintenance provider must inspect the aircraft system, and a DOT approved facility must hydrostatic test the oxygen bottles, before use. Before each flight, the pilot should thoroughly inspect and test all oxygen equipment. The inspection should be accomplished with clean hands and should include a visual inspection of the mask and tubing for tears, cracks, or deterioration. The regulator should also be checked for valve and lever condition and positions. Also be sure to check the oxygen quantity as well as the location and function of oxygen pressure gauges, flow indicators, and connections. The mask should be donned and the system tested. After any oxygen use, verify that all components and valves are shut off.
c. Pressurization Systems. Cabin pressurization is the compression of air in the aircraft cabin in order to maintain a cabin altitude lower than the actual flight altitude. Because of the ever-present possibility of decompression, the aircraft still requires supplemental oxygen.
Pressurized aircraft meeting specific requirements of 14 CFR part 23 or 25 have cabin altitude warning systems, which activate at a cabin altitude of 10,000 ft. Pressurized aircraft meeting the more stringent requirements of part 25 have automatic passenger oxygen mask-dispensing devices that activate before exceeding 15,000 ft cabin altitude. It should be noted that some aircraft require that the flightcrew disable the automatic passenger oxygen mask-dispensing devices prior to landing at airports over 10,000 ft MSL to prevent inadvertent deployment when the cabin depressurizes at landing. The system is then re-armed after departure.
(1) Pressure Control. Turbine aircraft use a steady supply of engine bleed air for cabin pressurization. In the case of most pressurized light aircraft, the air supply is sent to the cabin from the turbocharger’s compressor or from an engine-driven pneumatic pump. In any case, an outflow valve regulates the flow of compressed air out of the cabin, which keeps the pressure constant by releasing excess pressure into the atmosphere. The cabin altitude selection can be manual, or if available, electronic. A gauge that indicates the pressure difference between the cabin and ambient altitudes helps the pilot to monitor the cabin altitude. A manually set backup control automatically controls the rate of change between these two pressures.
(2) Pressure Differentials. Each pressurized aircraft has a determined maximum pressure differential, which is the maximum differential between cabin and ambient altitudes that the pressurized section of the aircraft can support. The pilot must be familiar with these limitations, as well as the manifold pressure settings recommended for various pressure differentials. Some aircraft have a negative pressure relief valve to equalize pressure in the event of a sudden decompression or rapid descent to prevent the cabin pressure from becoming higher than the ambient pressure. Reducing exposure to low barometric pressure lowers the occurrence of decompression sickness and eliminates the need for an oxygen mask as a full-time oxygen source above certain altitudes. Many airplanes are equipped with automatic visual and aural warning systems that indicate an unintentional loss of pressure.
d. High-Altitude Components. Thin air at high altitudes has created a need for specialized aircraft components that are adapted to this environment.
Par 2-9 Page 35
(1) Magnetos. Thin air at high altitudes makes the unpressurized magneto susceptible to crossfiring. Therefore, we recommend the use of pressurized magnetos. Sealed caps and plugs that keep the electrodes contained within the body compose the high-tension pressurized system.
A pressure line extends directly from the turbodischarger to the magneto. Pressurized magnetos perform better at high altitudes where low pressure and cold atmosphere have a detrimental effect on electrical conductivity. Avoid flight above 14,000 ft MSL with an unpressurized magneto because of its higher susceptibility to arcing.
(2) Vacuum Pump. Another airplane component recommended for flight at high altitudes is the dry vacuum pump. Engine-driven wet vacuum pumps cannot create a sufficient vacuum to drive the gyros in the low air density found at high altitudes. Furthermore, oil contamination from the wet pump system, which uses engine oil for lubrication and cooling, can ruin gyros and rubber deicing boots. Dry vacuum pumps are lightweight, self-lubricating systems that eliminate oil contamination and cooling problems. These pumps can power either a vacuum or pressure pneumatic system, allowing them to drive the gyros, deice boots, and pressurize the door seals.
2-10. AERODYNAMICS AND PERFORMANCE FACTORS. Thinner air at high altitudes has a significant impact on an airplane’s flying characteristics because surface control effects, lift, thrust, drag, and horsepower are all functions of air density.
a. Aircraft Controls. The reduced weight of air moving over control surfaces at high altitudes decreases their effectiveness. As the airplane approaches its absolute altitude, the controls feel sluggish, making altitude and heading difficult to maintain. For this reason, most airplanes that fly at above 25,000 ft MSL are equipped with an autopilot.
b. Horsepower. The engine uses a determined weight of air to produce an identified amount of horsepower through internal combustion. For a given decrease of air density, horsepower decreases at a higher rate, which is approximately 1.3 times that of the corresponding decrease in air density.
c. Speed and Velocity. For an airplane to maintain unaccelerated level flight, drag and thrust must be equal. Because density is always greater at sea level, the velocity at altitude given the same AOA will be greater than at sea level, although the IAS will not change. Therefore, an airplane’s TAS increases with altitude while its IAS remains constant. In addition, an airplane’s rate of climb will decrease with altitude.
2-11. EMERGENCIES AND IRREGULARITIES AT HIGH ALTITUDES. All emergency procedures in the AFM should be reviewed before flying any airplane, and be readily accessible during every flight. A description follows of some of the most significant high-altitude emergencies and remedial action for each.
a. Decompression.
(1) Causes and Results. Decompression is the inability of the aircraft’s pressurization system to maintain its designed pressure schedule. Decompression can be caused by a malfunction of the system itself or structural damage to the aircraft. A rapid decompression will often result in cabin fog because of the rapid drop in temperature and the change in relative
Page 36 Par 2-11 humidity. A decompression will also affect the human body. Air will escape from the lungs through the nose and mouth because of a sudden lower pressure outside of the lungs. Differential air pressure on either side of the eardrum should clear automatically. Exposure to wind blast and extremely cold temperatures are other hazards the human body may face with decompression.
(2) Cabin Volume and Decompression Rates. Decompression of a small cabin volume pressurized aircraft is more critical than a large one, given the same size hole or conditions, primarily because of the difference in cabin volumes. Table 2-8, Aircraft Cabin Volume Ratios, is a comparison of cabin volume ratios between several large transport airplanes and some of the more popular GA turbojet airplanes in current use. Table 2-8 also shows that, under the same conditions, a typical small, pressurized aircraft can decompress on the order of 10 to 200 times faster than large aircraft. The B-747/Learjet comparison is an extreme example in that the human response, TUC, and the protective equipment necessary are the same.
Actual decompression times are difficult to calculate due to many variables involved (e.g., the type of failure, differential pressure, cabin volume, etc.). However, it is more probable that the crew of the small aircraft will have less time in which to take lifesaving actions.
TABLE 2-8. AIRCRAFT CABIN VOLUME RATIOS Aircraft Type Cabin Volumes in Cubic Ft Ratio DC-9 vs CE-650 5,840 vs 576 10:1 B-737 vs LR-55 8,010 vs 502 16:1 B-727 vs NA-265 9,045 vs 430 21:1 L-1011 vs G-1159 35,000 vs 1,850 19:1 B-747 vs Learjet 59,000 vs 265 223:1 Data Source: Physiological Considerations and Limitations in the High-altitude Operation of Small Volume Pressurized Aircraft. E. B. McFadden and D. de Steigner, Federal Aviation Administration (FAA) Civil Aeromedical Institute (CAMI).
(3) Types of Decompression.
(a) Explosive Decompression. A change in cabin pressure faster than the lungs can decompress. Most authorities consider any decompression that occurs in less than 0.5 seconds as explosive and potentially dangerous. This type of decompression is more likely to occur in small volume pressurized aircraft than in large pressurized aircraft and often results in lung damage.
To avoid potentially dangerous flying debris in the event of an explosive decompression, properly secure all loose items such as baggage and oxygen cylinders.
(b) Rapid Decompression. A change in cabin pressure where the lungs can decompress faster than the cabin. The risk of lung damage is significantly lower in this decompression compared to an explosive decompression.
(c) Gradual or Slow Decompression. A gradual or slow decompression is dangerous because it may not be detected. Automatic visual and aural warning systems generally provide an indication of a slow decompression.
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(4) Recovery. Recovery from all types of decompression is similar. Regardless of the type of decompression, recognition of the decompression, awareness of cabin altitude/cabin pressurization warning systems, or recognition of hypoxia symptoms are essential for a safe recovery.
WARNING:
If hypoxia is suspected, immediately don oxygen mask and breathe 100 percent oxygen slowly. Descend to a safe altitude. If supplemental oxygen is not available, initiate an emergency descent to an altitude below 10,000 ft MSL. If symptoms persist, land as soon as possible.
(a) Oxygen. Because hypoxia can drive an increase in respiration as a reaction to make up for a lack of oxygen, you should carefully control your breathing rate and depth. The excessive rate and depth of respiration (hyperventilation) can cause symptoms similar to hypoxia. To recover from hypoxia, breathe 100 percent oxygen; it will not worsen hyperventilation. Thus, for either hypoxia or hyperventilation, the recovery procedures are the same—breathe 100 percent oxygen and slow the breathing rate. In some cases, particularly recovery from an extreme hypoxia exposure, the administration of 100 percent oxygen can cause an apparent increase in severity of symptoms lasting for 15–60 seconds. Even if this reaction occurs, the recovery procedures remain as described above. Breathe 100 percent oxygen and breathe slowly.
(b) Descent. Although top priority in such a situation is reaching a safe altitude, pilots should be aware that cold-shock in piston engines can result from a high-altitude rapid descent, causing cracked cylinders or other engine damage. The time allowed to make a recovery to a safe altitude before loss of useful consciousness is, of course, much less with an explosive decompression than with a gradual decompression.
b. Turbocharger Failure or Malfunction. Increased oil temperature, decreased oil pressure, and a drop in manifold pressure could indicate a turbocharger malfunction or a partial or complete turbocharger failure. The consequences of such a malfunction or failure are twofold.
The airplane may not be capable of sustaining altitude without the additional power supplied by the turbocharged system. The loss in altitude in itself would not create a significant problem, weather and terrain permitting, but you must notify ATC of the descent.
CAUTION:
Loss of cabin pressurization can occur if the pressurization system is dependent on the turbocharger compressor. Careful monitoring of pressurization levels is essential during the descent to avoid the onset of hypoxia from a slow decompression.
c. Vapor Lock. Another potential problem associated with turbochargers is fuel vaporization. Engine-driven pumps that pull fuel into the intake manifold are susceptible to vapor lock at high altitudes. Most high-altitude aircraft are equipped with tank-mounted boost
Page 38 Par 2-11 pumps to feed fuel to the engine-driven pump under positive pressure. These pumps should be turned on if fuel starvation occurs as a result of vapor lock.
d. In-Flight Fire. Because of the highly combustible composition of oxygen, if a fire breaks out during a flight at high altitude, pilots should initiate an immediate descent to an altitude that does not require oxygen. Follow the procedures in the AFM.
e. Flight Through Thunderstorm Activity or Known Severe Turbulence. When pilots anticipate flight through severe turbulence and/or it is unavoidable, we recommend the following procedure:
CAUTION:
Flight through thunderstorm activity or known severe turbulence should be avoided, if possible.
(1) Airspeed. Airspeed is critical for any type of turbulent air penetration. Use the AFM-recommended turbulence penetration target speed or, if unknown, fly an airspeed below maneuvering speed. Use of high airspeeds can result in structural damage and injury to passengers and crewmembers. Severe gusts may cause large and rapid variations in IAS. Do not chase airspeed.
(2) Altitude. Penetration should be at an altitude that provides adequate maneuvering margins in case severe turbulence is encountered to avoid the potential for catastrophic upset.
(3) Autopilot. If severe turbulence is penetrated with the autopilot on, the altitude hold mode should be off. If the autopilot has a pitch hold mode, it should be engaged. The autopilot pitch hold mode can usually maintain pitch more successfully than a pilot under stress. With the autopilot off, the yaw damper should be engaged. Controllability of the aircraft in turbulence becomes more difficult with the yaw damper off. Rudder controls should be centered before engaging the yaw damper.
(4) Lightning. When flight through a thunderstorm cannot be avoided, turn up the intensity of panel and cabin lights so lightning does not cause temporary blindness. White lighting in the cockpit is better than red lighting during thunderstorms.
(5) Attitude and Heading. Keep wings level and maintain the desired pitch attitude and approximate heading. Do not attempt to turn around and fly out of the storm because the speed associated with thunderstorms usually makes such attempts unsuccessful. Use smooth, moderate control movements to resist changes in attitude. If large attitude changes occur, avoid abrupt or large control inputs. Avoid, as much as possible, use of the stabilizer trim in controlling pitch attitudes. Do not chase altitude.
2-12. FLIGHT TRAINING. Flight training required to comply with § 61.31(g) may be conducted in a simulator that meets the requirements of part 121, § 121.407 or a high-altitude airplane. The simulator should be representative of an airplane that has a service ceiling or maximum operating altitude, whichever is lower, above 25,000 ft MSL. The training should
Par 2-12 Page 39 consist of as many flights as necessary to cover the following procedures and maneuvers. If an airplane is being used, each flight should consist of a preflight briefing, flight planning, a preflight inspection, demonstrations by the instructor of certain maneuvers or procedures when necessary, and a postflight briefing and discussion.
a. Preflight Briefing. The instructor should verbally cover the material that will be introduced during the flight. If more than one flight is required, previous flights should be reviewed at this time. The preflight briefing is a good time to go over any questions the trainee may have regarding operations at high altitudes or about the aircraft. The instructor should encourage questions by the trainee during all portions of the flight training.
b. Preflight Planning. Complete a thorough flight plan for a predetermined route. The flight plan should include a complete weather briefing. Contact a Flight Service Station (FSS) (1-800-WX-BRIEF) or use one of the various internet-based services such as Direct User Access Terminal System (DUATS) to obtain a weather briefing. Determine the altitude for the flight using winds, pilot reports, the freezing level, and other meteorological information obtained from the briefing to determine the best altitude for the flight. Retain the information to aid in subsequent calculations made during flight.
(1) Course Plotting. Plot the course on a high-altitude navigation chart, noting the appropriate jet routes and required reporting points on a navigation log. Low-altitude charts should be available for planning departures and arrivals to comply with airspace and airspeed requirements. Identify and note alternate airports.
(2) AFM. Review the AFM with particular attention to Weight and Balance (W&B), performance charts, and emergency procedures. Calculate oxygen requirements, airspeeds, groundspeeds, time en route, and fuel burn using the AFM and weather data, when applicable.
Plan fuel management and descents at this time. The AFM should be readily accessible in the cabin in the event of an emergency.
(3) Flight Planning. Complete a flight plan using appropriate jet routes from the en route high-altitude chart. File the flight plan with the nearest FSS, ATC facility, or with one of the online services, such as DUATS.
c. Preflight Inspection. The aircraft checklist should be followed carefully. Particular attention should be given to the aircraft’s fuselage, windshields, window panels, and canopies to identify any cracks or damage that could rupture under the stress of cabin pressurization. The inspection should also include a thorough examination of the aircraft oxygen equipment, including available supply, an operational check of the system, and assurance that supplemental oxygen is in a readily accessible location.
• Preflight the oxygen equipment so that the system is ready for use upon donning the mask.
• Demonstrate crew communications using the oxygen equipment, both between crewmembers (if more than one required pilot) and with ATC.
d. Runup, Takeoff, and Initial Climb. Follow the procedures in the AFM, particularly the manufacturer’s recommended power settings and airspeeds, to avoid overboosting the
Page 40 Par 2-12 engine. Standard callout procedures are highly recommended and should be used for each phase of flight where the airplane crew consists of more than one crewmember.
e. Climb to High Altitude and Normal Cruise Operations While Operating Above 25,000 ft MSL. Perform the transition from low to high altitude repeatedly to assure familiarity with appropriate procedures. Specific oxygen requirements should be met when climbing above 12,500 ft MSL and pressurization should be adjusted with altitude. When passing through FL 180, the altimeter should be set to 29.92 inHg and left untouched until descending below that altitude. Comply with reporting points, and also with appropriate altitude selection for direction of flight. Throughout the entire climb and cruise above 25,000 ft MSL, emphasis should be given to monitoring cabin pressurization.
f. Simulated Emergencies. Training should include at least one simulated rapid decompression and emergency descent. Do not actually depressurize the airplane for this or any other training. Actual decompression of an airplane can be extremely dangerous and should never be done intentionally for training purposes. Simulate the decompression by donning the oxygen masks, turning on the oxygen controls, configuring the airplane for an emergency descent, and performing the emergency descent as soon as possible. Coordinate with ATC in advance of any practice emergency descent. Do not vacate any assigned altitude without clearance.
g. Descents. Gradual descents from altitude should be practiced to provide passenger comfort and compliance with procedures for transitioning out of the high-altitude realm of flight.
Follow the airplane manufacturer’s recommendations with regard to descent power settings to avoid stress on the engine and excessive cooling. Particular emphasis should be given to cabin pressurization and procedures for equalizing cabin and ambient pressures before landing.
Emphasis should also be given to changing to low-altitude charts when transitioning through FL 180, obtaining altimeter settings below FL 180, and complying with airspace and airspeed restrictions at appropriate altitudes.
h. Engine Shutdown. Allow the turbocharged engine to cool for at least 1 minute and assure that all shutdown procedures in the AFM are followed. Before exiting the airplane, always check that all oxygen equipment and oxygen valves have been turned off or placed in the position recommended by the manufacturer.
i. Postflight Discussion. The instructor should review the flight and answer any questions the trainee may have. If additional flights are necessary to ensure thorough understanding of high-altitude operations, preview the material for the next flight during the postflight discussion.
Par 3-1 Page 41 CHAPTER 3. MACH FLIGHT AT HIGH ALTITUDES
3-1. PURPOSE. To present certain factors involved in the high-speed flight environment at high altitudes. It is the lack of understanding of many of these factors involving the laws of aerodynamics, performance, and Mach speeds that has produced a somewhat higher accident rate in some types of turbojet aircraft.
3-2. CRITICAL ASPECTS OF MACH FLIGHT. In recent years, a number of corporate jet airplanes have experienced catastrophic loss of control during high-altitude/high-speed flight. A significant causal factor in these accidents may well have been a lack of knowledge by the pilot regarding critical aspects of high-altitude Mach flight. Maximum operating altitudes of some General Aviation (GA) turbojet airplanes now reach 51,000 ft mean sea level (MSL). These types of accidents will become less common as pilots gain experience and also learn to respect the more critical aspects of high-altitude, high-speed flight. It is imperative to have a thorough knowledge of their specific make and model of aircraft and its unique limitations.
a. Mach. From the pilot’s viewpoint, Mach is the ratio of the aircraft’s traffic advisory system (TAS) to the local speed of sound. At sea level, on a standard day (59 °F)/15 °C), the speed of sound equals approximately 660 knots (kts) or 1,120 feet per second (ft/s). Mach 0.75 at sea level is equivalent to a TAS of approximately 498 kts (0.75 x 660 kts) or 840 ft/s. The temperature of the atmosphere normally decreases with an increase in altitude. The speed of sound is directly related only to temperature. The result is a decrease in the speed of sound up to about 36,000 ft mean sea level.
b. Airspeed Limitations. The sleek design of some turbojet airplanes has caused some operators to ignore critical airspeed and Mach limitations. There are known cases in which corporate turbojet airplanes have been modified by disabling the airspeed and Mach warning systems to permit intentional excursions beyond the FAA-certificated maximum operating limit speed (VMO/MMO) limit for the specific airplane. Such action may critically jeopardize the safety of the airplane by setting the stage for potentially hazardous occurrences. The compulsion to go faster may result in the onset of aerodynamic flutter, excessive G-loading in maneuvering, and induced flow separation over the ailerons and elevators. This may be closely followed by the physical loss of a control surface, an aileron buzz or snatch, coupled with yet another dangerous phenomenon called Mach tuck, leading to catastrophic loss of the airplane and the persons onboard.
(1) Mach Tuck. Mach tuck is caused principally by two basic factors:
(a) Flow Separation. Shock wave-induced flow separation, which normally begins near the wing root, causes a decrease in the down wash velocity over the elevator, and produces a tendency for the aircraft to nose down.
(b) Center of Pressure Movement. The center of pressure moves aft. This tends to unbalance the equilibrium of the aircraft in relation to its center of gravity (CG) in subsonic flight. The airplane’s CG is now farther ahead of the aircraft’s aerodynamic center than it was in slower flight. This dramatically increases the tendency of the airplane to pitch more nose-down.
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(2) Pressure Disturbances. Pressure disturbances in the air caused by an airfoil in high-altitude, high-speed flight result from molecular collisions. These molecular collisions are the result of air that moves over an airfoil faster than the air it is overtaking can dissipate. When the disturbance reaches a point at which its propagation achieves the local speed of sound, Mach 1 is attained. One hundred percent of the speed of sound at MSL with a temperature of 15 °C is 760 statute miles (sm) or 660 nautical miles (NM) per hour. This speed is affected by temperature of the atmosphere at altitude. Thus, optimum thrust fuel and range considerations are significant factors in the design of most GA turbine-powered airplanes that cruise at some percentage of Mach 1.
(3) Mach Compensating Device. Most turbojet airplanes capable of operating in the Mach speed ranges are designed with some form of trim and autopilot Mach compensating device (stick puller) to alert the pilot to inadvertent excursions beyond its certificated MMO. If for any reason there is a malfunction that requires disabling the stick puller, the aircraft must be operated at speeds well below MMO, as prescribed in the applicable AFM procedures for the aircraft.
CAUTION:
The stick puller should never be disabled during normal flight operations in the aircraft.
(4) Coffin Corner. An airplane’s indicated airspeed (IAS) decreases in relation to TAS as altitude increases. As the IAS decreases with altitude, it approaches the airspeed for the low-speed buffet boundary where pre-stall buffet occurs for the airplane at a load factor of 1.0 G.
The point where high-speed Mach, IAS, and low-speed buffet boundary IAS merge is the airplane’s absolute or aerodynamic ceiling. Once an aircraft has reached its aerodynamic ceiling, which is higher than the altitude limit stipulated in the AFM, the aircraft can neither be made to go faster without activating the design stick puller at Mach limit nor can it be made to go slower without activating the stick shaker or pusher. This critical area of the aircraft’s flight envelope is known as coffin corner.
(5) Mach Buffet. Mach buffet occurs as a result of supersonic airflow on the wing.
Stall buffet occurs at angles of attack that produce airflow disturbances (burbling) over the upper surface of the wing, which decreases AOA lift. As density altitude increases, the AOA that is required to produce an airflow disturbance over the top of the wing is reduced until a density altitude is reached where Mach buffet and stall buffet converge (described in subparagraph 3-2b(4) as coffin corner). When this phenomenon is encountered, serious consequences may result, causing loss of control of the aircraft.
(6) Mach Buffet Limits. Increasing either gross weight or load factor (G factor) will increase the low-speed buffet and decrease Mach buffet speeds. A typical turbojet airplane flying at 51,000 ft MSL altitude at 1.0 G may encounter Mach buffet slightly above the airplane’s MMO
(0.82 Mach) and low-speed buffet at 0.60 Mach. However, only 1.4 G (an increase of only
0.4 G) may bring on buffet at the optimum speed of 0.73 Mach and any change in airspeed, bank angle, or gust loading may reduce this straight and level flight 1.4 G protection to no protection.
Par 3-2 Page 43 Consequently, a maximum cruising flight altitude must be selected that will allow sufficient buffet margin for the maneuvering necessary and for gust conditions likely to be encountered.
Therefore, it is important for pilots to be familiar with the use of charts showing cruise maneuvering and buffet limits. Flightcrews operating airplanes at high speeds must be adequately trained to operate them safely. This training cannot be complete until pilots are thoroughly educated in the critical aspect of aerodynamic factors pertinent to Mach flight at high altitudes.
3-3. AIRCRAFT AERODYNAMICS AND PERFORMANCE. Pilots who operate aircraft at high speeds and high altitudes are concerned with the forces affecting aircraft performance caused by the interaction of air on the aircraft. With an understanding of these forces, the pilot will have a sound basis for predicting how the aircraft will respond to control inputs. The importance of these aerodynamic forces and their direct application to performance and execution of aircraft maneuvers and procedures at altitude will be evident. The basic aerodynamics definitions that apply to high-altitude flight are contained in paragraph 1-4.
a. Wing Design.
(1) Induced Lift. The wing of an airplane is an airfoil or aircraft surface designed to obtain the desired reaction from the air through which it moves. The profile of an aircraft wing is an excellent example of an efficient airfoil. The difference in curvature between the upper and lower surfaces of the wing generates a lifting force. Air passing over the upper wing surface moves at a higher velocity than the air passing beneath the wing because of the greater distance it must travel over the upper surface. This increased velocity results in a decrease in pressure on the upper surface. The pressure differential created between the upper and lower surfaces of the wing lifts the wing upward in the direction of the lowered pressure. This lifting force is known as induced lift. Induced lift may be increased, within limits, by:
• Increasing the AOA of the wing or changing the shape of the airfoil; changing the geometry (e.g., aspect ratio).
• Increasing the wing area.
• Increasing the free-stream velocity.
• A change in air density.
(2) Critical Mach Number (MCR). The pilot may have only varying degrees of control over these factors. Thus, the pilot must keep in mind that an aircraft will obey the laws of physics just as precisely at its high-speed limits as it does during a slower routine flight. So, regardless of wing shape or design, Mach range flight requires precise control of a high volume of potential energy without exceeding the critical Mach number (MCR). The MCR is important to high-speed aerodynamics because it is the speed at which the flow of air over a portion of a specific airfoil design reaches Mach 1. This results in the formation of a shock wave and drag divergence.
(3) Wing Sweep. Sweeping the wings of an airplane is one method used by aircraft designers to delay the adverse effects of high Mach flight and bring about economical cruise with an increase in the MCR. Sweep allows a faster airfoil speed before critical Mach is reached when compared to an equal straight wing. This occurs because the airflow now travels over a different cross-section (camber) of the airfoil. This new cross-section has less effective camber,
Page 44 Par 3-3 which results in a reduced acceleration of airflow over the wing, thus allowing a higher speed before critical Mach is reached. Sweep may be designed either forward or rearward; the overall effect is the same. However, rearward sweep appears to be somewhat more desirable, since it has presented fewer problems to manufacturers of models of GA aircraft in terms of unwanted design side effects. In effect, the wing is flying slower than the airspeed indicator indicates and, similarly, it is developing less drag than the airspeed indicator would suggest. Since less drag is being developed for a given IAS, less thrust is required to sustain the aircraft at cruise flight.
(a) There is a penalty, however, on the low speed end of the spectrum. A swept-wing airplane has a higher landing/stall speed when compared with a similar straightwing aircraft. This means higher touchdown speed, longer runway requirements, and more tire and brake wear as opposed to a straight-wing design. A well-stabilized approach with precise control of critical “V” speeds is necessary. Swept-wing drag curves are approximately the reverse of the lift curves. A rapid increase in drag component may be expected with an increase of AOA with the amount being directly related to the degree of sweep or reduction of aspect ratio.
(b) The extension of trailing edge flaps and leading edge devices may, in effect, further reduce the aspect ratio of the swept wing by increasing the wing chord. This interplay of forces should be well understood by the pilot of the swept-wing aircraft. Raising the nose of the aircraft to compensate for a mild undershoot during a landing approach at normal approach speeds will produce little lift and may instead lead to a rapid decay in airspeed, reducing the margin of safety.
(4) High-Speed Laminar Airflow Airfoil. Another method of increasing the MCR of an aircraft wing is through the use of a high-speed laminar airflow airfoil in which a small leading edge radius is combined with a reduced thickness ratio. This type of wing design is more tapered with its maximum thickness further aft, thus distributing pressures and boundary layer air more evenly along the chord of the wing. This tends to reduce the local flow velocities at high Mach numbers and improve aircraft control qualities.
(5) Vortex Generators and Boundary Layer Energizers. Several modern straight-wing, turbojet aircraft use another design method to delay the onset of Mach buzz and obtain a higher MMO. These aircraft sometimes incorporate the use of both vortex generators and small triangular upper wing strips as boundary layer energizers. Both systems seem to work equally well, although the boundary layer energizers generally produce less drag. Vortex generators are small vanes affixed to the upper wing surface, extending in height approximately 1 to 2 in. This arrangement permits these vanes to protrude through the boundary layer air. The vortex generators deflect the higher energy airstream downward over the trailing edge of the wing and accelerate the boundary layer aft of the shock wave. This tends to delay shock-induced flow separation of the boundary layer air, which causes aileron buzz, and thus permits a higher MMO.
(6) Lift Characteristics Related to Swept Wings. The lift characteristics of straight-wing and swept-wing airplanes related to changes in AOA are more favorable for sweptwing airplanes. An increase in the AOA of the straight-wing airplane produces a substantial and constantly increasing lift vector up to its maximum coefficient of lift and, soon thereafter, flow