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 true airspeed (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 .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 MSL.
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 Federal Aviation Administration (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 downwash 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 Airplane Flight Manual (AFM) procedures for the aircraft.
CAUTION:
The stick puller should never be disabled during normal flight operations in the aircraft.
(4) Mach Buffet. The airflow separation behind a shock wave pressure barrier caused by airflow over flight surfaces exceeding the speed of sound.
(5) 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 prestall 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. 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.
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
Par 3-2 Page 43 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) 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 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 lift. 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). Mcr 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 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, which results in a reduced acceleration of airflow over the wing, thus allowing a higher speed before critical Mach is reached.
(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.
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(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 swept-wing 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 separation (stall) occurs with a rapid deterioration of lift. By contrast, the swept wing produces a much more gradual buildup of lift.
(7) Airflow and Airspeed. Regardless of the method used to increase the critical Mach number, airflow over the wing is normally smooth. However, as airspeed increases, the smooth flow becomes disturbed. The speed at which this disturbance is encountered will be determined by the shape of the wing and the degree of sweep. When the aircraft accelerates, the airflow over the surface of the wing also accelerates until, at some point on the wing, it becomes sonic. The aircraft March number at which this occurs is the Mcr for that wing.
b. Jet Engine Efficiency.
(1) Fuel Economy. The efficiency of the jet engine at high altitudes is the primary reason for operating in the high-altitude environment. The specific fuel consumption of jet engines decreases as the outside air temperature decreases for constant revolutions per minute (rpm) and TAS. Thus, by flying at a high altitude, the pilot is able to operate at flight levels where fuel economy is best and with the most advantageous cruise speed. For efficiency, jet aircraft are typically operated at high altitudes where cruise is usually very close to rpm or exhaust gas temperature limits. At high altitudes, little excess thrust may be available for maneuvering. Therefore, it is often impossible for the jet aircraft to climb and turn simultaneously, and all maneuvering must be accomplished within the limits of available thrust and without sacrificing stability and controllability.
(2) Compressibility. Compressibility also is a significant factor in high-altitude flight.
The low temperatures that make jet engines more efficient at high altitudes also decrease the speed of sound. Thus, for a given TAS, the Mach number will be significantly higher at high altitude than at sea level. This compressibility effect, due to supersonic airflow, will be encountered at slower indicated speeds at high altitude than at low altitude.
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c. Controllability Factors.
(1) Stability. Stability is the inherent flight characteristic of an aircraft to return to equilibrium after being disturbed by an unbalanced force or movement.
(2) Controllability. Controllability is the ability of an aircraft to respond positively to control surface displacement and to achieve the desired condition of flight.
(3) Air Density. At high-flight altitudes, aircraft stability and control may be greatly reduced. Thus, while high-altitude flight may result in high TAS and high Mach numbers, calibrated airspeed is much slower because of reduced air density. This reduction in density means that the AOA must be increased to maintain the same coefficient of lift with increased altitude. Consequently, jet aircraft operating at high altitudes and high Mach numbers may simultaneously experience problems associated with slow-speed flight such as Dutch roll, adverse yaw, and stall. In addition, the reduced air density reduces aerodynamic damping, overall stability, and control of the aircraft in flight. Swept-wing and airfoil design alone, with boundary layer energizers such as the vortex generators described earlier, has reduced the hazardous effect of the problems described below. However, these problems are still encountered to some extent by the modern turbojet airplane in high-altitude flight.
(a) Dutch Roll. Dutch roll is a coupled oscillation in roll and yaw that becomes objectionable when roll, or lateral stability, is reduced in comparison with yaw or directional stability. A stability augmentation system is required to be installed on the aircraft to dampen the Dutch roll tendency when it is determined to be objectionable, or when it adversely affects control stability requirements for certification. The yaw damper is a gyro-operated autocontrol system installed to provide rudder input and aid in canceling out yaw tendencies such as those in Dutch roll.
(b) Adverse Yaw. Adverse yaw is a phenomenon in which the airplane heading changes in a direction opposite to that commanded by a roll control input. It is the result of unequal lift and drag characteristics of the down-going and up-going wings. The phenomena are alleviated by tailoring the control design by use of spoilers, yaw dampers, and interconnected rudder and aileron systems.
(4) Results of Supersonic Flow. Supersonic flow over the wing is responsible for:
(a) Shock Waves. The formation of shock waves on the wing, which results in drag rise.
(b) Aft Shift. An aft shift in the center of lift resulting in a nosedown pitching moment called Mach tuck.
(c) Airflow Separation. Airflow separation behind the shock waves resulting in Mach buffet.
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(5) G-Force and Safety Margins. In general, this discussion has been confined to normal level, unaccelerated 1.0 G-flight, when rotating or maneuvering about the pitch axis;
however, acceleration of G-forces can occur while maintaining a constant airspeed. As G-forces increase, both the aircraft’s aerodynamic weight and AOA increase.
(a) Maneuvering Flight G-Loading. The margin over low-speed stall buffet decreases, as well as the margin below Mach buffet, because of the increased velocity of the air over the wing resulting from the higher AOA. This, in effect, could lower the aerodynamic ceiling for a given gross weight.
(b) Non-Maneuvering Flight G-Loading. Increased G-loading can also occur in non-maneuvering flight because of atmospheric turbulence or the lack of fine-touch skill by the pilot. Pilots flying at high altitudes in areas where turbulence may be expected must carefully consider acceptable safety margins necessary to accommodate the sudden and unexpected vertical accelerations that may be encountered with little or no warning. How wide is the safety margin between low-speed and high-speed buffet boundaries for an altitude and weight in a 30 bank? The answer may be easily determined by reference to the Cruise Maneuver/Buffet Limit Chart for a particular aircraft. For example, in a typical jet aircraft, the 1.0 G buffet-free margin at flight level (FL) 350 is 135 knots (kts); at FL 450, this speed is reduced to a mere 26 kts. Thus, the safety margin in airspeed spread diminishes rapidly as the aircraft climbs and leaves little room for safety in the event of an air turbulence encounter or accidental thunderstorm penetration.
(6) Thunderstorm Penetration. If a thunderstorm cannot be avoided, follow high-altitude thunderstorm penetration procedures and avoid over-action of thrust levers. When excessive airspeed buildup occurs, pilots may wish to use speed brakes. The use of aerodynamic speed brakes, when they are part of the lateral control system, may change the roll rate any time there is a lateral control input.
(7) Information. For detailed information concerning the operation of specific turbojet aircraft, refer to the aircraft’s AFM.
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Subject: AC 61-107B CHG 1, Aircraft Operations at Altitudes Above 25,000 Feet Mean Sea Level or Mach Numbers Greater Than .75.
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