Mach buffet near the ceiling and the coffin corner
A business jet (Mmo 0.85, Vmo 320 KIAS, certificated ceiling FL450) is cruising at FL410 at M 0.82 and 8,500 kg. ISA temperature is +10°C. The high-speed buffet onset for the current weight and 1g is M 0.80; the low-speed buffet onset is M 0.68. The autopilot is in altitude hold. The aircraft enters moderate turbulence; the Mach number rises to 0.84 while the indicated airspeed remains about 240 KIAS. The autopilot trims nose-up to hold altitude, and the pilot sees a buffet. The pilot's first reaction is to pull back to reduce speed and to increase thrust. The aircraft is 40 nm from a suitable airport at FL410; weather at the destination is fine. The pilot asks why the aircraft is buffeting at a relatively low indicated airspeed.
1. Distinguish high-speed and low-speed buffet and explain how the aircraft can be at M 0.84 with only 240 KIAS at FL410. Use the relationship between IAS, TAS, Mach number and temperature in your answer.
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Low-speed buffet is the warning of an approaching stall: the wing is reaching its critical angle of attack at high lift coefficient. High-speed buffet is caused by shock-induced separation on the wing as local flow reaches the speed of sound, and it occurs at a Mach number that depends on angle of attack, thickness and lift coefficient.
IAS measures dynamic pressure, not true speed. For the same IAS, TAS increases strongly with altitude as air density falls. Mach number is TAS divided by the local speed of sound. The speed of sound depends on temperature, a = √(γRT), so colder air at high altitude gives a lower speed of sound for a given TAS. At FL410, a modest IAS therefore corresponds to a high TAS and a high Mach number. The aircraft is above its high-speed buffet onset of M 0.80, so the buffet is a high-speed Mach buffet, not a low-speed stall buffet.
2. Explain the likely aerodynamic cause of the buffet and any pitch tendency (Mach tuck) at M 0.84, and evaluate the pilot's proposed response of pulling back and adding thrust.
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At M 0.84 the wing has regions of supersonic local flow and shock waves. Shock-induced separation causes the buffet, and as Mach increases the shock moves and the centre of pressure tends to move aft, producing a nose-down pitch tendency known as Mach tuck. Lift may also be lost behind the shock.
Pulling back increases angle of attack, strengthens the shock and increases the likelihood of deeper separation and buffet. It may briefly oppose the nose-down tendency, but it worsens the high-speed aerodynamic problem and can lead to a pitch-up or loss of control. Adding thrust increases speed and pushes the aircraft further above Mmo and the buffet onset.
The correct response is to reduce thrust, extend speedbrakes carefully if needed, lower the nose slightly to reduce Mach below the buffet onset, and descend to a lower altitude where the buffet margins are wider.
3. Assess the 'coffin corner' at FL410 in this scenario and outline the recovery actions to regain a safe margin, including the effect of turbulence on both buffet boundaries.
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The coffin corner is the narrowing gap between the low-speed buffet/stall boundary and the high-speed Mach buffet boundary. Here the 1g boundaries are M 0.68 and M 0.80, a margin of only 0.12 Mach. In turbulence, load factor increases. A higher load factor requires more lift coefficient, so the low-speed buffet speed rises. The higher angle of attack also lowers the Mach number at which shock-induced separation occurs, so the high-speed buffet boundary falls. The safe corridor therefore becomes even narrower.
The recovery is to descend to a lower flight level, reduce Mach to a comfortable margin below 0.80, and maintain smooth, small control inputs. Avoid abrupt pull-ups, do not climb, and use speedbrakes cautiously. The aim is to restore a clear margin above the low-speed buffet and below the high-speed buffet, not merely to chase the indicated airspeed.