High-Speed Descent and Mach Trimming
You are flying a light jet at FL410, Mach 0.78, when ATC instructs a descent to FL310 at 2,500 ft/min. The aircraft has a Mach trimmer and a stick shaker with a low-speed (high-altitude) mode. As you retard the thrust levers and lower the nose, the Mach number rises toward 0.82 and the aircraft begins to buffet lightly; the Mach trimmer is annunciated as active. The autopilot is engaged in altitude hold and the speed is being controlled by pitch. At FL310 the temperature is ISA +10°C. The aircraft's Mmo is 0.83 and the high-speed buffet onset at this weight is about 0.80. The pilot's intention is to keep the descent at 2,500 ft/min to meet a crossing restriction.
1. Explain what is happening aerodynamically as the Mach number approaches 0.82, and why the buffet is a warning rather than a nuisance.
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As the local Mach number over the upper wing surface exceeds the critical Mach number, a shock wave forms and the boundary layer behind it thickens and may separate. The result is shock-induced separation, felt as buffet, and a rearward shift of the centre of pressure, which produces a nose-down pitching moment. At Mmo 0.83 the margin to the high-speed buffet onset of about 0.80 is already being consumed. The buffet is the aerodynamic warning that the wing is close to the drag-rise and pitch-break region; beyond it, control effectiveness can change abruptly and the aircraft may tuck under. The Mach trimmer is working to counter the pitch-up/down tendency, but it cannot restore the lost lift or the separated flow. The correct response is to reduce Mach by reducing the rate of descent or adding power, not to trim against the buffet.
2. The descent is being flown at a fixed 2,500 ft/min with the autopilot in altitude hold. Why is this a poor technique at high altitude, and what would you change?
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At high altitude the speed of sound and the true airspeed for a given Mach both change with temperature, and a fixed rate of descent with pitch controlling speed means the aircraft's Mach and IAS are coupled to the descent rate. As you descend into warmer air at FL310 (ISA +10°C), the speed of sound rises, so a constant Mach corresponds to a higher TAS, and the aircraft accelerates unless the pitch is raised. Holding 2,500 ft/min forces a particular pitch attitude that may not give the Mach you want. The better technique is to descend at a target Mach (or a Mach/IAS schedule) with the autopilot in a speed mode, letting the rate of descent vary, or to use flight-level change with a Mach target and accept a lower rate. If the crossing restriction is tight, ask ATC for a lower speed or a revised restriction rather than chasing both Mach and rate with pitch.
3. If the stick shaker activates during the level-off at FL310, what does that tell you about the high-altitude low-speed regime, and what is the recovery?
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At FL310 the indicated airspeed for a given Mach is low, and the wing is operating close to its buffet boundaries at both ends of the envelope, so the safe speed band is narrow. A stick shaker here indicates that the angle of attack has reached the stall warning threshold, which at high altitude can occur at an IAS that looks comfortable on the gauge. The recovery is to lower the nose to reduce angle of attack and add thrust smoothly, while avoiding a large pitch input that could produce an accelerated stall or a Mach overshoot on the other side. Do not retract speedbrakes abruptly or make large thrust changes, which can cause a pitch transient. The key point is that the high-altitude envelope is bounded on both sides, and the pilot must fly the middle of the band rather than either edge; if the shaker and the high-speed buffet are both close, the aircraft is too high for its weight and a descent is the correct remedy.