081 Principles of Flight topic guide
Shock Waves and Mach Buffet
The critical Mach number, Mcrit, is the free stream Mach number at which the local airflow somewhere over the aeroplane, usually over the upper wing surface where the flow has already accelerated the most, first reaches Mach 1, even though the aeroplane itself is still flying slower than the speed of sound. Above Mcrit, that local supersonic pocket keeps growing and its boundary is a shock wave.
The shock does more than sit there: it can separate the boundary layer just downstream of itself, producing buffet, and it can shift lift and pitching moment in ways that need dedicated systems to manage. Sweep and modern aerofoil shapes push Mcrit higher, but they never remove the phenomenon, only move where it starts.
How the shock forms and what it does to the boundary layer
As free stream Mach number rises past Mcrit, the local supersonic region over the wing terminates in a shock wave, across which pressure rises suddenly. If that pressure rise is steep enough, the adverse pressure gradient just behind the shock is more than the boundary layer can survive attached, and the flow separates there, an effect called shock induced separation. The unsteady, separated flow buffets the airframe, felt as Mach buffet, distinct from the low speed, high angle of attack buffet caused by ordinary flow separation near the stall.
As Mach number rises further, the shock strengthens and moves aft along the chord, and the separated region behind it grows, worsening both the buffet and the drag rise associated with it.
Tuck under and the pitching moment
Shock induced separation over the wing reduces the downwash reaching the tailplane and shifts the wing's centre of pressure aft, and both effects push the nose down. This nose down pitching tendency beyond Mcrit is known as tuck under, or Mach tuck, and it can steepen a descent if uncorrected. Swept wing jets commonly carry a Mach trim system that automatically compensates as speed rises into the affected range, so the aeroplane's handling stays predictable rather than requiring constant manual correction.
Buffet margins and the aerodynamic ceiling
The buffet margin is the gap between the low speed buffet boundary, set by angle of attack, and the high speed, Mach buffet boundary, set by Mcrit, usually expressed in terms of available manoeuvring margin at the current altitude and weight. Climbing at a constant weight needs a higher lift coefficient as air density falls, which pushes the aeroplane closer to the low speed boundary while also nudging Mcrit itself down a little, since a higher lift coefficient raises the local peak suction. The two boundaries close in on each other, and the altitude at which they meet, leaving no usable margin, is informally called the aerodynamic ceiling or coffin corner.
Wing sweep reduces the component of airflow the wing effectively feels, and supercritical aerofoil sections flatten the upper surface to delay and weaken the shock. Both raise Mcrit and widen the practical cruise envelope, but neither removes the shock or the eventual convergence of the two boundaries at high enough altitude.
Worked example
Worked example: what narrows the buffet margin at altitude
Not quite. The correct answer is B.
Correct answer: B
- A. This describes a real change in true airspeed but draws the wrong conclusion. Mach number, not true airspeed, is what the high speed buffet boundary is measured against, and the Mach number here is held constant by the question.
- B. Correct. Thinner air demands a higher lift coefficient for the same weight, and that higher lift coefficient both approaches the low speed boundary and edges the critical Mach number down, closing the margin from both ends.
- C. This confuses Mach number, which is already the speed relative to the local speed of sound, with the critical Mach number, which is set by the aerofoil's shape and lift coefficient, not by the ambient speed of sound value itself.
- D. This is a thrust and performance argument, not the aerodynamic cause of the margin narrowing. The buffet margin is set by the aerodynamic boundaries themselves, not by whether enough thrust exists to hold the cruise condition.
Step by step
- At a constant Mach number and weight, climbing means thinner air, so the dynamic pressure available at that Mach number falls.
- To generate enough lift for the same weight in thinner air, the wing must operate at a higher lift coefficient.
- A higher lift coefficient raises the local peak suction over the wing, which is reached at a slightly lower free stream Mach number, so the critical Mach number itself edges down.
- The same higher lift coefficient also sits closer to the aeroplane's low speed, high angle of attack buffet boundary.
- Both boundaries are converging on the current flight condition from opposite directions, which is exactly what the aerodynamic ceiling, or coffin corner, describes.
Common mistakes
Treating the critical Mach number as a fixed constant for the aeroplane
Mcrit shifts with lift coefficient, and lift coefficient shifts with weight and altitude, so a single memorised Mcrit figure is only valid at one flight condition, and questions that change weight or altitude are testing whether you know it moves.
Confusing Mach buffet with ordinary low speed stall buffet
Both feel similar in the cockpit and both are separated flow buffet, but Mach buffet is shock induced separation at high speed, and the correct response is to slow down or descend, the opposite of the low speed case, so misreading the stem's speed regime reverses the correct action.
Assuming sweep and supercritical sections remove the Mach buffet limit
Sweep and supercritical aerofoils delay and soften the shock, letting an aeroplane cruise closer to Mcrit with a usable margin, but they do not eliminate the shock or the coffin corner, only move it higher, which trips up any option claiming a swept wing has no Mach buffet limit.
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Last reviewed July 2026