The high-speed module built two walls: the low-speed stall on one side and the shock stall on the other. This lesson maps the corridor between them, because at cruise altitude both walls announce themselves the same way, as buffet, and the distance to each is a quantity crews actually manage, read from a chart, and choose their cruise levels by. It ends at the place where the corridor closes entirely: the aerodynamic ceiling, better known by its grimmer name, the coffin corner.
Two buffets, one cause
Buffet is always the same physical event: separated flow shaking the airframe, reached from two opposite directions. Fly too slowly and the angle of attack climbs toward the critical angle: separation spreads from the trailing edge, and the low-speed buffet of the stall lessons begins. Fly too fast at altitude and the shock system of the high-speed module strengthens until the pressure jump tears the boundary layer off behind it: shock-induced separation, felt as the Mach buffet (high-speed buffet). The Mach buffet is not a new phenomenon but the first sign of the approaching shock stall, and it arrives in company: the same growing, aft-marching shocks that separate the flow also drive the centre of pressure aft, so Mach buffet and Mach tuck share one root and tend to arrive together. Buffet from either wall is the airframe's announcement that separation has begun; which wall it is depends only on which boundary the aircraft approached.
The buffet margin and what moves it
At any moment in cruise the aircraft sits somewhere between the two walls, and the buffet margin is its distance to the nearer one: usually expressed as the manoeuvre (load factor or bank) or the speed change the aircraft can absorb before buffet begins. The exam's parameter list all works through one idea: anything that pushes the wing toward either boundary eats the margin.
Angle of attack — more α is simply closer to the low-speed boundary; at high altitude, where the 1g α is already high, there is less to give.
Mach number — more Mach is closer to the shock-buffet boundary; less Mach, closer to the low-speed one. Cruise speed is chosen between them.
Pressure altitude — the squeeze itself: climbing raises the 1g stalling IAS (compressibility eating ) while the Mach limit arrives at an ever lower IAS. Both walls close in with every foot of climb.
Mass — a heavier aircraft needs more lift coefficient at the same level and speed: the low-speed boundary moves toward it and, as with load factor, the higher lift coefficient strengthens the shocks, nudging the Mach wall in too. The altitude where the walls meet drops; margins are always quoted for a mass.
Load factor and bank angle — a manoeuvre demands lift like extra mass does, instantly: at n = 1.3 (about 40° of bank) the low-speed boundary has moved substantially toward the aircraft, and the Mach boundary also closes (the extra lift strengthens the shocks). Manoeuvring spends margin from both sides.
CG location — a forward CG adds tail download the wing must carry: slightly more lift demanded, slightly less margin, the small correction, but the exam lists it.
The buffet onset chart
All of this is drawn on one certification graph, the buffet onset boundary chart: for a given mass and pressure altitude, load factor against Mach number, with the two buffet boundaries rising toward each other. Everything the crew needs is read straight off it. Along the 1g line, the gap between the boundaries is the buffet-free speed range: the Mach window available in level flight. Rising vertically from the cruise Mach, the load factor at which the boundary is struck is the manoeuvre capability: convertible directly into the bank angle at which buffet begins. And sliding the whole chart up through the altitudes, the boundaries pinch together until they meet: the chart is also where the aerodynamic ceiling is found.