Which condition is associated with Coffin Corner at high altitude?

• A. Thin air
• B. Increased air density
• C. High humidity
• D. Cold weather only

Answer: (A)

Coffin Corner occurs when two flight boundaries compress into one at very high altitude: the low-speed stall boundary and the high-speed Mach boundary. In extremely thin air, the lift needed to stay aloft means you must fly at higher true airspeeds, while the indicated airspeed at which stall occurs stays about the same for a given weight. At the same time, compressibility effects near the speed of sound become significant, so you can reach the Mach limit even at relatively modest indicated speeds. The result is very little margin between stalling and Mach buffet, which is why this situation is tied to thin air. The other factors—increased density, humidity, or cold weather only—do not create this narrowing of margins.