Why does Coffin Corner occur at high altitudes?

• A. Thin air
• B. High humidity
• C. Low temperature
• D. Increased air density

Answer: (A)

Thin air is the key. At very high altitudes the air is extremely sparse, so lift drops for a given speed. To generate the same lift you must fly faster through the air (true airspeed increases as density falls). That raises the low-speed limit in terms of the actual airspeed needed to stay aloft. At the same time, the upper-speed limit is governed by compressibility effects and the onset of Mach buffet. In thinner air, shock waves and buffet can occur at lower true airspeeds because the speed of sound changes with temperature and the flow reaches critical Mach more readily. As a result, the low-speed stall boundary and the high-speed Mach buffet boundary move toward each other at high altitude, pinching the usable flight envelope into a narrow Coffin Corner.