Which factor primarily causes Coffin Corner at high altitudes?

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

Answer: (A)

Coffin Corner arises because at very high altitudes the air is incredibly thin, which squeezes the flight envelope from both ends. Lift depends on air density and speed, so as density drops, the speed needed to generate enough lift to stay level increases. That pushes the stall speed higher in true airspeed (and often in indicated airspeed as well). At the same time, there’s a limit set by compressibility effects—the maximum speed before you hit Mach-related problems—which becomes a tighter cap as altitude increases. When these two limits converge, there’s almost no room to maneuver: you can’t fly fast enough to avoid a stall, and you can’t fly fast enough to stay away from Mach effects. The thinning air is the primary factor driving this narrowing of the flight envelope. Humidity, low temperature, or wind shear can influence handling, but they don’t create the fundamental Coffin Corner condition like thin air does.