Which description best matches Coffin Corner?

• A. Condition where the margin between Mcr and Vs reduces until the Flight Envelope boundaries intersect
• B. A temporary flight restriction due to weather
• C. A fault in the instrument configuration
• D. A peak maneuver limit during takeoff

Answer: (A)

Coffin Corner is the situation at high altitude where the safe speed range shrinks because the stall speed (Vs) and the high-speed Mach limit (often the critical Mach number, Mcr) move closer together. As altitude increases, the aircraft’s stall speed climbs while the maximum allowable Mach number doesn’t drop away quickly, so the margin between Vs and Mcr decreases. When those two flight envelope boundaries meet, there’s effectively no practical buffer for safe flight—pushing airspeed up risks Mach buffet near Mcr, while slowing down risks a stall. That description—the shrinking margin between the critical Mach number and stall speed until the envelope boundaries intersect—best matches Coffin Corner. The other options describe weather restrictions, instrument issues, or takeoff maneuver limits, which are unrelated to this high-altitude envelope pinch.