Coffin Corner is defined as what?

• A. Condition where margin between Mcr (critical Mach) and Vs (stall speed) reduces until the Flight Envelope boundaries eventually intersect
• B. A phase of maximum climb rate
• C. A stall at sea level
• D. The aircraft’s center of gravity moves outside limits

Answer: (A)

Coffin Corner describes a situation where the safe airspeed window shrinks because two flight-envelope boundaries move toward each other: the high-speed limit set by the critical Mach number (where compressibility effects cause loss of normal lift) and the low-speed limit set by stall speed. As altitude increases, these limits converge, and the margin between them disappears, so there’s little to no safe airspeed range to fly within. That’s why the correct description is that the margin between the critical Mach number and stall speed reduces until the flight envelope boundaries intersect. The other options point to different flight phases or conditions (like maximum climb rate, a sea-level stall, or a center-of-gravity issue) and don’t capture this high-altitude speed-margin phenomenon.