Two factors converge to coffin corner?

• A. Weight and drag.
• B. Altitude and temperature.
• C. Maximum Mach decreases while true stall speed increases with altitude, narrowing speed window.
• D. Engine thrust and fuel flow.

Answer: (C)

Coffin corner happens when, at high altitude, two opposite speed boundaries squeeze together: the upper limit set by Mach number and the lower limit set by stall speed. As you climb, the maximum safe Mach number falls because compressibility effects bite earlier in thinner air, while the true stall speed climbs since the airplane needs more true airspeed to produce enough lift in less dense air. The result is a shrinking speed window in which the aircraft can safely fly. The best description of this is that maximum Mach decreases while true stall speed increases with altitude, narrowing the speed range. Other options don’t describe these opposing limits or how they interact to tighten the operating envelope.