FAA Airline Transport Pilot (ATM)Transport Aerodynamics and High-Altitude OperationsMedium
A transport-category jet is cruising at FL390 at Mach 0.78. The low-speed (stall) buffet margin is 0.15 Mach and the high-speed (Mach) buffet margin is 0.04 Mach above cruise speed. As the aircraft climbs to FL410 with no change in weight or Mach number, the low-speed buffet margin shrinks rapidly while the high-speed buffet margin remains nearly unchanged. What is this narrowing altitude/speed window called?
- AAbsolute ceiling
- BTropopause transition
- CRegion of reverse command
- DCoffin corner
Show answer & explanationAnswer & explanation
Correct answer: D. Coffin corner
As altitude increases at constant weight, the low-speed stall buffet Mach rises (higher AOA needed to maintain lift in thinner air) while the high-speed buffet/Mmo boundary stays roughly fixed, squeezing the usable speed range into a narrow band commonly called coffin corner.
Why the other options are wrong
- A. Absolute ceiling is the altitude where climb rate reaches zero, a different concept.
- B. The tropopause is a temperature layer boundary, unrelated to buffet margins.
- C. Region of reverse command relates to low-speed drag/power requirements, not high-altitude buffet.
Coffin Corner
The narrow speed range at high altitude where low-speed stall buffet and high-speed Mach buffet nearly converge, leaving little margin for maneuvering.
- Low-speed buffet Mach rises with altitude at constant weight
- High-speed buffet/Mmo boundary stays relatively fixed
- Occurs near an aircraft's maximum certificated altitude
Memory trick: Corner closes as both walls squeeze the jet