FAA Airline Transport Pilot (ATM)Aircraft Performance and Weight and BalanceMedium
A turbojet transport is cruising at a constant Mach number as it climbs to a higher altitude to improve fuel efficiency. As altitude increases at constant weight and Mach number, what happens to the margin between the low-speed (stall) buffet boundary and the high-speed (Mach) buffet boundary?
- AThe margin remains constant regardless of altitude
- BThe margin widens, giving the crew more speed flexibility
- CThe margin narrows, eventually converging toward a single speed known as 'coffin corner'
- DOnly the high-speed buffet boundary changes; the low-speed boundary is unaffected by altitude
Show answer & explanationAnswer & explanation
Correct answer: C. The margin narrows, eventually converging toward a single speed known as 'coffin corner'
As altitude increases at a given weight, the airplane must fly at a higher angle of attack (closer to stall/low-speed buffet) to maintain lift in the thinner air, while the critical Mach number for high-speed buffet remains essentially fixed. The two boundaries converge, narrowing the usable speed range — the 'coffin corner' phenomenon.
Why the other options are wrong
- A. The margin is altitude-dependent, not constant.
- B. The margin actually narrows with increasing altitude, not widens.
- D. Both boundaries are affected; the low-speed buffet speed increases with altitude too.
Coffin Corner
The high-altitude condition where the low-speed (stall) buffet speed and high-speed (Mach) buffet speed converge, leaving little margin for safe maneuvering.
- Occurs at high altitude and/or heavy weight.
- Low-speed buffet speed increases with altitude (thinner air needs higher AOA).
- High-speed buffet (critical Mach) speed stays roughly constant.
- Reducing weight or descending restores buffet margin.
Memory trick: Climb too high and the stall and Mach walls squeeze you into the coffin corner.