Definition
The narrow range of airspeeds at very high altitude where the low-speed stall and the high-speed Mach buffet converge. As an aircraft climbs, the indicated airspeed required to avoid a stall increases while the indicated airspeed at which Mach buffet begins decreases. At the altitude where these two speeds meet, the airplane can neither slow down without stalling nor speed up without entering Mach buffet, leaving no usable speed margin.
Plain English
It's the altitude where the airplane's slowest safe speed and its fastest safe speed come together. Fly any slower and you stall; fly any faster and you hit shockwave-related buffeting. There is no safe room in between.
Context Anchor
Encountered in high-altitude performance discussions, especially when studying Mach buffet, maximum operating altitude, and safe speed margins in faster airplanes.
Derivation
The phrase comes from World War II–era high-altitude flight testing. Pilots called it 'coffin corner' because the squeezed-together speed limits at extreme altitude could trap the aircraft with no safe way out — slow stall on one side, high-speed buffet on the other.
Why Pilots Care
Exceeding either limit at altitude can produce violent pitch-up or roll-off that is difficult to recover from in thin air.
Analogy
Imagine walking on a path between two cliffs that get closer together the higher you climb. Eventually the path becomes too narrow to stand on — that's coffin corner.
Intuition Check
Coffin corner does not mean a physical corner in the airplane or on a chart. It means a narrow and risky speed range at high altitude, where the airplane is close to both its slow-speed and high-speed limits.
Example Sentence 1
At her airplane's maximum certified altitude, the pilot had to manage power and pitch carefully to stay clear of coffin corner.
Example Sentence 2
Turbulence in coffin corner can instantly push the wing into either stall buffet or Mach buffet.