Coffin corner (aerodynamics)

The "corner" refers to the triangular shape at the top of a flight envelope chart where the stall speed and critical Mach number are within a few knots of each other.

Consideration of statics shows that when a fixed-wing aircraft is in straight, level flight at constant-airspeed, the lift on the main wing plus the force (in the negative sense if downward) on the horizontal stabilizer is equal to the aircraft's weight and its thrust is equal to its drag.

[2] As an airplane moves through the air faster, the airflow over parts of the wing will reach speeds that approach Mach 1.0.

At such speeds, shock waves form in the air passing over the wings, drastically increasing the drag due to drag divergence, causing Mach buffet, or drastically changing the center of pressure, resulting in a nose-down moment called "mach tuck".

The flight envelope is a plot of various curves representing the limits of the aircraft's true airspeed and altitude.

The top-right boundary of the envelope is the curve representing critical Mach number in true airspeed terms, which decreases as altitude increases.

[3] The above explanation is based on level, constant speed, flight with a given gross weight and load factor of 1.0 G. The specific altitudes and speeds of the coffin corner will differ depending on weight, and the load factor increases caused by banking and pitching maneuvers.

Similarly, the specific altitudes at which the stall speed meets the critical Mach number will differ depending on the actual atmospheric temperature.

In either case, as the airplane falls, it could gain speed and then structural failure could occur, typically due to excessive g forces during the pullout phase of the recovery.

This was pointed out by a report describing the effect of ice crystals on pitot-tube airspeed indications at high altitude: " .

For example, a flight test project conducted by the National Research Council of Canada titled “Aerodynamic Low-Speed Buffet Boundary Characteristics of a High-Speed Business Jet” and presented at the 24th International Congress of the Aeronautical Sciences involved an intermediate capacity, high-speed business jet with highly swept wings to conduct low-speed buffet testing.

In other words, be wary of your pitch attitude while at high altitudes because of the limited range of AOA due to Mach effects.