What is coffin corner?
Coffin corner occurs at an aircraft’s absolute ceiling, where the speeds at which Mach number buffet and prestall buffet occur are coincident, and although trained for, in practice, they are difficult to distinguish between.
Therefore, a margin is imposed between an aircraft’s operating and absolute ceiling.
Mach number and the slow-speed stall buffet are coincident at coffin corner because a stall is a function of indicated air speed (IAS) and Mach number is a function of the local speed of sound (LSS), which itself is a function of temperature. For a constant Mach number (which is the normal mode of speed man-agement), the IAS decreases with altitude due to the decreasing LSS. To prevent the IAS from decreasing to its stall speed, the Mach number must be increased, which results in an increasing IAS.
For a constant IAS, the Mach number increases with altitude due to a decreasing LSS and temperature to a point where the Mach number exceeds Mcrit. To prevent the Mach number from exceeding Mcrit, the IAS must be reduced, which results in a decreasing Mach number. Therefore, there comes a point at the aircraft’s absolute ceiling where the aircraft can go no higher. This is so because it is bounded on one side by the low-speed buffet and on the other by the high-speed buffet because the stall IAS and the Mcrit values are equal. This is coffin corner, and this effect restricts the altitude attainable by the aircraft.