Coffin’s corner represents the narrow margin at high altitudes where an aircraft’s stall speed and its critical Mach number (Mcrit) converge, leaving pilots with minimal room for error between low-speed stall and high-speed Mach buffet.
What is Coffin’s Corner?
At higher altitude the indicated stall speed (the minimum speed for safe flight) and the Mach limit (maximum safe speed before hitting transonic effects) draw closer together. Operating in this tiny “corner” is dangerous: fly just a bit slower, and the wing stalls; fly a bit faster, and the aircraft risks encountering shockwaves and flow separation.
Mcrit: The Critical Mach Number
The critical Mach number (or Mcrit) is the lowest Mach number at which some airflow over any point on the aircraft reaches Mach 1, or the local speed of sound. When this happens, a small shock wave forms, increasing drag and potentially disturbing lift and control, even though the aircraft itself is traveling below Mach 1. Approaching or exceeding Mcrit leads to the onset of transonic effects and marks the upper speed limit at altitude before entering coffin’s corner. The maximum operating Mach number (MMO) set by manufacturers is always below the aircraft’s true critical Mach number to maintain a safe margin.
Effect of Altitude on Stalling Speed and MMO
As the altitude increases, we observe an increase in Stalling speed of an aircraft. This is because the density of air reduces as the altitude increases, and therefore we need to fly faster to maintain the necessary airflow above the wings.
We also observe that MMO decreases with increase in altitude. This is because Mach Number is inversely proportional to local speed of sound at that altitude. The speed of sound in air is proportional to the square root of the air temperature, so colder air at higher altitudes results in a lower speed of sound. Pressure and density also decrease with altitude, but their effects cancel out, leaving temperature as the main factor controlling the speed of sound. This means that as we go higher, it is easier to reach local speed of sound and creating a shockwave.
Operational Hazards and Flight Planning
Coffin’s corner is a regime where only a few knots separate stall and overspeed, meaning turbulence or small control inputs can quickly push the aircraft into aerodynamic stall or high-speed Mach buffet. Pilots are trained to carefully monitor airspeed and Mach number, especially during cruise at maximum altitudes, and avoid operating near these limits. In turbulence or heavy maneuvering, crews descend to restore a safety buffer between stall and Mach limits. Watch this video of Global 5000 cruising near Coffin’s Corner.
