What’s coffin corner?

Reference：https://airwaysmag.com/coffin-corner-aviation/ , https://www.boldmethod.com/learn-to-fly/aerodynamics/coffin-corner-where-vne-and-mmo-meet/

Ans:

Coffin Corner can be explained by using below illustration. It is talking about when when the aircraft fly to an altitude where the aircraft reduces the speed, it will stall while the aircraft increase its speed, it will over the Mmo. Therefore, the margin for error is low.The coffin corner is not exactly same for different aircraft. However, it has significance in the safety of operations. 

Further question – please explain more about the illustration?

Lift formula = CL * S * 1/2 * air den * V^2

Fly higher, the air den drop, to maintain the same amount of lift, the AoA need to increase or compensate by increase the TAS.

For left hand line (Vs):

Since the wing will only stall when it reach critical AoA! When fly higher, the TAS must be higher to generate same amount of lift, so the so-called Vs (stall speed) will increase! (explained by Lift formula)

For right hand line (Vmo):

In commercial airline industry!!! We usually use(fly) Mach number at high altitude.

The problem comes:

Mach Number (MN) = TAS/LSS

LSS is affected by temperature and air den

Fly higher, TAS increases is must, as mentioned before, and the LSS will decrease which means the MN will higher (eg 4/2 is 2 and 6/1 is 6), so when fly higher, the MN is actually higher which mean it is more easy to reach the so called Mach crit