Originally Posted by mad jock
swept wing jet pilot myself which the coffin corner mainly effects
- not so - it can affect
all aircraft at high altitude. In simple terms, it involves MCrit and V stall 'coinciding', and since all 'high-level' aircraft have both..................... If my memory serves me right, I think 'Coffin Corner' came to prominence with the NA B45 (straight-wing) bomber, and it would be of interest to you to know that I believe the U2 (straight-wing) has/had a very small IAS margin at cruise altitude - something like 2 or 4 kts, I think.
To answer to OP - Stall. There is always a safety 'margin' on the Mach/IAS limit on any aircraft, but once you are stalled..................