Mcrit/Critical Mach No. is the free stream mach number at which the local velocity (velocity of air over any part of the aircraft, mostly the wing) reaches sonic speed.
Critical Mach No. is affected by Thichness:chord ratio and by AOA. Therefore if your weight is increased you will be flying at a higher AOA which reduces the Critical Mach No.
I don't think aircrafts operate at or above M.crit because the drag rise with the formation of the shock wave at M.crit is huge. So I think a/c's fly close to the M.crit.
Coffin Corner is the altitude at which your lowspeed buffet (stall) and high speed buffet (M.crit) speeds are same. At this alt you can fly only at one speed if you decelerate you stall and if you accelerate you exceed M.crit.
Last edited by flyjet787; 28th Feb 2010 at 15:07.