Since no one else has replied to this one...
What happens to a stall at very high altitudes?
As an aircraft climbs, TAS becomes much greater than CAS. This occurs because CAS is an airspeed based on dynamic pressure, but assuming sea level air density. It is not an accurate measure of airspeed at high altitude (low air density).
As an aircraft climbs the stall speed (CAS value does not change much with altitude) gets closer and closer to the maximum allowable speed - based on TAS or Mach Number.
At an aircraft's absolute ceiling the airspeed indicator will show a value just above the aerodynamic stall buffet, but TAS will be high enough to be just below max TAS or Mach buffet.
If the aircraft were to be allowed to stall, how will the pilot recover? You must lower the nose to reduce angle of attack, but this will inevitably result in an increase in airspeed so you exceed max TAS or even get Mach buffet. Both of these (of course) are very bad for the aircraft structure, and may result in structural limits being exceeded and/or bits falling off.
To fix this, aircraft capable of flying at very high altitudes have a certificated maximum altitude to avoid getting near this flight regime (sometimes called "coffin corner".)
Note 1: there are many other reasons why an aircraft may have a specified max altitude - this is just one aerodynamic one.
Note 2: as far as I know supersonic aircraft do not have the same problem - it's a civil air transport issue.
O8
