Rather than saying "If you fly slower, you will stall" might it be more accurate to say "If you fly slower, the steepest angle of attack that would avoid a stall will be insufficient to maintain altitude"? A plane with upward vertical momentum one might pass the maximum altitude where it could sustain level flight, but if the pilot kept the airspeed and angle of attack within safe limits the plane would simply descend until it was in thick enough air. If there are no mountains in the way I wouldn't think the plane should "notice" the changing vertical velocity.

@supercat I think that is more accurate indeed. The plane will "notice" changing vertical velocity though. The change is caused by an imbalance between lift and weight; the wing loading is different.

@Iceman I've added a paragraph mentioning engine flame out at high altitudes.

@DeltaLima: Strain gauges on the wings could detect the quantitative change in lift, but if one didn't have a calibrated accelerometer or know how much the plane weighed, and couldn't sense the ground, I don't think the behavior of the plane would qualitatively change at maximum altitude (unlike, e.g. if the plane exceeds maximum airspeed or maximum angle of attack, either of which would cause its behavior to change markedly).

@supercat If you are flying below the minimum speed for sustained level flight and keep the angle of attack at the maximum, then the weight exceeds the lift thus you are accelerating downwards. That will increase the angle of attack beyond the maximum unless you decrease the pitch angle constantly to match the change in flightpath angle. That is a qualitative change in behaviour isn't it? It is straight and level flight vs parabolic flight.

@DeltaLima: Yeah, you're right. I forgot to consider the required change in attitude necessary to maintain stable flight. Even so, if a pilot was trying to ride along the altitude ceiling, overshooting it slightly would merely require a minor change in attitude. By contrast, exceeding the critical angle of attack would cause a sudden radical change of behavior which could only be reversed by a significant change in the other direction; while maximum airspeed isn't a single critical value (the more stress one is willing to accept, the faster one could fly, to the point that the plane...

...which would be able to survive for millions of hours under low-stress conditions might only survive for a few minutes or seconds before it falls apart). In any case, I would think the primary issue with the altitude ceiling is that it doesn't represent a threshold where something bad will happen if it's exceeded, so much as it represents a limit of how high a plane will go if other parameters are kept in range.

@supercat Before you are going to try out your theory and kill yourself may I suggest you investigate high altitude operations, especially: mach stall, backside of the power curve, longitudinal stability, effect of temperature variations on mach number and effects of turbulence on speed and load factor to start with? There are very good reasons for the certified ceiling of an aircraft to be well below the absolute ceiling. The name coffin corner does not come out of the blue (well maybe it does, but I guess you get the point)

@DeltaLima: Fair point, from the perspective that safe operation of an aircraft requires that one always have an "escape plan" in case of things like unexpected turbulence, and simultaneously pushing the limits on two parameters at once could leave one in trouble if the air conditions into which one flies aren't exactly as expected. Still, I would think (correct me if I'm wrong) that if one avoids pushing the limits for airspeed or angle of attack, the fact that the airframe reached an altitude where it would no longer accelerate upward would not make anything particularly dramatic happen.

@supercat If the aircraft has enough power the aircraft can be climbed above the maximum altitude. The problem is that above that altitude, for loadfactors >= 1 there is no speed that is less than the maximum (critical) mach number and above the stall speed.

The only solution is to get there in a parabolic flight, such that the load factor < 1

Assuming you do not exceed the maximum angle of attack (which by the way is surprisingly low at high mach numbers) you will come back below the maximum altitude with a) a negative vertical speed and b) nose down attitude.

To recover from this descent you will need to have create a load factor > 1

@DeltaLima: I would think that the big danger at high altitudes would be that the only way to get there is to have both airspeed and angle of attack near their limits. Normally if one needed to rapidly reduce airspeed, one could nose up, and if one needed to reduce angle of attack one could nose down. But if both values are near their limits, attempting to fix one problem would trigger the other or make it worse.

@DeltaLima: I would also expect that airplanes would specify an altitude which the plane should be expected to be able to fly over (meaning that if there's a mountain which is above that altitude, the plane might make it over in some weather conditions but not on others), as well as an altitude which, if the plane stays below it, should be far enough below the combined limits of angle-of-attack and airspeed...

Exactly, and even if you succeed in preventing a stall by lowering the nose, you would end up a lot lower than during normal stall recovery because you can't pull up out of your descent as quickly because there is very little stall margin.

The certified maximum altitude allows for a load factor of 1.3

...that one could recover from turbulence that put one over one limit without hitting the other. BTW, would I be correct in guessing that there's probably a huge difference in safety between flying high over low terrain for the sake of achieving a high altitude, versus flying at an altitude above that where the plane would be most efficient, for the purpose of getting over high terrain?

So that gives a safe margin from both mach stall and low speed stall.

If one finds at an altitude of 30,000 feet in air conditions that support a maximum altitude of 29,000 feet and won't be able to recover until 25,000 that might not be a problem if the ground is at 200 feet, but could be a huge problem if the ground is at 26,000?

There is indeed a difference in safety. Not only because the altitude loss you can tolerate for recovery is different but also because of the turbulence that high terrain generally creates.

But if an aircraft is altitude limited to 30000 feet, I think that would usually be due to power limitations. So that is not as big of a problem as the coffin corner would be.

BTW, are altitude limits expressed in terms of altimeter readings, or distance from a mean-sea-level ellipsoid [e.g. measured with GPS]? The plane's immediate behavior would be a function of the altimeter reading, but if plane that's in a high-pressure zone passes through a front, its apparent altitude could increase suddenly.

The performance related altitude limits are expressed in altimeter reading under standard conditions with corrects for temperature to be applied.

The apparent altitude will change stepwise when the aircraft passes through a front, it will be gradual.

The mach number can jump when you pass through a front or through an updraft.

Thanks for the information. Ping me if you want to chat more about anything.

You're welcome, it's always a pleasure to exchange ideas.