We all know what happens when a plane goes overspeed... but what happens when a plane goes above their max. altitude? And why are planes certified maximum altitudes anyway? Is it because there won't be enough lift to keep them at that altitude? I know it's not because they don't want the aircraft to get lost in space, because the max. alt. of the Airbus A350 is at FL420, which is not even close to space. Anyone ave an answer?
There is a variety of reasons for a maximum altitude:
- Most common is insufficient engine performance to climb any higher. Then your question never arises. The aircraft simply cannot go higher.
- For more powerful aircraft the partial pressure of the oxygen supply in an unpressurized cabin would drop below the value to keep the pilot(s) alert. For GA aircraft without supplemental oxygen this would be 15,000 ft (short term) or 12,500 ft (long term).
- For many airliners it is the pressure limit of the cabin which is certified for a maximum differential to outside pressure. If the airplane climbs higher, the internal pressure must be reduced to less than what is deemed the acceptable minimum for the passengers.
- For special subsonic high altitude aircraft with sufficient engine power the limit is the coffin corner when minimum speed and maximum Mach fall together. When that height is exceeded, the aircraft stalls and exceeds its Mach limit at the same time. The result is a shallow dive which can only be recovered a few thousand feet lower.
- In some cases the limit is given by minimum combustion chamber pressure inside the engine to keep the flame burning. There are cases when aircraft climbed too high, suffered a flame-out and could not restart.
- With supersonic aircraft the limit might be set by maximum flight Mach number, when higher speed means stagnation heat exceeds structural limits. The high Mach number is needed to sustain minimum dynamic pressure to produce enough lift.
The maximum altitude specifications that an airplane has are based on the practical limits on staying airborne as the plane climbs higher and higher. For ordinary planes like airliners and private planes, those limits do not mean something "breaks" if you fly too high, the way it would if you flew too fast in a dive for example. It means that if you try to fly higher than a certain altitude, the engine's performance is degraded enough that the plane can't climb with its full rated payload any higher than that.
A typical airplane needs two things to stay airborne: sufficient airspeed (specifically, indicated airspeed, which is directly related to dynamic pressure which is what makes wings generate lift), and to stay subsonic. The latter criterion is due to the design of the airplane; supersonic airplanes do not have this limit and can typically fly much higher.
As you go higher, for a given true airspeed, the indicated airspeed decreases. To get enough dynamic pressure on your wings, you need to increase speed. However, the speed of sound decreases even faster with altitude, which means at some point you'll need to go near supersonic to get enough airspeed to keep enough dynamic pressure on your wings to keep aloft. Go faster and you'll go supersonic and plummet out of the sky; go slower and you'll stall and plummet out of the sky. This point is called the coffin corner, and typically, you want to stay away from it (even though there's much sky underneath you to plummet into).