What happens if a passenger plane flies too high?
A look at what happens if a commercial passenger jet flies too high…
If a passenger jet flies too high it reaches a point called ‘Coffin Corner’. This is the point at which the aircraft’s low speed stall and high-speed buffet meet and the plane can no longer maintain its altitude which forces it to descend. For a regular passenger jet this occurs at a height of around 40,000ft – 45,000ft but it can be higher or lower depending on the aircraft’s weight and environmental conditions.
A low speed stall is where there is not enough air passing over the wing to keep the aircraft flying and high-speed buffet is where the air over the wing is going fast enough to cause a shockwave which can result in aircraft control issues. At the altitude at which Coffin Corner occurs, the plane can’t speed up, slow down or climb; the only way to keep the aircraft flying safely is to reduce the altitude and go down. Pilots are aware of what the maximum altitude is and ensure they do not reach it.
What limits a plane’s altitude?
The maximum altitude of a passenger aircraft is limited by 3 factors and any one of these factors can be the limiting one on any given day depending on air temperature and aircraft weight.
The air at high altitude is very thin (less dense). It gets so thin that at a certain altitude not enough air can pass through the engine for it produce enough thrust to keep the aircraft climbing. The density of the air is dictated by the temperature. So, on hot days, the aircraft’s maximum altitude is lower than on cold days. The engine limited altitude is said to be the altitude at which the aircraft can no longer achieve a rate of climb of at least 300 feet per minute.
Cabin Pressure Differential
As the air at high altitude is so thin, it doesn’t contain enough oxygen for a person to survive if they had to breath it in. It’s also very cold reaching temperatures as low as -60c. Therefore, compressed heated air is supplied by the engines (before it’s mixed with fuel) to the aircraft cabin. By forcing air into the cabin, it increases the air pressure and allows passengers to breath normally.
This creates a difference in pressure between inside the cabin and outside environment which is contained by the aircraft fuselage. The higher the aircraft flies, the higher this pressure difference becomes. The maximum pressure differential between the inside of the aircraft and outside is about 9 PSI. This limit is reached when the aircraft reaches around 43,000ft. If the aircraft flew any higher, the maximum pressure differential could be exceeded and this could cause structural failure of the fuselage.
Coffin Corner / Aerodynamic Altitude
A margin of safety is applied to ensure coffin corner is never reached. Typically, a 1.3g margin is used to determine the aircraft’s maximum aerodynamic altitude. This altitude varies with the weight of the aircraft.
Your pilots are aware of which altitude is limiting on any given day and will ensure the aircraft is operated within these limits. Pilots are also given extensive training on how to recover from an event where any of the above issues occured.
If you found this article of interest then check out our page on ‘How Fast do Commercial Jets Fly‘.