Most commercial jets operate at similar maximum altitudes. Maximum levels are defined for each aircraft type based on performance and safety considerations. What are these limits, and how are they determined?
Which aircraft fly the highest
Before we get into the reasons, we will look at the limits for commercial aircraft today. All aircraft have a specified ‘service ceiling’ that defines the maximum height that aircraft should operate them at. For most modern commercial jets today, this is around 41,000 feet. The exact level will depend mainly on the engines’ performance (it is designed to be the maximum that still allows efficient operation).
Many large widebodies have a ceiling of up to around 43,000 feet (12,500 meters). The A380, for example, is 43,100 feet and the A350 and Boeing 787 are the same (although the larger 787-10 and A350-1000 are lower at 41,100 feet and 41,450 feet).
The newest 737 MAX is certified to 41,000 feet (although the Original and Classic series 737s are rated only 37,000 feet). The A320 family is rated slightly lower, at 39,100 to 39,800 feet for the neo series.
There are some higher limits, though, outside current commercial aircraft. Concorde (of course, no longer operational) was rated to fly up to 60,000 feet. And many private jets operate up to around 45,000 to 51,000 feet. And if you introduce military jets, the SR71 holds the record (for routine flight) at 85,000 feet.
Why fly so high?
This raises the question of why aircraft fly at 35,000 to 40,000 feet in any case. The main reason for this is performance. The air is less dense at higher altitudes, producing less resistance (and less fuel). Jet engines also operate more efficiently at such heights.
There is a limit to this, though. If altitude is increased too much, then the jet engines will produce less thrust and lift from the wings decreases. These need to be sufficient to maintain flight. Stress on the fuselage is also a consideration. With a pressurized cabin, stress on the fuselage increases with altitude (external air density decreases, and internal pressure remains the same).
The exact altitude chosen in flight, though (up to this maximum), will depend on several factors. The wind is a significant factor, and taking into high altitude jet streams is very important for optimal flight. Turbulence, weather, and other air traffic restrictions also affect flight levels.
The limits on flying high commercially
There is another reason why limits are set for commercial aircraft and why engines are optimized to fly at that height. This is related to safety.
In the event of cabin decompression, the aircraft will descend quickly to a lower altitude. This takes longer from a higher altitude, and critically passengers would lose consciousness much quicker at a higher height. Allowing sufficient time for passengers and crew to react and fit oxygen masks before losing consciousness is vital.
How then can private jets and Concorde fly higher? Private aircraft are not subject to the same ‘Time of Useful Consciousness’ limits and often have larger engines than aircraft weight.
Concorde was different. It excelled at higher altitudes, removing drag allowing higher speed, and with it more lift. It also minimized the risk of decompression by having a system to assist with rapid emergency descent. With a delta wing, it could descend much faster. Its small windows would also lessen the rate of depressurization in the event of failure.