You stand in line to use the restroom, hands in your pockets, a constant hum in your ears. You look out the window and see a swollen foot, the head of a goat, maybe the fin of a dolphin, all of them white and fluffy, shapes that seem to be morphing. Then the clouds open up and you see the coastline. Sunlight bouncing off the water’s surface. “How high are we flying,” you wonder, “and why?”
When it comes to commercial airplanes, economics play an important role, almost at the same level as safety’s. Every meter a plane ascends, the layer of air gets thinner; there is less oxygen. Between 10,500 and 12,000 meters, the so-called “cruising altitude”, aircraft face less air resistance, can travel faster and also use less fuel. The more they save, the better business airlines do.
Commercial aircraft engines are of the turbofan type, which have a higher thrust as they face less air resistance. But they also need atmospheric oxygen to sustain combustion, so exceeding 12,000 meters would already compromise the efficiency of the turbines.
The difference between the outside pressure and the pressurized cabin inside is achieved at the limit of the aircraft’s structural strength. To go higher, the aircraft would need to be strengthened, which would add weight and therefore negate the economic advantage of a higher flight level.
The final altitude depends on the weight of the aircraft: the heavier it is, the lower it flies. An Airbus A380 or a Boeing 747 will have a lower altitude than a Boeing 737 or an Airbus A319, but they always operate in that cruising altitude range.
Another reason is related to safety: in case of problems with the engines or any other component of the aircraft, pilots have a margin of time to fix them. If a plane were to fly 1,000 meters above the ground and its engines stopped, the impact would be almost immediate. At ten times that altitude, pilots can get the plane to glide 50 to 80 kilometers higher.
In a sudden depressurization event at 12,000 meters, the average passenger has between 7 and 10 seconds of TUC (Time of Useful Consciousness) before suffering the disabling effects of hypoxia.
Most atmospheric phenomena (rain, lightning, wind, hail, dense clouds) occur in the troposphere, the atmospheric layer that goes from the surface to 10,900 meters.
Although an airplane can go through storms and deal with winds, it’s a known fact that turbulence generates a lot of fear and complicates operations. The best practice is to avoid bad weather conditions and fly above them. At that altitude there are no birds that can impact the aircraft. This is a common problem during takeoffs and landings, and can cause accidents.
Below 10,000 meters is the altitude that helicopters and smaller aircraft, such as propeller-driven aircraft, typically fly. Any higher altitude and passengers, who do not have a pressurized cabin, would have problems.
