The maximum height that a commercial airplane is allowed to reach when they fly is 42,000 feet, as this is the universally approved maximum altitude. This max altitude for airplanes is known as the “service ceiling.” Most commercial air jets fly at such a high altitude because it is known to optimize efficiency.

For commercial passenger jets, the "service ceiling" is typically between 41,000 and 43,000 feet. Most cruise at 33,000 to 38,000 feet (FL330 to FL380) to optimize fuel efficiency and engine performance. Smaller business jets can fly higher, often reaching 51,000 feet, which allows them to bypass commercial traffic and weather. In general aviation, most unpressurized small planes are restricted to below 12,500 feet unless the pilot and passengers have supplemental oxygen. The "absolute" maximum for any winged aircraft was set by the rocket-powered North American X-15, which reached 354,200 feet (over 100 km), technically entering space. For standard operations, the limit is dictated by the "Armstrong Limit" (~60,000 feet), where atmospheric pressure is so low that water (and blood) would boil at body temperature without a pressurized suit or cabin.