Plain English Meaning

Flying high enough that the thin air starts to affect both the pilot and the airplane, so extra oxygen, a pressurized cabin, and special training and procedures become necessary.

Full Definition

Flight conducted at altitudes where the reduced air pressure and lower oxygen partial pressure require supplemental oxygen, pressurization, or both to keep occupants safe and the aircraft performing as expected. In FAA practice, the term generally refers to operations above 10,000 feet MSL, with stricter oxygen and equipment requirements applying above 12,500 feet, 14,000 feet, and 25,000 feet, and additional required for flight above 25,000 feet in .