High Altitude Operations
Oxygen rules to memorize cold
Under 14 CFR 91.211, required crew must use supplemental oxygen when the cabin pressure altitude is between 12,500 and 14,000 ft MSL for more than 30 minutes, and continuously above 14,000 ft MSL. Passengers must be provided oxygen above 15,000 ft MSL. For pressurized aircraft the same rule applies to cabin pressure altitude. The time-of-useful-consciousness concept explains the rule: at 18,000 ft it is roughly 20–30 minutes, at 25,000 ft roughly 3–5 minutes, at 35,000 ft 30–60 seconds, and above 40,000 ft measured in seconds.
Physiology and traps
Hypoxia symptoms are insidious - euphoria, tingling, impaired judgment - and the pilot is the worst judge of their own hypoxia. Hyperventilation mimics hypoxia; the treatment is to slow the breathing rate, while hypoxia requires oxygen and descent. A rapid decompression demands oxygen on, and an emergency descent to a breathable altitude (typically 10,000 ft or the minimum safe altitude, whichever is higher).
Airspace and performance
Class A airspace begins at 18,000 ft MSL: IFR only, with an operating transponder and altitude reporting. True airspeed climbs roughly 2 percent per 1,000 ft for a given indicated airspeed, so a Vne in indicated terms stays constant while true airspeed rises - the coffin corner is where high-speed and low-speed limits converge. Lean aggressively for cruise at altitude per the POH, and remember that go-around power is limited by the engines' ability to accelerate.
Endorsement
A high-altitude endorsement (61.31) is required to act as PIC of a pressurized aircraft capable of high-altitude operations, or a jet, and it requires ground and flight training plus a review of the knowledge areas every 24 calendar months.
Rule of thumb
12,500–14,000 for 30 minutes; above 14,000 continuous for crew; above 15,000 for passengers.