At what flight level must a flight crewmember use an oxygen mask?

Oxygen masks are a critical safety component for flight crew members, particularly at high altitudes where the availability of breathable air diminishes. The regulation states that at flight levels above a certain altitude, a flight crewmember must be equipped with and use an oxygen mask to ensure adequate oxygen supply in the event of cabin depressurization or other emergencies.

At flight levels above FL390, the use of oxygen masks becomes mandatory for all flight crew members. This requirement is established because at these altitudes, the atmospheric pressure is so low that the partial pressure of oxygen is insufficient to sustain normal physiological functions without supplemental oxygen. Utilizing an oxygen mask at or above this altitude helps protect against hypoxia, which can impair cognitive and motor functions necessary for the safe operation of the aircraft.

Therefore, the requirement for oxygen mask usage aligns with safety protocols established by aviation authorities to minimize risks associated with high-altitude flight.