At what altitude must the remaining pilot wear oxygen if the other pilot leaves the flight deck?

The requirement for a pilot to wear oxygen when flying at high altitudes is dictated by regulations concerning the safety of flight operations. In this context, when another pilot leaves the flight deck, it is essential for the remaining pilot to wear supplemental oxygen if the flight is operating above a certain altitude to ensure that they remain alert and capable of responding to any in-flight emergencies.

At 25,000 feet mean sea level (FL250), the partial pressure of oxygen is significantly reduced, which can lead to hypoxia—a condition that impairs cognitive and physical functions due to a lack of oxygen. Therefore, the regulations stipulate that the remaining pilot must use supplemental oxygen at this altitude, particularly in a multi-pilot environment where one pilot may leave the cockpit.

This requirement is established to ensure flight safety, particularly in scenarios involving potential in-flight emergencies, where the remaining pilot must maintain full operational capability. It is vital for pilots to be aware of these thresholds to comply with safety regulations and safeguard themselves and their passengers.