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The requirement for at least one pilot to wear an oxygen mask above Flight Level 350 (FL 350) is primarily rooted in safety protocols established to address the risks associated with high-altitude flying where oxygen levels are insufficient for normal physiological function. When an aircraft is cruising at or above this altitude, the cabin pressure may drop due to various circumstances like cabin decompression.

The reasoning behind the necessity of one pilot wearing an oxygen mask in the scenario of two pilots not being at their stations is related to maintaining continuous control of the aircraft. In the event that one pilot is incapacitated or away from their duties, the remaining pilot must be prepared to assume full operational control. Wearing an oxygen mask ensures that the active pilot maintains adequate oxygen levels to function effectively, should a situation arise that requires immediate action.

While it is valuable for all crew members to be prepared for emergencies or to follow certain safety protocols, the specific need for one pilot to wear an oxygen mask when the other is not present underscores the crucial aspect of maintaining flight safety and control continuity above FL 350.