Which of the following is true about oxygen mask usage?

Using oxygen masks in an aircraft is crucial for maintaining safety in specific situations, particularly when altitude increases. When one pilot leaves the flight deck at high altitude, the oxygen mask becomes necessary. This is primarily due to the risks posed by reduced cabin pressure or potential hypoxia, which can occur if there is a failure in pressurization.

The requirement for using oxygen masks ensures that any crewmember who may be exposed to lower oxygen levels while at altitude can maintain adequate oxygen supply. This is especially vital for continued operation and control of the aircraft, as one pilot's absence could significantly impact the flight's safety.

While the use of oxygen masks may also be dictated by other factors, the specific context of one pilot leaving the flight deck at high altitude makes the need for an oxygen mask particularly important in that scenario. This practice aligns with aviation safety standards that prioritize crew health and operational capability during critical phases of flight.