At what altitude is the flight crew required to always wear oxygen?

The requirement for flight crew to wear oxygen is mandated by regulations pertaining to altitude and the potential effects of reduced atmospheric pressure on the human body. Specifically, flight crew members are required to use supplemental oxygen when operating at altitudes above 14,000 feet for more than 30 minutes. This regulation is in place to prevent hypoxia, which can result from insufficient oxygen levels at higher altitudes, ensuring that crew members maintain optimal cognitive function and physical capacity during flight.

While supplemental oxygen is recommended at lower altitudes (10,000 feet and above) for passenger comfort and safety, the definitive regulation regarding mandatory use for flight crew is established above 14,000 feet. Therefore, the correct response is that flight crew is required to always wear oxygen above this altitude to comply with safety standards and avoid the adverse effects of altitude on performance.