Above what cabin altitude must oxygen be provided for all crewmembers during the entire flight?

The requirement for providing oxygen to all crew members is based on the altitude at which the cabin environment can affect human physiological function. Specifically, oxygen must be supplied for all crewmembers when the cabin altitude exceeds 12,000 feet. At this altitude, the partial pressure of oxygen decreases significantly, which can lead to hypoxia, impaired performance, and increased risk of incapacitation.

Above this threshold, it is essential to ensure that all crew members have access to supplemental oxygen to maintain their cognitive and physical functions effectively. This regulation is part of ensuring flight safety and is grounded in aeromedical research that highlights the risks associated with higher altitudes and lower oxygen availability.

The other altitude options, while relevant to specific regulations and recommendations, do not apply universally to the entire crew. For instance, oxygen systems may be needed at lower altitudes for certain operations, but 12,000 feet is the critical threshold where supplemental oxygen becomes a statutory requirement for all crew members throughout the flight.