Passenger oxygen masks not approved for sustained operation above what flight level?

• A. FL350
• B. FL250
• C. FL400
• D. FL300

Answer: (B)

Emergency passenger oxygen systems are certified for sustained use only up to a certain altitude. The oxygen supply and regulators are sized to keep occupants oxygenated long enough to descend to a breathable altitude during a depressurization event, not for long-duration use at very high altitude. That certification limit is up to flight level 250, so the system isn’t approved for sustained operation above that level. In practice, this means masks will provide oxygen, but the system isn’t certified to support continuous use at higher altitudes, and crew would descend to a lower altitude where the system remains within its certification. The other altitude options fall outside this approved sustained-use limit.