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

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

• A. FL400
• B. FL350
• C. FL450
• D. FL300

Answer: (A)

The main idea here is that crew oxygen systems have certification limits for how long they can be used. The masks provided for crew are designed for emergency or short-duration use and are not approved for sustained operation beyond a certain high altitude. That ceiling is the point at which the oxygen delivery and system performance are no longer guaranteed for long periods, so you would switch to the aircraft’s primary high-altitude oxygen system or descend to a lower altitude. So, the correct threshold is the altitude corresponding to flight level 400. Above this level, sustained use of the crew masks isn’t approved. In practice, crews ensure adequate oxygen by using the dedicated high-altitude oxygen system or descending if needed.

The main idea here is that crew oxygen systems have certification limits for how long they can be used. The masks provided for crew are designed for emergency or short-duration use and are not approved for sustained operation beyond a certain high altitude. That ceiling is the point at which the oxygen delivery and system performance are no longer guaranteed for long periods, so you would switch to the aircraft’s primary high-altitude oxygen system or descend to a lower altitude.

So, the correct threshold is the altitude corresponding to flight level 400. Above this level, sustained use of the crew masks isn’t approved. In practice, crews ensure adequate oxygen by using the dedicated high-altitude oxygen system or descending if needed.