How long is supplemental oxygen required for above FL250?

Supplemental oxygen is required for pilots operating above Flight Level 250 (FL250) to ensure that they maintain proper physiological function in an environment where the partial pressure of oxygen is significantly decreased. The Federal Aviation Administration (FAA) mandates that pilots must use supplemental oxygen if they are at altitudes above FL250 for more than a certain duration.

The requirement specifies that at altitudes above FL250, pilots must be using supplemental oxygen after 30 minutes. However, if a pilot were to descend back below FL250, supplemental oxygen should be used for at least 10 minutes as the body adjusts to the rapid decrease in oxygen levels. This ensures the pilot has enough time to acclimatize to the lower oxygen levels and avoid hypoxia.

Therefore, the correct answer reflects the regulatory requirements for oxygen use at high altitudes, where maintaining proper oxygen levels is crucial for flight safety. This ensures pilots are physically and cognitively prepared to operate the aircraft effectively in environments where oxygen levels are insufficient for human endurance.