With quick-donning oxygen masks available but not worn, what is the maximum altitude the aircraft can fly?

• A. FL100
• B. FL450
• C. FL410
• D. FL250

Answer: (C)

The key idea is how oxygen availability and altitude affect a pilot’s ability to operate safely. Above certain heights, the air is too thin for unassisted breathing, so oxygen must be supplied. Quick-donning masks are designed to be used quickly during a depressurization or high-altitude situation, but if they’re not worn, you’re relying on the aircraft’s system to keep crew safe long enough to don them.

Most airliners are certified to operate with crew oxygen available but not immediately in use up to about 41,000 feet. Beyond that, the time you could remain conscious without oxygen becomes too short to manage safely if masks aren’t being worn right away. Therefore, with the masks available but not worn, the practical maximum altitude is around 41,000 feet (Flight Level 410). The other options are either well below the safe operating envelope or exceed it when the crew isn’t donning oxygen immediately.