Above 25,000 feet, oxygen must be used by how many pilots unless quick-donning masks are available?

• A. Both pilots
• B. No oxygen
• C. All crew
• D. One pilot must use oxygen unless quick-donning masks are available

Answer: (D)

When flying above 25,000 feet, oxygen is required to prevent hypoxia in the crew. The rule is that at least one pilot must be using oxygen, so the crew is protected if a single pilot is donning a mask. Quick-donning (QD) masks change the situation: if these masks are available, both pilots can be supplied with oxygen quickly, so the requirement isn’t limited to just one pilot.

So the best answer states that one pilot must use oxygen unless quick-donning masks are available. This captures the standard protection level without the rapid-donning capability, while acknowledging that the presence of QD masks allows for immediate oxygen for more than one crew member if needed.