How is an oxygen mask supply regulated for crew during high-altitude operations?

• A. Crew oxygen systems provide a regulated supply via masks with flow control and alarms; ensure mask fit and flow is active when required.
• B. Oxygen is supplied unregulated and constant.
• C. Oxygen masks are optional for crew when cabin altitude is high.
• D. Oxygen masks supply only to pilots, not cabin crew.

Answer: (A)

Oxygen for crew is regulated by a controlled supply that is activated when the mask is donned and kept at a safe, usable flow through a regulator at the mask or in the oxygen system. This regulator reduces the high-pressure stored oxygen to a safe delivery rate and, in many designs, provides a demand-style flow so oxygen is delivered mainly as you inhale. Flow control ensures the wearer gets enough oxygen without waste, and alarms monitor for issues such as low pressure, blocked flow, or a compromised mask seal so the crew can respond quickly. A proper fit is essential because a good seal ensures the oxygen actually reaches the lungs rather than leaking away. The system is designed for crew throughout the aircraft, not just pilots, and it is not unregulated or optional at high altitude.