At what cabin altitude do the crew oxygen masks provide 100% oxygen regardless of switch position?

• A. 10,000 ft cabin altitude
• B. 20,000 ft cabin altitude
• C. 30,000 ft cabin altitude
• D. 40,000 ft cabin altitude

Answer: (C)

As altitude increases, the air inside the cabin becomes thinner and the brain needs more oxygen to stay tissue-sufficient. The crew masks have a switch to set the oxygen level under normal operation, but a safety mechanism kicks in at a high enough altitude and forces the masks to deliver pure 100% oxygen no matter what the switch is set to. This automatic override occurs around 30,000 feet because, beyond that point, the partial pressure of oxygen in the cabin would be inadequate for safe breathing unless the supply is full oxygen. So, at about 30,000 feet cabin altitude, the masks provide 100% oxygen regardless of switch position.