When the cabin altitude is above what height does the crew oxygen mask supply pure oxygen?

The correct answer is based on the operation of the oxygen system in the aircraft. The cabin altitude threshold for the crew oxygen masks to automatically supply pure oxygen is set at 30,000 feet. This is crucial for ensuring the safety and health of the crew at high altitudes where the air pressure is insufficient to support normal breathing.

When the cabin altitude exceeds 30,000 feet, the masks deploy pure oxygen to prevent hypoxia, a condition caused by insufficient oxygen levels in the body. At altitudes above this threshold, breathing ambient air becomes unsafe, as the partial pressure of oxygen diminishes significantly. Providing pure oxygen ensures that crew members can maintain consciousness and perform their duties until a safe altitude is reached or the situation is resolved.

The other options represent altitude levels that are lower than the threshold where pure oxygen is required, which would not provide adequate protection against the risks associated with cabin altitudes exceeding 30,000 feet. Thus, it is essential for aviation safety protocols to have this specific threshold in place.