When should oxygen be used for crew members?

The correct choice is based on standard aviation regulations and practices concerning the use of supplemental oxygen in an aircraft. Crew members are required to use oxygen above 14,000 feet primarily to ensure proper physiological conditions for safe operation. As altitude increases, the partial pressure of oxygen in the atmosphere decreases, leading to a decline in the available oxygen for breathing.

When operating at an altitude above 14,000 feet, the risk of hypoxia – a deficiency in the amount of oxygen reaching the tissues – increases significantly. Supplemental oxygen helps mitigate these risks effectively, ensuring that crew members maintain optimal cognitive and physical performance. This altitude threshold is consistent with both federal aviation regulations and accepted safety protocols in aviation.

The other options fall below this critical altitude where the need for oxygen becomes paramount. Use of oxygen at lower altitudes may not be necessary for healthy individuals, as they can typically maintain adequate oxygen saturation levels.