A pilot and flight crew operating above 14,000 feet MSL must use supplemental oxygen how often?

The correct reasoning behind the necessity for pilot and flight crew use of supplemental oxygen above 14,000 feet MSL centers on maintaining physiological safety and performance. At altitudes above 14,000 feet, the partial pressure of oxygen decreases significantly, which can lead to hypoxia—an inadequate supply of oxygen to the tissues. This condition can impair cognitive functions, decision-making, and overall physical abilities, which are critical for safe flight operations.

To mitigate this risk, pilots and flight crew are required to use supplemental oxygen constantly when operating at or above this altitude. Continuous use ensures that they maintain adequate oxygen levels, enabling them to perform their duties effectively and respond to any in-flight emergencies that may arise. This regulation is in place to ensure safety is prioritized in the demanding environment of high-altitude flying.

When compared to other options, using supplemental oxygen only during takeoff, as needed, or just during emergencies does not provide the continuous safeguard necessary for the entire duration of a flight at such high altitudes. Each of those alternatives could lead to crucial moments of impaired performance where oxygen levels are insufficient, increasing the risk of accidents.