What is the required oxygen protocol for flight crews at altitudes above 14,000'?

At altitudes above 14,000 feet, pilots and flight crew are required to use supplemental oxygen at all times to ensure adequate oxygen saturation in the bloodstream. At these elevations, the atmospheric pressure decreases, leading to lower partial pressures of oxygen in the air. This reduction can result in hypoxia, a condition where the body does not receive enough oxygen to function properly.

The use of supplemental oxygen mitigates the risk of hypoxia, allowing crew members to maintain cognitive function and physical performance critical for safe flight operations. For aviation safety, regulations clearly outline the need for constant oxygen supply in such environments, thus reinforcing the importance of option A.

The other choices reflect inadequate safety measures under these conditions: using oxygen only during specific phases of flight does not account for the sustained risks associated with prolonged exposure to high altitude; not requiring oxygen fails to acknowledge the physiological changes that occur at higher elevations, and only one pilot using oxygen does not ensure that both crew members are functioning within their optimal capacities when required to respond in various scenarios. Therefore, consistent use of supplemental oxygen is imperative for safety above 14,000 feet.