According to 14 CFR part 91, when must supplemental oxygen be used by the required minimum flightcrew?

The requirement for supplemental oxygen use by the required minimum flightcrew, as outlined in 14 CFR part 91, is specific to cabin pressure altitudes between 12,500 feet MSL and 14,000 feet MSL. During this range, the regulation mandates that pilots require supplemental oxygen when operating for more than 30 minutes above these altitudes since the hypoxic effects of lower oxygen levels become significant at these heights. The rationale stems from the physiological challenges that arise due to reduced air pressure and oxygen availability at higher elevations, which can impair cognitive function and performance—a critical aspect for safety in aviation operations.

Both higher and lower altitudes present different regulations regarding oxygen use. For example, at altitudes above 14,000 feet MSL, there are additional requirements for oxygen use, while at altitudes below 12,500 feet MSL, standard atmospheric pressure typically maintains sufficient oxygen levels for the average individual. This distinction helps ensure that flightcrew is adequately prepared to manage their physiological health while performing their duties in varying air pressure environments.