When is minimum flight crew oxygen required according to part 91?

Minimum flight crew oxygen is required at altitudes of 12,500 feet and above. This regulation is in place due to the physiological effects of high altitude on the human body. At altitudes above this threshold, reduced atmospheric pressure can lead to hypoxia, a condition where the body is deprived of adequate oxygen.

By requiring oxygen for flight crews at 12,500 feet, the regulation ensures that pilots maintain adequate cognitive function and physical ability to operate the aircraft safely. While the need for supplemental oxygen becomes critical at lower altitudes in various situations, this specific requirement is set to safeguard against the risks associated with reduced oxygen levels that occur at 12,500 feet and higher.

The other altitude options either fall below this threshold or are not applicable to the minimum oxygen requirement as stipulated in the regulations.