At what altitude must all flight crew have supplemental oxygen for flights above 14,000 MSL?

The requirement for all flight crew to have supplemental oxygen at altitudes above 14,000 feet MSL is rooted in safety regulations related to hypoxia, which can occur due to reduced atmospheric pressure at high altitudes. At 14,000 feet and above, the partial pressure of oxygen decreases, and without supplemental oxygen, pilots and crew might experience impairments in cognitive and physical functions, leading to unsafe flight operations.

The regulation mandates that flight crew members require supplemental oxygen for the entire flight above this altitude to ensure they remain alert and capable of performing necessary duties. This precaution helps mitigate the risks associated with high-altitude flight, where oxygen availability diminishes significantly.

The other options might suggest circumstances where supplemental oxygen could be considered unnecessary, but they do not align with the safety standards intended to protect the crew and overall flight operation above the critical altitude of 14,000 feet MSL.