Above what altitude MSL must all occupants of an unpressurized aircraft be provided with supplemental oxygen?

The requirement for supplemental oxygen in unpressurized aircraft is primarily based on the physiological effects of altitude on human performance and safety. At altitudes above 12,000 feet MSL, the partial pressure of oxygen decreases, which can lead to hypoxia, a condition caused by inadequate oxygen supply to the body.

At 14,000 feet and above, supplemental oxygen becomes even more critical. Specifically, at 14,000 feet MSL, it is mandatory for pilots and occupants to use oxygen if they are exposed to that altitude for more than 30 minutes. Furthermore, above 15,000 feet MSL, all occupants of the aircraft must be provided with supplemental oxygen. This regulation is in place to ensure safety and prevent the risk of hypoxia, which can impair cognitive functions and physical abilities necessary for a safe flight.

Thus, the correct altitude where all occupants must be provided with supplemental oxygen is 15,000 feet, as this regulation is designed to protect crew and passengers from serious health risks associated with high-altitude flight.