What altitude requires the remaining pilot to wear supplemental oxygen?

The requirement for a remaining pilot to wear supplemental oxygen is based on regulations concerning cabin pressure and altitude. When the altitude reaches FL250 (25,000 feet) and above, regulations typically state that pilots must use supplemental oxygen if they are flying alone in the cockpit (i.e., the remaining pilot after one has exited due to incapacitation). This requirement is especially important to ensure that the pilot has sufficient cognitive and physical capabilities to operate the aircraft safely in a reduced oxygen environment often found at higher altitudes.

Although the altitude at which these rules begin to apply can vary by specific regulations and conditions, the critical point is that at FL250, the likelihood of experiencing hypoxia, a deficiency of oxygen reaching the tissues, increases significantly. This poses a safety risk, making the use of supplemental oxygen essential for the remaining pilot to maintain alertness and prevent any impairment that would compromise flight safety.

Thus, at FL250, the requirement is explicitly clear that the remaining pilot must wear supplemental oxygen, ensuring they can effectively manage the aircraft in critical phases of flight.