Above what altitude must the remaining pilot wear oxygen if the other pilot leaves the flight deck?

The requirement for a remaining pilot to wear oxygen when the other pilot leaves the flight deck is established by aviation regulations and guidelines to ensure safety in the aircraft. Specifically, if one pilot departs the cockpit, the other must supplement their oxygen supply above an altitude of 25,000 feet. This threshold is set to guard against the risk of hypoxia, as the atmospheric pressure and oxygen levels decrease at higher altitudes, which can impair cognitive function and overall performance.

At altitudes above 25,000 feet, the likelihood of experiencing hypoxia increases significantly due to the reduced availability of oxygen. Therefore, the regulation mandates that the remaining pilot must utilize supplemental oxygen to maintain operational safety and effectiveness, thus ensuring they can adequately manage the flight responsibilities without impairment.