What altitude is regarded as critical for using supplemental oxygen at all times?

The correct altitude for using supplemental oxygen at all times is 15,000 feet MSL. At this altitude, the partial pressure of oxygen significantly decreases, which can lead to decreased oxygen saturation in the bloodstream.

According to FAA regulations, crew members are required to use supplemental oxygen if they are operating above 14,000 feet MSL; however, above 15,000 feet MSL, it becomes essential for all occupants to have oxygen available due to the increased risk of hypoxia and other physiological effects. This critical altitude is important for safety as it ensures that all individuals in the aircraft can maintain adequate oxygen levels, reducing the risk of incapacitation and other health issues associated with low oxygen levels.

Understanding this altitude threshold is crucial for pilots, as it plays a vital role in flight safety and passenger wellbeing during high-altitude operations.