What is the maximum altitude for unpressurized flight without oxygen masks available?

The maximum altitude for unpressurized flight without the availability of oxygen masks is indeed 13,000 feet above sea level (ASL). At altitudes above this level, the partial pressure of oxygen decreases, which can lead to hypoxia, a condition resulting from insufficient oxygen reaching the body's tissues.

The regulations are structured to ensure safety and manage the risk of exposure to low oxygen levels during flight. When flying above 12,500 feet, it becomes increasingly crucial to provide supplemental oxygen to the pilot and passengers for safety. It is generally advised that all crew members should utilize supplemental oxygen when flying above this altitude to avoid potential physiological impairments that can occur due to the thinner atmosphere.

This altitude marks the threshold where the risk of hypoxia becomes significant, consistent with safety standards in aviation. Thus, 13,000 feet ASL is the correct answer for maximum altitude for unpressurized flight without oxygen masks available.