Under what condition must oxygen be carried in unpressurized aircraft?

The requirement to carry oxygen in unpressurized aircraft is specifically mandated for operations at altitudes between 10,000 and 13,000 feet ASL if the flight duration at those altitudes exceeds 30 minutes. This regulation is in place to ensure the safety of occupants, as the physiological effects of altitude can begin to impair human performance, particularly beyond 10,000 feet.

Once an altitude exceeds 10,000 feet, the partial pressure of oxygen begins to decrease, and after approximately 30 minutes at this level, a lack of supplemental oxygen can lead to hypoxia (a deficiency in the amount of oxygen reaching the tissues). Thus, to mitigate the risks associated with prolonged exposure to decreased oxygen levels, the regulation specifies this carry-on requirement.

Other altitude ranges, such as below 10,000 feet or above 15,000 feet, have their own specific criteria for oxygen use, but for the context of this question, the correct choice emphasizes the critical period at 10,000 to 13,000 feet.