Final answer:
An aircraft may be flown at an altitude above 10,000 feet ASL but not more than 13,000 feet ASL without oxygen available for up to 30 minutes. This limit is set to prevent hypoxia, where prolonged exposure without supplemental oxygen can result in altitude sickness and cognitive impairment.
Step-by-step explanation:
An aircraft may be flown at an altitude above 10,000 feet ASL but not more than 13,000 feet ASL without oxygen being available for up to 30 minutes. This is because the partial pressure of oxygen at these altitudes may not be sufficient for all individuals to maintain adequate blood oxygenation. Prolonged exposure without supplemental oxygen can lead to hypoxemia, which is a deficiency of oxygen in the blood, potentially causing altitude sickness, hypoxia, and impaired cognitive function.
Above this threshold and up to 13,000 feet ASL, the use of supplemental oxygen is recommended, especially for flights exceeding 30 minutes, to ensure the safety and health of passengers and crew. The regulation is designed to prevent the potential for hypoxia, taking into account the varying responses of individuals to altitude based on their health status and acclimatization.