Which situation requires the use of oxygen for all occupants in an unpressurized aircraft?

The correct answer indicates that oxygen is required for all occupants in an unpressurized aircraft at altitudes of 15,000 feet. This requirement is based on regulations aimed at ensuring safety due to the decreasing levels of oxygen available in the atmosphere at higher altitudes. At 15,000 feet, the partial pressure of oxygen becomes insufficient to maintain adequate oxygen saturation in the blood for prolonged periods, which can lead to hypoxia and impair cognitive and physical functions.

The requirement becomes particularly critical because while pilots may be trained to handle these conditions, passengers might not be accustomed to high altitudes and their bodies may not respond as well to the lower oxygen levels. Therefore, regulations mandate that supplemental oxygen be provided to all occupants to prevent the adverse effects of hypoxia.

In contrast, other altitude thresholds mentioned do not impose the same requirement universally for all occupants. For instance, the requirement for crew members to use oxygen begins at 14,000 feet but does not extend to all passengers until higher altitudes, specifically 15,000 feet. Additionally, below 10,000 feet in controlled airspace does not necessitate oxygen use as the atmospheric conditions are generally safe for all occupants without supplemental oxygen.