What are the oxygen requirements for an IFR flight in an unpressurized aircraft?

The requirement stipulating that required crewmembers must use oxygen at all times reflects a critical safety consideration for IFR operations in unpressurized aircraft. In this context, "required crewmembers" typically refers to pilots and any personnel essential for the safe operation of the aircraft.

This rule is grounded in the understanding that at higher altitudes, particularly above 12,500 feet, the availability of breathable oxygen decreases significantly, which can impair cognitive and operational performance. The regulation aims to ensure that necessary crew members maintain sufficient oxygen levels to effectively manage the aircraft, respond to emergencies, and make sound decisions throughout the flight.

While it is true that passengers are also subject to oxygen requirements, especially above 10,000 feet or during certain segments of the flight, the primary focus of the correct answer is the continued responsibility of the crewmembers. The distinctions among the other choices highlight various aspects of oxygen requirements, but they do not emphasize the ongoing necessity for crew oxygen consumption during all phases of flight operation in an unpressurized environment as adequately as the correct choice does.