Under what condition should Aviox units be used?

The use of Aviox units is specifically recommended when the onboard oxygen supply, including Portable Oxygen Bottles (POBs) and other oxygen sources like the Passenger Portable Oxygen Bottles (PPOBs), has been depleted to 500 psi, while still having more than one hour of flight time remaining. This condition indicates that the standard oxygen supplies are no longer adequate for the duration of the flight, and using Aviox units can provide the necessary supplemental oxygen to ensure passenger safety.

In this scenario, having more than an hour of flight time left necessitates the need for a reliable source of oxygen, as the altitude and cabin pressure conditions could lead to potential hypoxia if proper precautions are not taken. Therefore, maintaining the safety and health of passengers under these circumstances justifies the necessity to use Aviox units.

While other conditions mentioned might seem relevant, they either do not adequately address the specifics of oxygen supply levels or do not account for the need to proactively address potential oxygen deprivation as the flight progresses.