What should be done first when using aviox for supplemental oxygen needs?

When managing supplemental oxygen needs using aviox, the priority is to ensure that there is a sufficient supply of oxygen available for emergencies. Depleting PPOBs (Passenger Portable Oxygen Bottles) to 500 PSI is the correct initial action because this allows for a proper reserve of oxygen to be maintained for passenger use. This ensures that, in the event of an emergency or a situation requiring supplemental oxygen, there is still an adequate amount of oxygen available for immediate use.

This approach is crucial for maintaining passenger safety, as running the PPOBs too low could leave insufficient supply for emergencies. The given PSI level indicates a balance between operational readiness and safety.

In this context, depleting POBs (Passenger Oxygen Bottles) to 300 PSI, depleting aviox to empty, or using POBs immediately do not prioritize maintaining a reserve that is crucial for ensuring that all passengers can access supplemental oxygen when necessary. Hence, managing the oxygen supply in a way that maximizes readiness and safety is the key rationale behind choosing to deplete PPOBs to 500 PSI first.