Which step is critical in preparing the Lav fire extinguisher before flight?

The critical step in preparing the Lav fire extinguisher before flight is ensuring that the gauge reads in the green zone. This gauge indicates whether the extinguisher is fully charged and ready for use. A reading in the green zone shows that the extinguisher has the appropriate pressure necessary to function effectively in the event of a fire.

If the gauge is not in the green zone, it may suggest that the extinguisher is either undercharged or overcharged, both of which can render it ineffective when needed. Therefore, this step is vital for ensuring the safety of both passengers and crew onboard the aircraft.

While confirming the extinguisher identification number, examining the fire safety manual, and calibrating the extinguisher are important tasks, they do not directly verify the operational readiness of the extinguisher like checking the gauge does. Hence, ensuring the gauge reads in the green zone is the most critical step before flight.