What occurs if the airplane battery voltage drops below 22 VDC during APU start?

When the airplane battery voltage drops below 22 VDC during an Auxiliary Power Unit (APU) start, an alert is triggered through the Aircraft Condition Alerting and Warning System (ACAWS), and a message indicating low oxygen quantity (LOX QTY) may appear. This mechanism is in place to warn the crew that the battery voltage is insufficient for reliable APU operation, which can impact the electrical system necessary for starting the APU effectively.

The significance of this alert is to prompt immediate attention from the crew to manage the electrical power situation and ensure that all systems remain functional. Staying informed about the battery's status is essential, especially during critical phases like APU starting, where adequate power is needed to support the necessary loads.

This alert serves as a safety measure, allowing the crew to take action before further complications arise, like potential failure to start the APU due to inadequate battery voltage.