General Authority of Civil Aviation (GACA) Aeronautical Information Publication (AIP) Practice Exam

How many batteries are needed to start the APU if bleed air is not available?

1. One battery

2. Two batteries

3. Three batteries

4. No batteries

The correct answer is: Two batteries

The requirement for two batteries to start the Auxiliary Power Unit (APU) when bleed air is not available is based on the additional power needed to engage the APU starter motor effectively. APUs often require a significant amount of electrical power to initiate the start sequence, particularly when the traditional bleed air method cannot be utilized. When bleed air is unavailable, the reliance on batteries increases, as they must provide all the required energy for the ignition and acceleration processes of the APU. This entails not only overcoming the resistance within the starter motor but also ensuring that there is sufficient reserve power for any electrical systems that might need to function during start-up, especially in larger aircraft which may have higher power demands. In contrast, if there were an adequate bleed air source, typically only one battery would be required to provide the necessary electrical support alongside the bleed air, thus making two batteries essential in the absence of that bleed air support. This understanding highlights the operational protocols in aviation regarding energy sources during APU start-up procedures.