How does the APU assist in aircraft engine starting?

The APU plays a crucial role in aircraft operations, particularly in assisting with engine starting. Its primary function in this context is to provide compressed air, which is required to start the engines. When an aircraft's engines are not running, the APU can generate compressed air to drive the starter motor of the main engines. This is essential because the gas turbine engines require a significant amount of airflow to initiate the combustion process during start-up.

The APU serves as a ground-based power source only when the aircraft is not operating in flight, supplying the necessary energy to initiate the engine. This reliance on compressed air from the APU ensures that engine starts can be accomplished efficiently and reliably even on the ground, where external ground support equipment is not available.

While the other options relate to the operations of the APU or other aircraft systems, they do not directly pertain to its function in engine starting. For example, generating hydraulic pressure and powering radio systems are important functions within the aircraft but are not involved in the initiation of engine operations. Additionally, the APU does not fuel the engines; instead, it may require fuel to operate itself, serving as an auxiliary source of power and compressed air.