What does the APU provide to ensure effective engine starting?

The APU provides compressed air to ensure effective engine starting. This compressed air is crucial, particularly during the engine start sequence, as it generates the necessary pressure to crank the engines and initiate combustion. The APU operates as a small jet engine and is designed to produce this high-pressure air, which can flow through the engine's start system to assist in turning the engine's turbines.

This capability allows the aircraft to start its main engines without reliance on ground support equipment, making operations more efficient, especially in remote areas or during missions where quick turnaround times are essential. The use of compressed air from the APU is a standard practice in large aircraft like the C-5, facilitating a reliable means to initiate engine operation in various conditions.