What is involved in the starting sequence of the C-5 APU?

The starting sequence of the C-5 APU involves turning the APU turbine with a starter motor. This is an essential process since the starter motor is specifically designed to provide the initial rotational energy required to start the APU.

When the APU is initiated, the starter motor engages to spin the turbine until it reaches a sufficient speed for the main combustor to ignite, allowing the APU to generate power for systems on board the aircraft. This method of starting is reliable and is part of the basic operation of APUs in many aircraft, ensuring a seamless transition from powered-off to operational status.

In contrast, engaging a hydraulic system is not a primary function in starting the APU, as hydraulic systems operate independently of the APU startup process. Using external battery power may be a consideration for supporting systems but is not a direct part of the APU's self-contained starting mechanism. Similarly, jump-starting from another aircraft is not a common or standard procedure for starting an APU. The starter motor serves as the definitive starting component in this process, making it the correct answer.