What is the correct starting sequence for the APU?

The correct starting sequence for the Auxiliary Power Unit (APU) is to activate the control switch first, which enables the fuel flow, and then ignites the engine. This sequence is essential for ensuring that the APU starts properly and safely.

Starting with the activation of the control switch initiates the APU's internal processes, allowing fuel to flow to the combustion chamber. This step is crucial because without proper fuel flow, igniting the engine would be ineffective and could potentially cause a failure to start. Following the activation of the control switch, the next step of igniting the engine ensures there is fuel available for combustion, facilitating a successful start.

This methodical approach helps to prevent issues that may arise from attempting to ignite the engine without ensuring that the necessary fuel flow is established, which can lead to incomplete starts or, in a worse case, damage to the APU system. Therefore, the prescribed sequence reflects best practices in APU operation to ensure reliability and safety in airframe operations.