What is used to provide air pressure to start the main engine when the APU is inoperative?

The air start option is utilized to provide the necessary air pressure to initiate the main engine when the Auxiliary Power Unit (APU) is not operational. When the APU, which typically supplies air and electrical power for engine starting, is unavailable, the air start method allows for sufficient compressed air to be supplied directly to the engine to begin the ignition process. This method is critical in ensuring that the aircraft can get operational even in the absence of the APU, ensuring continuity and efficiency in ground operations.

In situations where the APU is non-functional, the GPU (Ground Power Unit), while used primarily to provide electrical power rather than air pressure, does not assist in starting the engine directly. Similarly, PCA (Pre-Conditioned Air) supplies air for climate control within the cabin but is not used for engine starts. The tow-bar, which is used to move the aircraft on the ground, does not provide any engineering function with respect to starting the engines. Thus, in the context of initiating engine operation without APUs, an air start method is the correct and effective solution.