During engine start, which part of the hydraulic system is normally on?

The auxiliary pump is typically the part of the hydraulic system that is normally on during an engine start. The function of the auxiliary pump is to provide hydraulic pressure to the system when the primary engine pump is not operating, such as during the initial phases of engine start-up. This is crucial for ensuring that essential hydraulic systems, which include flight controls and landing gear operations, are functional from the moment the engines begin their start sequence.

When the engines are operating, the primary engine pump takes over once the hydraulic pressure reaches a sufficient level. However, at the start of the engine operation, the auxiliary pump ensures that the hydraulic system remains available, thus supporting any necessary operations that might be required during this critical phase.