Which engine should be started first when using a GPU for start?

Starting the left engine first when using a Ground Power Unit (GPU) is typically recommended for operational consistency and safety reasons. This procedure stems from standard operating protocols, which aim to maintain a systematic approach to starting aircraft engines. By starting the left engine first, pilots can ensure that the systems powering the left engine are engaged and operational before proceeding to the right engine.

This practice also helps in managing the load on the GPU and allows for a smoother operation during the startup sequence, reducing potential electrical or mechanical issues. In many aircraft, the left engine is often designated as the "primary" engine due to operational protocols that dictate load management and system checks.

If the engines are started simultaneously or if the right engine is started first, it may create unbalanced loads or might complicate the checks that need to be performed as the engines come online. Hence, the preference for starting the left engine aligns with maintaining efficiency and procedural integrity during the startup process.