What must be connected when hooking up external power to the aircraft?

When hooking up external power to an aircraft, it is essential to connect the ground cable. The ground cable serves a critical safety function by establishing a clear electrical connection between the aircraft and the ground. This connection helps prevent electrical shocks and ensures that any static electricity is safely discharged before power is supplied to the aircraft systems.

Connecting the ground cable first minimizes the risk of arcing that could occur if the power source is linked directly to the aircraft without grounding. It also protects the aircraft's electronic systems from potential surges or faults present in the external power source. When the ground cable is properly connected, it creates a safe pathway for electrical current, which is crucial for the overall safety of personnel and equipment during the power-up process.

Other cables, such as control, battery, or data cables, do not fulfill the same grounding function and are not prioritized in the procedure of connecting external power. Prioritizing ground connections reflects adherence to safety protocols essential in aviation ground operations.