What is the correct action to take after engine start when using external power?

After starting the engine while using external power, the correct action is to call "hands clear." This is important because it ensures the safety of the ground personnel, particularly those who may be in close proximity to the aircraft. The call alerts everyone that the engine is about to be engaged, indicating that the propellers may start to turn or that there is a change in engine output, thus requiring everyone to maintain a safe distance to prevent accidents or injuries.

While signaling the crew chief and notifying air traffic control are also important tasks in the overall pre-flight checklist, they are not the immediate priority right after engine start. Checking all systems is a critical step but typically follows once personnel are clear and it is safe to analyze aircraft function. The primary focus right after starting the engine is ensuring safety for both the crew and any ground personnel.