What must be done if any engine is left running in an aircraft?

When an engine is left running in an aircraft, it's essential for safety that someone competent occupies the pilot’s seat. This practice is crucial because an engine running could lead to unintentional movements or actions if left unattended. Having a competent person in the pilot’s seat ensures that there is someone able to manage the aircraft's controls, should any unexpected events occur, such as an increase in RPM, the need to execute a quick maneuver, or if there is loss of external safety conditions.

In many cases, the regulations or company procedures may require that while the aircraft is running, a qualified individual remains ready to react if needed to prevent accidents. This is particularly important during maintenance or other ground operations where engines may run for testing or other purposes.

For safety protocols, it is also important that the aircraft be secured properly when not in operation, and passengers typically should be seated with seatbelts fastened, but the presence of a competent pilot in command of the controls during engine run-up or taxi situations takes precedence in maintaining operational safety.