Under what condition may an engine be left running in an aircraft?

An engine may be left running in an aircraft when a pilot’s seat is occupied by a skilled person, such as a qualified pilot or mechanic capable of operating the aircraft. This condition ensures that the aircraft can be properly controlled should any unexpected situation arise, such as a need for immediate maneuvering or an emergency. Keeping the engine running with a skilled person in the pilot's seat allows for swift response and management of the aircraft, which is crucial for safety.

In contrast, leaving an engine running while the aircraft is parked typically poses safety risks, as there may not be anyone present with the necessary skills to control the aircraft. During refueling, engines are generally required to be shut down to avoid fire hazards and to comply with safety protocols. Likewise, simply having all passengers on board does not justify keeping the engine running, as passenger safety takes precedence and the engines pose possible operational hazards.