Definition
An airplane that begins to move under its own engine power without a pilot at the controls, typically as a result of unsafe hand propping — for example, the engine starting at a high power setting with no one in the cockpit to manage the throttle, brakes, or controls.
Plain English
An airplane whose engine has started up and is now rolling away on its own, with nobody in the cockpit to stop it.
Context Anchor
Seen in hand-propping procedures, especially when discussing why the airplane must be secured before the engine is started by turning the propeller by hand.
Derivation
The phrase borrows from the older sense of a 'runaway' horse or carriage — something that has gotten loose and is moving on its own with no one in control. Applied to an airplane, it describes the same situation: a machine moving under its own power that nobody is steering.
Why Pilots Care
A runaway airplane can injure people, damage other aircraft, or collide with obstacles before it can be stopped.
Grounding Statement
Picture the engine starting while the airplane is not tied down and no qualified person is at the controls; the airplane can begin rolling immediately.
Intuition Check
Do not read “runaway” as meaning the airplane is flying away or acting on purpose. Here it means the airplane is rolling on the ground without safe human control.
Example Sentence 1
Before hand propping the trainer, the instructor set the parking brake, placed chocks in front of the wheels, and put a qualified person at the controls to prevent a runaway airplane.
Example Sentence 2
Setting the brakes and placing chocks prevents the airplane from becoming a runaway airplane during a hand start.