Definition 1 of 2
Definition
The minimum throttle setting used in flight, where the engine is producing the lowest power it is rigged to deliver while the airplane is airborne. It is set higher than ground idle so the engine continues to respond promptly when power is reapplied and supports systems such as the alternator, vacuum pump, and hydraulics during low-power flight phases like descents and approaches.
Plain English
The lowest throttle position used while flying. The engine is still running and ready to respond, just producing very little power.
Context Anchor
Used during power-off accuracy approaches, descents, and landing practice when the pilot reduces power but keeps the engine running.
Derivation
"Idle" comes from the older sense of being inactive or doing little work. "Flight idle" specifies the idle setting used in flight, distinguishing it from ground idle, which is set even lower for taxi and shutdown.
Why Pilots Care
Using flight idle prevents the engine from cooling too rapidly during a power-off descent while still allowing a steep enough approach for accuracy landings.
Analogy
It is like a car engine idling while the car is stopped: the engine is still running, but it is not being asked to do much work.
Intuition Check
Do not read idle as engine off. Flight idle means the engine is still running, just at its lowest normal in-flight power setting.
Example Sentence 1
On final approach for a power-off accuracy landing, the pilot reduced the throttle to flight idle and used pitch alone to control the glide path.
Example Sentence 2
Maintaining flight idle during the descent kept the engine warm enough for a possible go-around.