Definition
The lowest stable engine RPM at which the engine continues to run smoothly with the throttle fully closed. In a piston aircraft, idle speed is set during engine adjustment so the engine will not quit when the throttle is brought all the way back, while still producing the minimum practical thrust.
Plain English
The slowest the engine will run on its own when you pull the throttle all the way back without it stopping.
Context Anchor
In a steep spiral, the pilot may reduce power to idle speed so the airplane can descend while airspeed and flight path are controlled with the flight controls.
Derivation
“Idle” originally means not doing useful work. In aviation engine use, it does not mean the engine is stopped; it means the engine is running at its lowest normal power setting.
Why Pilots Care
Proper idle speed prevents engine stalling on the ground or in flight and ensures smooth transitions during power changes.
Analogy
It is like a car engine running while the car is stopped at a light: the engine is still on, but it is not being asked to move the vehicle.
Intuition Check
Do not read “idle” as “off.” At idle speed, the engine is still running; it is just producing very little power.
Example Sentence 1
Entering the steep spiral, the pilot reduced the throttle to idle speed and established the target glide airspeed.
Example Sentence 2
In the steep spiral, the pilot reduced power to idle speed to control the descent rate.