Definition
The fuel-to-air ratio delivered by an aircraft engine's carburetor or fuel injection system when the throttle is at the idle position. Idle mixture is set during ground adjustment so the engine runs smoothly at low RPM without being too rich (excess fuel) or too lean (excess air).
Plain English
How much fuel the engine gets compared to air when the throttle is pulled all the way back to idle. It has to be just right or the engine won't run smoothly at low power.
Context Anchor
Pilots may encounter this term during engine ground checks, maintenance discussions, or troubleshooting when an engine runs roughly at low power.
Derivation
Idle originally meant inactive or not doing work. In engine use, it does not mean stopped; it means the engine is running but producing very little power. Mixture comes from a word meaning to mix, which fits the fuel and air being combined before burning in the engine.
Why Pilots Care
Correct idle mixture keeps the engine from dying during taxi, after landing rollout, or when power is reduced, avoiding a rough or stalled engine on the ground.
Intuition Check
Idle does not mean the engine is off; it means the engine is running at very low power. Mixture here means the balance of fuel and air going into the engine, not just any general blend.
Example Sentence 1
During the run-up, the pilot briefly leaned the mixture at idle and saw a 25 RPM rise, confirming the idle mixture was set correctly.
Example Sentence 2
With the correct idle mixture set, the engine continued running smoothly when the pilot closed the throttle after landing.