Definition
A condition in which the pilot has reduced the proportion of fuel relative to air being delivered to the engine, by moving the red mixture control toward the IDLE CUTOFF position. After landing and clearing the runway, leaning the mixture during taxi reduces fuel flow to a level appropriate for low-power ground operation, helping prevent spark plug fouling and conserving fuel.
Plain English
The pilot has pulled back the red mixture knob so the engine is getting less fuel for the amount of air it's drawing in. On the ground after landing, this is done to keep the spark plugs clean and avoid wasting fuel while taxiing.
Context Anchor
Seen in after-landing and taxi checklist items after the airplane is clear of the runway and stopped.
Derivation
‘Mixture’ refers to the fuel-air mixture burned in the cylinders. ‘Leaned’ comes from the everyday sense of ‘lean’ meaning ‘thin’ or ‘not rich’ — here, less fuel relative to air. A ‘rich’ mixture has more fuel; a ‘lean’ mixture has less.
Why Pilots Care
Correct leaning improves fuel economy, prevents spark plug fouling, and maintains smooth engine operation; failure to lean can cause roughness or excessive fuel consumption.
Grounding Statement
Picture the mixture control moved back from full rich while the airplane taxis at low engine power.
Intuition Check
Leaned does not mean the airplane is tilted. It means the fuel-air mixture has been adjusted to use less fuel relative to air, and it does not mean the engine is shut off.
Example Sentence 1
After clearing the runway, the pilot completed the after-landing flow: flaps up, transponder to standby, and mixture leaned for taxi.
Example Sentence 2
At the high-elevation airport the pilot leaned the mixture before takeoff to avoid a rough-running engine.