Definition
The act of reducing the amount of fuel supplied to the engine relative to the air entering it, by moving the mixture control toward the lean (out) position. This is done to maintain the correct fuel-to-air ratio as air density decreases with altitude or higher temperatures, since less dense air contains less oxygen and therefore needs less fuel to burn efficiently.
Plain English
Cutting back the amount of fuel going into the engine so it matches the thinner air at higher altitudes. As you climb, the air gets thinner, so the engine needs less fuel to keep burning cleanly.
Context Anchor
Seen when using the red mixture control during taxi, cruise, high-altitude operations, and engine shutdown procedures in piston-engine airplanes.
Derivation
‘Lean’ comes from Old English hlǣne, meaning thin or having little fat. A lean mixture is literally a ‘thin’ fuel mixture — less fuel relative to air. The image carries over directly: leaning the mixture makes the fuel side of the blend thinner.
Why Pilots Care
Correct leaning prevents engine overheating or fouling, improves fuel economy, and maintains smooth operation.
Grounding Statement
As altitude increases, the engine breathes less air, so the same fuel setting can become too much fuel unless the pilot reduces it.
Intuition Check
Leaning the mixture does not mean tilting anything or making the engine “weak.” It means reducing the fuel part of the fuel-and-air mixture.
Example Sentence 1
After leveling off at 8,500 feet, the pilot began leaning the mixture until the engine ran smoothly at peak performance.
Example Sentence 2
On a long cross-country flight the pilot leaned the mixture further once level at cruise altitude to save fuel.