Definition
A fuel-air mixture in a piston aircraft engine that contains a lower proportion of fuel relative to air than the chemically ideal ratio. Leaner mixtures contain less fuel per unit of air, producing higher combustion temperatures and improved fuel economy, but reduced power and increased risk of detonation if leaned too far.
Plain English
A fuel-and-air blend going into the engine that has less fuel and more air than the perfect balance.
Context Anchor
Encountered in piston engine operation, carburetor or fuel injection adjustment, mixture control use, and engine troubleshooting.
Derivation
Lean comes from the Old English hlǣne, meaning thin or lacking fat. Applied to a fuel mixture, it describes a blend that is thin on fuel — not generous with it.
Why Pilots Care
Allows better fuel economy and cooler cylinder-head temperatures when used correctly at altitude.
Grounding Statement
Picture the engine taking in plenty of air, but not receiving enough fuel to burn smoothly with that air.
Intuition Check
Lean does not mean the engine is lighter or more efficient by itself. Here, lean means the fuel portion of the fuel-air mixture is low compared with the air portion.
Example Sentence 1
As the pilot climbed to cruise altitude, they leaned the mixture to maintain smooth engine operation in the thinner air.
Example Sentence 2
At altitude the engine ran smoothly once the mixture was leaned to the recommended setting.