Definition
A power setting on certain large reciprocating aircraft engines, typically radial engines with pressure carburetors, in which the carburetor automatically supplies a leaner fuel-to-air mixture suitable for cruise operation. In Auto Lean, the mixture is metered for best fuel economy at reduced power settings, as opposed to the richer mixture used at high power settings.
Plain English
A mixture control position on older big piston engines that tells the carburetor to automatically run the engine on a lean (less fuel) mixture for efficient cruising.
Context Anchor
Seen on some older or larger aircraft engine controls, operating manuals, and checklists, often alongside settings such as Auto Rich and Idle Cutoff.
Derivation
"Auto" because the carburetor automatically adjusts the mixture as conditions change; "Lean" because the fuel-to-air ratio is reduced compared to a rich setting. Together: an automatic lean-mixture setting.
Why Pilots Care
Prevents an overly rich mixture at altitude that wastes fuel and fouls spark plugs.
Intuition Check
“Lean” does not mean the airplane is tilting or banking here. It means the engine is receiving less fuel in the fuel-air mixture than it would in a richer setting.
Example Sentence 1
After leveling off at cruise altitude and reducing power, the pilot moved the mixture control from Auto Rich to Auto Lean.
Example Sentence 2
With the control in auto lean, the engine stayed smooth as the aircraft climbed through ten thousand feet.