Definition
The maximum distance an aircraft can fly on a given quantity of fuel when the engine is operated with the fuel/air mixture leaned to the most economical setting recommended by the manufacturer. Lean range represents the upper limit of range performance, achieved by reducing the proportion of fuel relative to air during cruise so that less fuel is burned per mile flown.
Plain English
How far the airplane can go on its fuel when the pilot adjusts the engine to use less fuel for the same air, squeezing the most miles out of every gallon.
Context Anchor
Seen in aircraft performance and cruise planning, especially when comparing how far the airplane can fly at different power settings and mixture settings.
Derivation
The word 'lean' here comes from the everyday sense of something containing less of a richer substance — a 'lean' cut of meat has less fat. In engines, a lean mixture has less fuel relative to the air. 'Range' simply means the distance the aircraft can travel. Together, lean range is the distance achievable when running the engine on the leaner, more efficient mixture setting.
Why Pilots Care
It directly affects how far a flight can be planned without refueling and is used when calculating fuel reserves for cross-country trips.
Grounding Statement
For the same fuel on board, proper leaning can let each gallon carry the airplane farther.
Intuition Check
Lean does not mean the airplane is banking or that it is lightweight here. It means the engine is receiving less fuel in the fuel-air mixture. Range means flight distance, not a set of values.
Example Sentence 1
Planning the cross-country, she used the lean range figure from the POH to confirm the airplane could reach the destination with adequate reserves.
Example Sentence 2
At 65 percent power and best lean range, the aircraft covered 450 nautical miles on 40 gallons.