Definition
The fuel-to-air mixture setting that produces the greatest number of miles per gallon of fuel burned, achieved by leaning the mixture until the engine is operating at or slightly lean of peak exhaust gas temperature (EGT). It produces less power than a best-power mixture but uses fuel most efficiently for a given amount of work done.
Plain English
The mixture setting that gets the most flying done per gallon of fuel. The pilot leans the fuel back until the engine is running as efficiently as possible, accepting a small loss of power in exchange for better fuel burn.
Context Anchor
Seen in engine operation, cruise performance, mixture-control procedures, and fuel planning for piston-engine aircraft.
Derivation
“Economy” comes from an older idea of careful management of resources. In this term, the resource being managed is fuel: the goal is not maximum power, but the most useful power for the least fuel burned.
Why Pilots Care
Using best-economy mixture extends range and reduces fuel costs on long flights but requires careful monitoring to avoid engine damage from excessive leaning.
Intuition Check
“Best-economy” does not mean the best mixture for every situation. It means the fuel-saving mixture for conditions where the aircraft or engine procedure says it is appropriate.
Example Sentence 1
Once established in cruise at 7,500 feet, the pilot leaned to best-economy mixture to stretch fuel for the long leg ahead.
Example Sentence 2
The handbook calls for best-economy mixture whenever operating at or below 65 percent power.