A control installed in a reciprocating-engine-powered aircraft that allows the pilot to vary the fuel-air mixture ratio while the engine is running.

A reciprocating engine requires the proper amount of fuel to be mixed with the air on the basis of the weight of the air. But most fuel-metering systems measure the fuel on the basis of the volume, not the weight, of the air entering the engine. The density (weight per unit volume) of the air decreases as the aircraft goes up in altitude, and the pilot must be able to decrease the amount of fuel metered into the less dense air. This adjustment is made with the mixture control.