MIXTURE CONTROL
The Mixture control changes the fuel/air ratio. During take off and at flight below 3000 ft it is normally left pushed in (the fully rich position). Air density decreases with height therefore as the aircraft climbs it may be necessary to lean off the mixture by easing the mixture control out slightly. This decreases the amount of fuel entering the engine and restores the correct fuel to air ratio, (given the reduced volume of air available). Leaning the mixture off in this way ensures that the engine continues to run smoothly and efficiently. After the aircraft has landed and parked it is normally the mixture control that is use to stop the engine. This is done, after certain checks are completed, by pulling the control fully out and thereby making the mixture so lean that the engine stops. Stopping the engine in this way helps ensure that the engine spark plugs do not get fouled up.