The fuel/air ratio of the combustible mixture delivered to the engine is controlled by the mixture control. This control, usually located adjacent to the throttle, is most often identified by a red knob (an indication to use it with caution). Like the throttle, it has no spring return and will remain in any position selected by the pilot. It also has a friction lock to prevent the control from "creeping" away from the desired setting.
The ratio of fuel to air is the most critical single factor affecting the power output of an engine. If the fuel/air mixture is too lean (too little fuel for the amount of air - in terms of weight), rough engine operation, sudden "cutting out," or an appreciable loss of power may occur. Lean mixtures must be avoided when an engine is operating near maximum output (such as takeoffs, climbs, and go arounds). At power settings in excess of 75% of the rated power, an excessively lean mixture will cause detonation, serious overheating, loss of power, and damage to the engine.
If the fuel/air mixture is too rich (too much fuel for the amount of air - in terms of weight), rough engine operation and an appreciable loss of power may also occur.
Carburetors and fuel control units normally are calibrated for sea level operation, which means that the correct mixture of fuel and air will be obtained at sea level with the mixture control in the "full rich" position. As altitude increases, the air density decreases; that is, a cubic foot of air will not weigh as much as it would at a lower altitude. Consequently, as the flight altitude increases, the weight of air entering the cylinders will decrease, although the volume will remain the same. The amount of fuel entering the cylinders is more dependent on the volume of air than on the weight of air. Therefore, as the flight altitude increases, the amount of fuel will remain approximately the same for any given throttle setting if the position of the mixture control remains unchanged. Since the same amount (weight) of fuel but a lesser amount (weight) of air is entering the cylinders, the fuel/air mixture becomes richer as altitude increases.
Moving the mixture control full forward to the RICH position provides the richest fuel mixture; this setting is used for all operation on the ground and at high power settings. Movement of the control aft toward the LEAN position progressively leans the mixture. The amount of leaning required varies with altitude. Moving the control full aft to IDLE CUTOFF shuts off all fuel flow at the carburetor or the fuel control unit.
The manufacturer's recommendations for leaning the fuel mixture in the particular airplane should always be followed.