The fuel mixture is the ratio of oxygen to fuel delivered to the engine. The stoichiometric setting, where the chemical reaction is perfect, is 1 part fuel to 14,7 parts oxygen. The mixture must not be allowed to reach very lean values, as this significantly reduces the power output and can cause damage to the cylinder bores.

In piston engines, the spark required for combustion is generated at the spark plugs, so if we continuously fly with the mixture at its most advanced position, there is a risk that the spark plugs will be flooded and their effectiveness reduced.

During take-off and landing, the mixture should be in a rich position, the lever should be advanced so that less air enters. In this way, we ensure that the maximum amount of fuel reaches the engine.

As we climb the air density decreases. If we leave the same fuel mixture setting, the amount of oxygen will become lower so the mixture will become richer. Once we have reached our cruising altitude, we must pull back the lever to adjust the amount of fuel reaching the engine. To adjust the mixture correctly, we have to consider several engine parameters: the RPM, the CHT and the EGT. CHT being the cylinder head temperature and EGT being the exhaust gas temperature.

When we are at our final altitude and start to trim the mixture, the RPM will be increased, as the engine will start to run more efficiently. CHT and EGT will also increase, as the fuel also serves as a coolant in the engine.

These values will continue to rise until the peak of the mixture is reached, the point at which the engine is running as efficiently as possible. If we continue to trim the mixture, the parameters we have already seen will start to drop.

With increasing altitude and unchanged mixture setting, the air/fuel mixture becomes richer.