Leaning the engine during flight can be intimidating for those pursuing their private certificate. During most training flights, our altitude is low enough that leaning is not recommended. However, we do lean during cross-countries, but often there is so much going on that we never quite understand why we are leaning the mixture. I will give you a quick overview of leaning, but do not consider this an all-inclusive guide to leaning.
For an engine to run properly, it must have the proper fuel-to-air ratio (about 1:14 in cruise). Air density drops as the altitude increases, and the amount of air going into the engine is reduced. The only way to keep the proper ratio is to reduce the amount of fuel going to the engine. We decrease it by leaning the mixture. During cruise, leaning is normally required when above 3,000 feet MSL. Leaning during a climb is also important at higher altitudes. The DA20 Flight Information Manual says expect “to require leaning at full throttle above 5000 ft pressure altitude.”
So, how much do we lean the engine? The most common method is using the exhaust gas temperature (EGT) gauge, which is the direct indicator of the fuel-to-air ratio in the engine. When you decide to start leaning the engine, pull the mixture back slowly. As the EGT temperature climbs, it will eventually slow down. Finally, the temperature will peak (reach the hottest temperature) and start dropping again. Watch for the temperature to start dropping. After finding that peak temperature, begin to richen the mixture to the recommendation in your airplane. In cruise, the usual recommendation is to lean the mixture 25-75 degrees rich of peak EGT.
When an aircraft is carbureted, or if it does not have an EGT gauge, people use a variety of methods to achieve the ideal mixture. The goal is still getting you in the ballpark of 75 degrees rich of peak EGT. On most aircraft, I pull the mixture back slowly, until the RPM starts to drop, then go rich 1/3 of the total range that achieved the peak RPM. Some of our instructors at Sweet Aviation go forward 1/2 an inch from peak RPM. Really, it depends on the aircraft, the pilot, and the manufacturer. Everyone has their own method and opinion.
When we are rich on the mixture, extra fuel is being used to cool the engine. The extra fuel is also being used to lubricate the engine a bit (since the fuel contains lead). For these reasons, whatever method you use, I recommend erring on the rich end of peak EGT. Since there are so many different opinions on leaning, don’t just listen to mine. Talk with your fellow pilots. Read your aircraft manufacturer’s recommendation. Develop an educated method for leaning and have fun flying!