Being able to properly lean the mixture for every single phase of flight is important not only performance-wise but also for the health of your engine. Here is a quick guide to easily lean the mixture while in flight.
We will not enter in deep theoretical concept about how the engine is supposed to work. There are plenty of resources available online in case you want to dig more into that subject.
To keep it simple, the engine needs the right amount of air and fuel to produce the ideal combustion: 15 parts of oxygen for 1 part of fuel is the perfect mixture and is called stoichiometric ratio.
When the plane climbs, the amount of oxygen in the air decreases and therefore the initial air to fuel mixture will change accordingly (i.e. the mixture gets richer meaning there is more fuel than ideally required). The consequences are:
There is a higher amount of fuel than needed for a proper combustion but the excess of fuel will help cooling the engine which is a nice thing.
However, the excess of fuel which is not burned will most likely cause spark plug fooling with consequently a less efficient spark emitted from the spark plugs.
Higher fuel consumption.
The engine yield is not at its maximum.
It is important to lean the engine on ground as well to prevent spark plugs fooling (the engine is running at very low RPM therefore we don't need to keep the mixture full rich (especially if the outside air temperature is high).
When the plane is climbing and reaches the cruise altitude (typicall above 3,000 ft MSL), leaning is required.
Determine the Peak EGT
The ideal 15:1 ratio is reached by reading the EGT gauge (Exhaust Gas Temperature). To reach it, lean the mixture (by pulling the red mixture lever) until the EGT reaches the maximum temperature called "Peak EGT".
When the Peak EGT is known, follow the rules below to properly lean the mixture. You don't want to be in the so called "Red box". The higher the RPM and therefore the closest you are from the Maximum Continuous Power MCP, the bigger the Red Box is. This is explained by the fact that when the engine produces high power, it should not overheat and it needs a sufficient amount of fuel to remain cool for safe operation.
ROP stands for Rich of Peak, which is a richer mixture as compared to the Peak EGT.
LOP stands for Lean of Peak, which is a leaner mixture as compared to the Peak EGT.
Let's consider the following example:
You're flying at 8,500 ft for instance with the engine sets at RPM corresponding to 78% of its MCP.
Determine your Peak EGT (the highest EGT you will reach by playing with the mixture lever).
Then to properly lean your engine: enrich to remain at least 180°F below the Peak EGT or lean to remain at least 40°F below Peak EGT.
You can print that cheat sheet and stick it on your home cockpit panel or keep it close to you when flying.
Happy New Year 2019!
TIPAH
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