Leaning an aircraft engine in flight seems a bit cruel. You slowly pull the red mixture knob out until the engine coughs and stutters, then you push the knob in again until the stumbling stops. No pilot (or passenger) enjoys a rough-running engine, even if it’s only momentary, so it’s legitimate to wonder whether all of this is really necessary.
The answer, of course, is yes. It is necessary. And leaning the engine in this primitive way is a form of tough love that turns out to be good for it, and you, in the long run.
First, understand what’s happening. As the airplane climbs, the engine takes in thinner, less-dense air at high altitude. Maintaining the proper fuel/air mixture requires less fuel. Slowly pulling the red mixture knob out reduces the amount of fuel that gets mixed with the same quantity of air.
The right fuel/air mixture is essential for optimum speed and range performance. Put another way, failing to lean the engine in flight will result in an excessively rich mixture that increases fuel consumption, reduces range, and may foul spark plugs.
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