The meaning of leaning
Leaning your aircraft engine is quit possibly the number one most misunderstood aviation topic. Don’t believe me? Ask two Certificated Flight Instructors (CFI) how to lean and you’re certain to open up a world of confusing replies. Here’s a recent example. Mike Busch, a renowned aircraft mechanic points out, “…no subject I know seems to trigger more discussion and debate among pilots, nor to provide more misinformation and bad advice.” No doubt this question opens the door to wildly different universes of techniques, why is that? There is a gross misunderstanding and lack of standardized teaching regarding why and how to lean. Ask any CFI how a wing stalls and you’ll get a similar answer, ask them how to lean and you get different answers. What fun!
Wondering what we’re talking about? Leaning an aircraft engine is the practice of refining the air / fuel mixture to compensate for the loss of air density as you ascend (or denser air when you descend). As you climb the air gets thinner so a pilot must adjust the fuel / air mixture to compensate. We adjust the mixture to be leaner (less fuel) in less dense air and richen the mixture (add fuel) in more dense air.
You probably came here hoping I had a magical answer for you. I don’t. Here’s why: to really “fix” the issue it requires more than rote understanding, it necessitates actual study and working knowledge. Maybe this is why many CFI’s don’t teach it. Maybe because a person’s ineptitude is passed down over and over. Maybe it’s because of conflated misunderstandings in a modern world of engineering where mixture in everything else is computer controlled. I don’t know.
What to do? First, find an instructor (CFI) who has a real opinion, this will indicate to you that she has studied beyond what is required and possesses a real knowledge of the importance of proper leaning. Second, I personally think the best way to overcome this learning / teaching gap is if you, the student, educates yourself. Armed with real information you’ll actually understand what you're doing when you pull that red knob out to a stumble during taxi and be able to defend your action with science.
Here are the best resources I know of to begin your study:
Start here: Leaning Basics Video from Savvy Aviation
This short and informative book from Mike Busch is invaluable: Manifesto, a revolutionary approach to airplane maintenance.
A primer to the book mentioned above is this article on AVWeb: The Savvy Aviator #59: EGT, CHT and Leaning
Another must have book is Pilot Workshop’s Airplane Engines, A Pilot-Friendly Manual
AOPA’s Ask the A&P’s podcast is always a fun listen and a great place to learn, in fact as of this writing (12/20) the most recent episode was I Went to the Church of Lean of Peak.