Here’s the quick mixture setting for takeoff that works almost all the time at our altitudes: lean about 1/4” to 3/8” lean of full rich. Period. No leaning to RPM drop, restoring, a few turns left, right, then cycling tanks, turning pitot heat on, checking software versions in the GPS, etc. I’m being silly but it’s really quite simple and we overthink it way too much. To understand why, and to help your student understand why, please read below. Also, please remember, you still need to lean for cruise - which is what most folks want to do for takeoff.
I agree with recent posts about folks having a lot of different methods for leaning for takeoff. It is being, and has been, overthought way too much. As for a single method or setting with respect to how it should be done, that’s hard to pin down because what we are leaning for during takeoff is MAXIMUM POWER. Max power settings change with density altitude so there isn’t a single setting to use for that. There is, however, a good average we can use that works well and keeps equipment happy. Keep reading but before we get to that remember that when you are in cruise, we lean for MAXIMUM EFFICIENCY. It’s a different process. Leaning for max efficiency is what people are doing most often during runup BUT leaning for max efficiency, especially during hot conditions, will lead cylinder head temps soaring and is really hard on the engine during climb at full throttle.
If we take what the POH for the 172 says regarding leaning for max power it reads, “Prior to takeoff from fields above 3000 feet elevation, the mixture should be leaned to give maximum RPM in a full throttle, static runup.” If you were to use this technique, you apply full mixture then full throttle then lean the mixture until the maximum RPM is achieved. Done. That’s max power. If RPM didn’t increase, you were at max power at full rich! You’ll notice that it is about 1/4”-3/8” lean in most conditions for most of our aircraft at our altitudes. Look at the expiration date on your AOA badge. Those numbers are between 1/4” and 3/8” tall for reference. This technique does not require leaning until RPM drop then richening a turn or two or three or whatever. Doing it that way sends engine temps soaring.
All that said, I do not want people doing full throttle runups all the time on every flight. It will chew the props to bits a lot quicker. Demonstrating this technique is fine but knowing that the sweet spot is about 1/4”-3/8” lean (from full rich mixture and at our altitudes) is also fine. Students should know where that setting came from. You can demo this really well on the simulator and run the thing at full throttle on the ground all you want.
Climb out at that richer setting to most altitudes where we work! If you’re going much above 8000, or so, you might want to lean the mixture just a little bit, like a quarter to half turn or so. If you hear the RPM increase, great, if not, richen it back to where it was. Remember, a lot of our targets for settings are a little dynamic due to the dynamic nature of the atmosphere. On average we’ll see about 10-12 GPH during climb if mixture is set properly.
So, again, short story: 1/4” to 3/8” lean of rich is typically an okay setting for max power for take off and climb at our altitudes. Longer story: during mag check leaning to RPM drop, richening, then arbitrary turns rich does not generate max power and instead encourages an overheated engine. Don’t do a full throttle runup on evert flight but understand where the rule of thumb comes from. See me for any question or to practice this. Thank you!
