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Garmin Lean Assist Questions


Well Known Member
I'm using Garmin Lean Assist. The manual is clear about how to lean but is lacking on re-leaning. By this I mean, if your leaned to the desired value (for me LOP) and for instance, you climb to a higher altitude that requires more leaning, what do you do? Do you restart the lean assist, so it's recalculating or does the previous calculations hold and just keep the +- numbers where you want them.
I suggest an experiment (assuming you have a CS prop):
- Climb to 6000’
- 2500 RPM
- Lean to ~20 degrees LOP
- Do not change anything and climb to 10,000’
- Note the fuel flow once stable at 10,000’ (flow #1)
- Go ROP, then use your tool to lean back to 20 degrees LOP.
- Note the fuel flow when stable (flow #2)
- Is fuel flow #1 about the same as fuel flow #2?

I just richen slightly and re lean, but dont change the lean assist setting. i tend to lean for peak EGT or slight drop (LOP); I dont lean to a set number from peak EGT.Ithink the efficiency is the same as long as you are leaner than peak EGT, then horsepower is proportional to fuel flow.
I've kind of given up on Lean Assist. When I change altitude or cruise power, I just:
  1. Enrich slightly and watch EGT
  2. If it goes down I know I'm ROP. If it goes up I know I'm already LOP.
  3. Find peak EGT and then lean/enrich as needed from there.
The only step Lean Assist seems to save is memorizing peak EGT at whatever altitude/power setting you have. You have to do the same thing with the red knob either way.
For a longer flight I would reset Lean Assist (5" job: LA off, go to rich, LA on, lean, Normalize), but on shorter flights will use FF and ear as better and direct indicators.
For a longer flight I would reset Lean Assist (5" job: LA off, go to rich, LA on, lean, Normalize), but on shorter flights will use FF and ear as better and direct indicators.
Coming back from Vegas this weekend, I was at 10F lean of peak. as I went under some high clouds 2K above me, the EGTs changed and I was at 0F lean of peak. It must have been a change in the atmosphere. Anyways, I just richen slightly and re engage lean assist. QED.
This is very simple with all our Lycoming engines. Carby or injected. If it is a small climb, say 500-1000' do nothing just climb at about 120KIAS.

If it is a climb from 1500 up to 9500, it's full rich, full RPM and WOT. Then a big mixture pull at TOC.

Lean Find Functions are a great concept teaching tool, but not a great operational tool. If you set say 25dF LOP with the lean find you will most certainly end up at 40-50df LOP if you go back and find peak from the lean side using raw data on the previously last to peak cylinder.

You can run LOP happily well above 75% power....no problem.

80% and above 60-80dF LOP
75% about 40dF LOP
70% about 25dF LOP
65% or less 10-15dF LOP

IMPORTANT NOTE: Do not use Lean Find functions as a normal operational tool. Great educational tool, but they all wind you up further LOP than you think.
Work out your richest cylinder and use it as your reference.

Next trap most people fall into. If you are 80% power when ROP, and you lean to an appropriate LOP power, that should be a 10% BMEP drop, like the big old radial days. That means you will end up at 72% power, which serendipitously is going to be around 25-30dF LOP.

If you are at 72% power and ROP, you will end up 65% power when LOP, and that will be around 10-15dF LOP.

The simple answer is read all John Deakins articles carefully, they are sticked somewhere here on VAF.