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I'm a new pilot (130 hours) and I'm flying Cessna 172's of the 1970's vintage. In my training we didn't spend much time on leaning the mixture except to taxi with the mixture control 1 inch out and to cruise "rich of peak". These planes don't have any gauges for exhaust gas or cylinder head temperature so the only guide is the tachometer.

Up until now I have been renting planes, and because I'm still green I get nervous about leaning the mixture as I find it unsettling if the engine begins to run a little rough. As a result I often don’t even bother touching the mixture control for my typical 1 hour flights (my home airport is 600 feet ASL near Toronto). Now that I’m close to purchasing a similar aircraft I need to get over this fear, especially with the high cost of fuel that I’ll now be paying for.


1) Can anyone offer advice on leaning?



2) There seems to be a lot of debate over whether to fly “lean of peak” vs. “rich of peak” and I would be interested to hear some of your thoughts on that.


Gary

Hi Gary,

I think a lot of pilots are in your shoes. We don't spend enough time discussing leaning procedures and the details of of the process past "lean over 3,000 ft".
Here are a few tips.

First of all. It the engine runs rough, really rough, and immediate increase of the mixture will remedy it. It's healthy to be cautious, but no need to fear. A little practice and you'll start feeling comfortable.
Consult the POH, but a general rule of thumb is 75% power or less, the mixture should be leaned. This could mean any altitude. Often in training, 3,000 ft is given as a guideline for safety factors, but you can lean according to the POH.
Something to note here, ALL of the performance charts are based on properly leaned mixtures. If you are doing any calculations based on endurance/range etc, they will be inaccurate if you haven't leaned...further reason to learn this practice, fuel exhaustion.

The leaning process, using a tachometer is to slowly decrease the mixture until you observe an increase in rpms-this is where the fuel to air ratio is optimal. After this, the engine power will decrease...and run rough. For engine cooling, it is recommended to run rich of peak as the extra fuel can be used for cooling and you error on the side too much fuel as opposed to too little which can cause engine damage as opposed to fouled spark plugs.

Lean of peak vs rich of peak. You are looking at best economy vs best power and once again the POH will guide you. The debate, to my understanding, originates around temperature and your ability to accurately determine it. The more complex the airplane, the more indicators of engine temp...EGT, CHT, TIT etc. These are all tools to achieve exactly what you want.
For yourself, I would run rich of peak and consult the POH about the details.

Hope that helps,
anais

check out the Pelicans Perch (written by John Deakin) articles on Avweb if you want a comprehensive and exhaustive discussion of mixture settings, power settings, and rich of peak vs. lean of peak.

_________________
Everyone must believe in something, I believe that I will go flying.

FWIW, I wouldn't try running LOP on anything that didn't have a 4-cyl (or 6) EGT/CHT gauge.

Lycoming has some advice in the Key Reprints on their web site. Check out the document titled "Operations".

_________________
Brad
RV-7: RV7Factory.com
Blog: PilotBrad.com

I agree with the proposed literature to read about LOP operations. But the short story is also that you shouldn't really try to run LOP unless you're flying a fuel-injection engine, PLUS the appropriate EGT/CHT gauges. Doing so risks flying in the zone of detonation and extreme heat.

Interestingly, if you study the Lycoming chart in their article, you realize that EGTs actually start to decrease just before you get to peak power, and it's that range from 50-100 degrees rich of peak where the EGT/CHTs are highest. So the typical leaning instructions for a Lycoming engine (carburetted, no gauges) has it running at max temperatures.

But in that engine, they're not worried about temperatures as much as they are the possibility of running slightly LOP, which is where the problems of detonation can lie. Truthfully, in the typical 160-180hp four bangers that most of us fly, detonation isn't really that much of a risk. But it's better to be on the safe side because the pilot can't really detect it in flight. And renters would likely never detect it. It's only us poor owners who would get the bad news during an annual inspection!

why? walter atkinson and the other GAMI guys regularly run their 172s and cherokees LOP. using slight carb heat to even out the fuel flows works to keep the operation smooth. it totally depends on the individual intake system though. the GAMI guys also tried pretty hard to get a flat 4 or 6 engine to detonate and just couldnt do it. it seems to be a hold over of the radial engine days (and those WILL detonate)

_________________
Everyone must believe in something, I believe that I will go flying.

The risk in running LOP in a carburetted engine, as I understand it, is just what you describe. It's difficult to regulate fuel flow to each cylinder such that you can maintain a relatively even temperature range across them. Some set-ups will do it, some won't, but without the appropriate gauges it's tough to tell.

I don't think the GAMI guys are proponents of running LOP in a stock carburetor with no gauges, but I could be wrong on that. It's been discussed at length in the Piper Owners forum so I'm just working on memory from those threads.

As someone with a 160hp 4-banger (Lycoming O-320 D3G) I feel that I get sufficient fuel economy using Lycoming's leaning technique (described earlier in this thread) and I don't ever see engine temps that are concerning. Plus, the extra heat seems to help keep the lower spark plugs from fouling. That's just my experience.

Jeff

The problem I always had with leaning 152s and 172Ns using the tach is that the vibration in the tach needle is often greater than the small shift in RPM that you're looking for. The leaning technique that I ended up using back then to get around this was basically:

1. Slowly lean the mixture until the engine just starts to run rough.
2. Back it off until it's smooth again.
3. Back it off another quarter turn.

The technique I use now is:

1. Fly a Diamond DA20-A1, which has automatic mixture control.

Stefan Vorkoetter
http://www.stefanv.com/aviation

if the fuel flows wont even out, the engine will run rough. but if you can add carb heat to the point where engine operation is smooth, then you know that the fuel flows (and therefore the power) is more or less even between all cylinders.

_________________
Everyone must believe in something, I believe that I will go flying.

We are doing a video-tip series on this subject. The first released last week and the next this monday. Let us know what you think.
a

hello all
I'm new here and also a new pilot (43 hrs) and I would like to learn more on leaning. I primarily rent and fly a 172N with no EGT/CHT. I fly out of field at 4607' and what my instructor taught me was to back mixture out about an inch after startup and then during runup at full power, back mixture out untill engine rpm starts to drop and then, turn back in 1 1/2 to 2 turns. As for cruise he said 1 turn out for every 2000' elevation gain and of course back in as you decend. Does this sound like a good method? I do sometimes rent newer 172's with guages but I love that old Cessna and at $77 an hr. How can I resist?

"I fly out of field at 4607' and what my instructor taught me was to back mixture out about an inch after startup and then during runup at full power, back mixture out untill engine rpm starts to drop and then, turn back in 1 1/2 to 2 turns. As for cruise he said 1 turn out for every 2000' elevation gain and of course back in as you decend."

What you are doing when you lean the mixture is looking for the optimal fuel flow to match the air density. Turning it back 1 1/2 ensures that you are running on the rich side. There is no reason not to use this process immediately after start rather than to wait until after run-up. During cruise, the process is the same. Your instructor may not want you to do this for fear of cutting the engine, however, to be efficient, this is the procedure. Go slow, and if you hear any significant decrease in rpm, then immediately remedy it by increasing the mixture.

Thanks Anais,
I guess its just that new pilot fear of doing anything in flight that might make the engine stop. I'm sure with time and more confidence leaning will become easier.

It absolutely will and I completely understand about not wanting to cut the engine. This is a safe fear

It's great you are asking questions. There is a safety factor here and the method you explained works just fine. To fine tune it even more, have your instructor go over leaning procedures and together at altitude watch the rpm increase and then enrichen. That should ease your fear.

Thanks for your post and we'll look to hear more.
a