When to lean mixture

Chief Captain

Chief Captain

Well-Known Member
I know the books (eg Cessna) usually say to lean above 3000ft, but is this density or true altitude? I ask because in my part of the world, sea level airports are at a density altitude of over 1500ft quite often.

Would I be wrong to lean the mixture before the altimeter indicated 3000ft on hot days?
 
Lean after engine start until the engine just about dies. It won't hurt anything as the engine is (should) be operating at a very low power setting. Do this at any altitude.
Lean above the DA altitude for takeoff. So if it says 3000' use a DA of 3000'.
Use the procedure recommended in the POH or the engine manual. I've seen some pretty strange ways of leaning prior to takeoff that are not in approved manuals; some of them result in extremely high CHTs.
John Deakin has some good articles on leaning.
http://www.avweb.com/news/pelican/
 
Blackhawk said:
Lean after engine start until the engine just about dies. It won't hurt anything as the engine is (should) be operating at a very low power setting.
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Carrying this habit into fuel injected engines is a great way to crack an engine block or fry some cylinders. Just ask the SR20 owner that's based on the field here with us.

If you're having to lean the engine to keep carbon deposits off the plugs, it's time to have your mechanic look at the idle mixture setting.
 
TFaudree_ERAU said:
Carrying this habit into fuel injected engines is a great way to crack an engine block or fry some cylinders. Just ask the SR20 owner that's based on the field here with us.

If you're having to lean the engine to keep carbon deposits off the plugs, it's time to have your mechanic look at the idle mixture setting.
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I disagree with you. I do it in our Cirrus as well as any other plane I fly and teach the same.

Of course I live at an altitude of 6000 feet, but just because a spark plug builds carbon deposits on a hot day or a high elevation airport does not mean the a mechanic needs to readjust the mixture settings.
 
TFaudree_ERAU said:
Carrying this habit into fuel injected engines is a great way to crack an engine block or fry some cylinders. Just ask the SR20 owner that's based on the field here with us.

If you're having to lean the engine to keep carbon deposits off the plugs, it's time to have your mechanic look at the idle mixture setting.
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Fuel injected, carburated, does not matter. If they have cracked cylinders it is not because they leaned during taxi. Your power levels are too low to cause damage. The cracked cylinders were caused by something else, such as not resetting the mixture prior to runup/takeoff. I've been doing this for years on both carburated and fuel injected engines and the technique is recommended by engine experts and is even recommended by engine manufacturers.
 
Fly_Unity said:
I disagree with you. I do it in our Cirrus as well as any other plane I fly and teach the same.

Of course I live at an altitude of 6000 feet, but just because a spark plug builds carbon deposits on a hot day or a high elevation airport does not mean the a mechanic needs to readjust the mixture settings.
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I suspect running the mixture too lean during climb and letting CHTs get above 400. This is too hot and will cause premature cylinder replacement.
 
Chief Captain said:
I know the books (eg Cessna) usually say to lean above 3000ft, but is this density or true altitude? I ask because in my part of the world, sea level airports are at a density altitude of over 1500ft quite often.

Would I be wrong to lean the mixture before the altimeter indicated 3000ft on hot days?
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Density altitude.

Remember that the "density" in density altitude refers to air density. Whether it's the efficiency of the prop, the lift on the wings, the airflow into the pitot tube or the fuel/air mixture, density altitude is really all your airplane knows.
 
I've been under the impression that it's a good thing to lean prior to take off under humid conditions as well, despite humidity having a lower impact on density altitude...does that sound about right? Something about the water vapor having an overall cooling effect on the cylinders?
 
Blackhawk said:
Fuel injected, carburated, does not matter. If they have cracked cylinders it is not because they leaned during taxi. Your power levels are too low to cause damage. The cracked cylinders were caused by something else, such as not resetting the mixture prior to runup/takeoff. I've been doing this for years on both carburated and fuel injected engines and the technique is recommended by engine experts and is even recommended by engine manufacturers.
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This. We lean very aggressively after start with our fleet. We actually had problems when we were not doing this. Ever since the change in procedure, the engines have had far less problems.
 
On the ground, lean lean lean lean lean, in flight, unless I'm equipped with a bunch of equipment (which most airplanes are not) I lean to the manufacturers suggested mixture setting plus a little (for cooling) or to what the company tells me to.

One Company told me, "don't lean the 207 at all unless you go above 3000' and since you're never getting above 1000' don't bother," another told me, "lean to the top of the green arc." Both of the guys flying at these places had multiple 1000s of hours, in fact, "Old Man Bill," had over 17,000hrs in 207s, and Mike had over 11,000hrs in 207s. Used both techniques, didn't really notice a difference. I've talked to a fair number of mechanics who've told me, "stay low, and don't lean" in regards to IO520s. They've said it'll keep the motor running longer. In IO540s, never had any real advice one way or they other. The IO540 can take a hell of a beating from everything I heard, I always tried to be gentle with it.
 
You should always lean for taxi , even at sea level. About 1 inch should work, just remember to set it full rich at sea level before takeoff. My home field is 6348 MSL, so we lean for best power for takeoff and landing. I use Cessna's reccomendation of 50F rich of peak EGT for cruise (that can be used even below 3000, generally it is close to best power, ehich is full rich at sea level), some of my partners lean to best economy (Peak EGT).
 
TFaudree_ERAU said:
Carrying this habit into fuel injected engines is a great way to crack an engine block or fry some cylinders. Just ask the SR20 owner that's based on the field here with us.

If you're having to lean the engine to keep carbon deposits off the plugs, it's time to have your mechanic look at the idle mixture setting.
Click to expand...

I do it daily on my TIO-540's, so does everyone else at my company and we have no problems.

Set 1000 RPM after start, then lean for maximum RPM (best power mixture if you will), then bring the power back down to 1000 for idle. "Slightly" enrichen the mix for taxi and runup. This works to protect against spark plug fowling. After nearly 600 hours flying here, I have yet to find a fowled plug during a runup!

To the original question, leaning is done after takeoff at 1000 AGL, set about 30gph, and watch the CHT's and enrichen as necessary. Cruise leaning is done after leveling off and is set at 75 degrees rich of peak. (of course this is for a large bore turbo charged Lyc, maybe not what youre flying, but hey... didnt specify what engine were talkin about!

I trust that my company knows whats best for these engines, and so does Lycoming and the FAA by giving us increased TBO (not sure what it is off the top of my head, but I want to say something in the neighborhood of 2200, where the standard TBO according to lycoming is 1800 hours the TIO-540)
 
A150K said:
You should always lean for taxi , even at sea level. About 1 inch should work, just remember to set it full rich at sea level before takeoff. My home field is 6348 MSL, so we lean for best power for takeoff and landing. I use Cessna's reccomendation of 50F rich of peak EGT for cruise (that can be used even below 3000, generally it is close to best power, ehich is full rich at sea level), some of my partners lean to best economy (Peak EGT).
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I know Cessna (and some other manufacturers), recommend 50F ROP, but this is one of the worst things you can do for your engine. From John Deakin's Pelican Perch #65:
The Dangerous Red Box


Just where is that "red box" I keep talking about? Some rough numbers, good (that is to say, BAD) for most of these engines -- these are "no fly zones," DO NOT set the mixture between them:

Red Box = No Fly Zone

​
  • At and below about 60% power, there is no red box. Put the mixture wherever you want it.
  • At about 65% power or so, 100ºF ROP to Peak.
  • At about 70%, 125ºF ROP to 25ºF LOP.
  • At about 75%, 180ºF ROP to 40ºF LOP.
  • At about 80%, 200ºF ROP to 60ºF LOP.


All those numbers are approximate! Please don't start splitting hairs, here!
You probably don't want to run your engine between those mixture settings. If you do, you are running very high peak pressures inside the combustion chambers, and that peak pressure is occurring too close to top dead center.
There's a chance you read too fast, and missed this very important point, so let me put it another way:



Outside the Box

​
  • At 65% power, use richer than 100 ROP, or leaner than peak EGT.
  • At 70%, use richer than 125ºF ROP, or leaner than 25ºF LOP.
  • At 75%, use richer than 180ºF ROP, or leaner than 40ºF LOP.
  • At 80%, use richer than 200ºF ROP, or leaner than 60ºF LOP.
 
I've read some of his articles, including one on running oversquared :D. Pretty interesting. I admit that I run it 50 ROP, because I don't really know where else to run it and that is what one of the partners in the 150/150 (O-320 E2D) who is also an A&P IA reccomended to follow and the POH.
 
A150K said:
I've read some of his articles, including one on running oversquared :D. Pretty interesting. I admit that I run it 50 ROP, because I don't really know where else to run it and that is what one of the partners in the 150/150 (O-320 E2D) who is also an A&P IA reccomended to follow and the POH.
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This is one of the examples where richer is probably better and where the POH is probably not best.
Another example occured recently with a Mooney Ovation that a client purchased. The POH recommends that the mixture be set to the bottom of the blue arc on the EGT indicator during takeoff/climb. The problem with this was noted as we climbed- the owner had a JPI engine monitor installed and the CHTs climbed to over 400F. The mixture had to be enrichend to lower the CHTs.
 
I lean right after start, and in anything with an O-235 I lean before start. I have never had an issue leaning on the ground in anything. I even aircraft with 540's and 580's in them but only a gallon or 2.
 
My home airport sits at 8390ft MSL (average density altitude of 11000ft), so if you don't lean, you don't go anywhere. And yes, even 152's take off quite easily !
 
I draw to your attention:
§ 91.9 Civil aircraft flight manual, marking, and placard requirements.

(a) Except as provided in paragraph (d) of this section, no person may operate a civil aircraft without complying with the operating limitations specified in the approved Airplane or Rotorcraft Flight Manual, markings, and placards, or as otherwise prescribed by the certificating authority of the country of registry.

Therefore, no matter what anyone says about leaning procedure works for them, what their chief pilot does, what some Joe who's the best mechanic in the world said in his blog, etc., you're probably going to get 91.9 thrown at you if you fry an engine such that the FAA finds out and you had leaned your engine in a manner other than what the POH suggests.

Aside from this, it is my understanding that the 3,000' mark for leaning is based upon the fact that the engine will not be developing sufficient horsepower above that altitude to cause damage when leaned. Leaning below that altitude at high power settings can cause damage.

One cannot damage a Lycoming or Continental engine with any mixture setting during taxi. This I relay firsthand from Lycoming and Continental reps on a panel discussion I attended.

So, my practice is to always follow the POH for leaning.

-A.S>
 
Pépé le Pilot said:
My home airport sits at 8390ft MSL (average density altitude of 11000ft), so if you don't lean, you don't go anywhere. And yes, even 152's take off quite easily !
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Same here. There is a freelance CFI on the field (6348) who owns a 150G with the O-200 and he seems to make it work fine (I did about half of my PPL training with him), but having a 172 (or better yet a 150/150 like I fly) definately increases your margin for error.
 
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