Ground leaning

AvNavCom said:
only very aggressive leaning (borderline ICO) would make a difference in fuel flow.
Click to expand...

That is the gist of my argument. A lot of pilots ground lean because they heard they should, and because they don't understand the function of the idle circuit or how to check it. They believe they are at full rich because the red knob is in.

The reality is that if your aircraft is set up correctly, you are already very close to peak, close enough that spending the time to lean further is not going to have a significant effect on fouling.

If your aircraft is NOT set up correctly, then yes, you need to ground lean.

So how many members of the church of ground leaning check their idle mixture adjustment? How many CFIs teach their students how and why to do this?
 
Ed Haywood said:
The reality is that if your aircraft is set up correctly, you are already very close to peak, close enough that spending the time to lean further is not going to have a significant effect on fouling.
Click to expand...
I already did the math for you, showing that 50rpm lower than peak on the rich side puts you at a 12:1 mixture (stoichiometric is 14.7:1), if not a bit richer. A warm engine will run fine with a lean 18:1 mixture, which means you're adding 50% more fuel into the engine, 50% more lead that is improperly scavenged because of the low combustion temperatures. And that 50% more lead is on top of the four times more lead contained in 100LL compared with 80/87.

Ed Haywood said:
So how many members of the church of ground leaning check their idle mixture adjustment?
Click to expand...
I did. Checked good. My fuel flow gauge also matched the theoretical calculations above.
 
AV8R_87 said:
No. Peak RPM (thus power) is richer than stoichiometric.
Click to expand...
It will increase the idle RPM. After all, when setting the idle mixture screw, we look for an RPM rise just before ICO, and that's where we set the mixture knob for aggressive ground leaning.
 
donjohnston said:
And what if you are not at sea level during ground ops?
Click to expand...
Where was the airplane when the idle mixture screw was set? It matters. If one sets it for a 25 to 50 RPM rise in Denver, that engine will be far too lean on the ground in Seattle. It might not want to run. If the opposite is true, set in Seattle but on the ground in Denver, it will be far too rich.

The carburetor is an imperfect device. It does not meter the fuel based on the weight (or density) of the air passing through it. It meters it according to velocity. So at full throttle on takeoff in Seattle, there's many more molecules of air going through it than there is on takeoff in Denver, which makes a takeoff in Denver risky if you didn't lean for best power before takeoff. The engine is getting too much fuel and the ratio is screwed up.

Aircraft fuel injection systems are similarly imperfect, which is why they still have mixture controls, and there is an idle mixture adjustment on the fuel servo. Cars have electronic fuel injection that measures a lot of stuff including mass airflow, air temperature, altitude, engine temperature, oxygen levels in the exhaust, and more. It can fine-tune the whole air:fuel ratio, but all that electronic stuff relies on the airplane's alternator and battery and on very clean and tight connections throughout the entire system. Mechanical fuel injection systems keep running when the airplane's electrical system dies. Electronic ones fail without some high-quality redundancies.

Fuel:air ratios are by weight. 8:1 is rich; eight pounds of air mixed with one pound of fuel. 12:1 is best power, typically. 15:1 tends to be stoichiometric, where all oxygen and all fuel is consumed. 18:1 is about as lean as the engine can take and still run. Some oxygen is left over.
 
Dan Thomas said:
Where was the airplane when the idle mixture screw was set? It matters. If one sets it for a 25 to 50 RPM rise in Denver, that engine will be far too lean on the ground in Seattle. It might not want to run. If the opposite is true, set in Seattle but on the ground in Denver, it will be far too rich.
Click to expand...

Which is the point. If the mixture is set based on the airfield in Denver, how will you fly to the east and west coast? So it seems to me that the idle mixture should be set to a value which would provide a slightly rich mixture at sea level.

Which means if you are at a field above sea level then leaning for ground ops would be needed.

I know that if I didn’t lean aggressively on the ground in my 182RG that my plugs would foul.
 
donjohnston said:
Which is the point. If the mixture is set based on the airfield in Denver, how will you fly to the east and west coast? So it seems to me that the idle mixture should be set to a value which would provide a slightly rich mixture at sea level.

Which means if you are at a field above sea level then leaning for ground ops would be needed.
Click to expand...

In theory it is a concern. In practice I haven’t found it to be much of a real problem.
 
donjohnston said:
Which is the point. If the mixture is set based on the airfield in Denver, how will you fly to the east and west coast? So it seems to me that the idle mixture should be set to a value which would provide a slightly rich mixture at sea level.
Click to expand...
That's the way. We set them toward the upper end of the RPM rise range so that at sea level they didn't get too rich.
donjohnston said:
Which means if you are at a field above sea level then leaning for ground ops would be needed.
Click to expand...
Most of the time it's not a big deal, but if you fly from the coast to an unusually high airport you will notice it.
donjohnston said:
I know that if I didn’t lean aggressively on the ground in my 182RG that my plugs would foul.
Click to expand...
We had one of those. Lycoming specified the HA-6 carb for that engine (O-540-J3C5D) and the rich setting was very rich. We had rough-running problems and big mag drops during runup until we started leaning it, and when carb heat was pulled on downwind it could get a b it scary, so we were leaning it then, too. I had trouble convincing the other instructors to lean it for smoothness in the pattern; they had been taught "always rich before landing" and I had to point out that one has to actually think a little sometimes. If it belches and bucks with carb heat, lean it out some and it will settle down.

I called Precision Airmotive, the carb manufacturer, and complained about the extra-rich settings in the HA-6. They told me that Lycoming had specified those settings and that they could not change them, They told me to just lean it as necessary.
mondtster said:
In theory it is a concern. In practice I haven’t found it to be much of a real problem.
Click to expand...
As above; it takes serious altitude changes, or temperature changes, to make a problem, and a pilot should be aware of it. Mechanics who don't take into account the season and local altitude will often set the idle mix to some spot where the pilot really notices it.
 
Or, here's a thought: maybe the pilot adjusts the idle mixture setting. :eek:

The procedure is in the operator's manual, not.the maintenance manual.

With some systems, you don't even need a tool. Just reach in the cowl and turn the wheel with your fingers.
 
Last edited:
Ed Haywood said:
Or, here's a thought: maybe the pilot adjusts it.
Click to expand...
EXACTLY!!!

I flew into an airport (now closed) in the ABQ area on my first X-US flight in a friend's Cherokee Six on my way from NJ to Las Vegas. The pattern was uneventful and I did everything I had been taught and the landing was actually a greaser... Rolled out towards the Taxiway and as the plane slowed as I turned off the Rwy, the prop stopped spinning. There was a whole lot of "what the ----" and then it struck me that I was at the highest airport I had ever landed at. Took a couple of tries to get the mixture lean enough, then the engine started right up.

Along those lines... A friend bought a biplane in the late fall on Long Island, NY where the airport couldn't have gotten any closer to Sea Level. He was flying the plane to DEN where he was gong to leave it for the winter before heading over the Rockies in the spring. As he got closer to DEN he realized he really couldn't lean the engine as much as he wanted to. Plane made it to DEN fine, but the mixture control was one of the top things on his list. He heard later that the mixture cable had be cut/adjusted so that it only went so far. And not nearly as far as he needed to fly in higher altitude he was based in. Down at Sea Level, it was probably an adjustment the prior owner didn't see as an issue and never changed the cable (and probably forgot about it).
 
donjohnston said:
I know that if I didn’t lean aggressively on the ground in my 182RG that my plugs would foul
Click to expand...
I have heard a number of 182 owners say this. For example, flew with a 182 owner years ago who leaned very aggressively. Not just a guesstimate, but back to engine roughness and forward to smoothness. I mentioned that I didn’t even do it that aggressively in Colorado (we were at SMO in California). He said, “I’m the only 182 owner I know who hasn’t had fouled plugs.”
 
Without jumping in to the details, I do the ground leaning thing. I also don’t think it does much at 800-1000 rpm, I do it anyway. I’ve not come close to forgetting, the engine would at least ‘stumble’ anyway.

For most takeoffs, I may be a tad less than ‘full rich’, may not be much.
 
sourdough44 said:
I’ve not come close to forgetting
Click to expand...
Neither have I. I also don’t forget boost pump, carb heat off, flaps, trim, etc. It’s been mentioned a few times, but whatever the mechanical rationale is one way or the other, the equivalent of “I don’t do a pre-takeoff check” might be the worst reason I can think of for not leaning for taxi. If that’s the reason, don’t lean in cruise either and, for heaven's sake, don’t fly a retract!
 
Last edited:
RyanB said:
The mixture should be leaned to the point that any power setting above what’s needed for taxi will cause the engine to stumble.
Click to expand...
I’m going to disagree with this statement. That sounds way over-lean to me.

For taxi setting (1000-1200), you can simply lean slowly until the RPM just starts to climb. That will be lean enough not to create an over-rich condition that builds up junk on the plugs, while not being so lean that you risk detonation - and you can bump the throttle briefly to accelerate when crossing a runway without having it stumble.

It will also still stumble clearly if you forget and try to take off without pushing the mixture in.

JMO, YMMV.
 
I’m just going to say I’ve always leaned (aggressively) for ground op’s. I see no downside. As for it being dangerous because you might try to takeoff with the mixture leaned, what about cowl plugs, control locks, pitot covers, prop controls and the myriad of things that must be done before taking off which insure a safe outcome?
 
I can't vouch for Lycoming (though I suspect it is a also true for them), but I guarantee that it is true for Continentals, you're not going to get anywhere near detonation at idle or even any taxi speeds no matter where you have the mixture control. Continental states that below max recommended cruise, you can run the mixture from full rich to so lean it's misfiring without concern.
 
Hang 4 said:
... When I was using flight school planes, where the red knob was basically never touched, I'd have to clear fouled plugs on every other run-up. My own plane, that I lean in all ground ops, I've never had to clear a fouled plug. Sample of 1, but I suspect others have had the same experience.
Click to expand...
Me too, and I suspect it's a sample of hundreds at least. Leaning during ground ops does indeed make a difference, but you need to lean it almost all the way to the idle cutoff.
 
midlifeflyer said:
He said, “I’m the only 182 owner I know who hasn’t had fouled plugs.”
Click to expand...

Alternative conclusion: most 182's do not have their idle mixture circuit adjusted properly, and most 182 owners do not know how to check.
 
The other sign your idle mixture is too rich is that after you've been flying a lot and you land and you pull the throttle back out to a reasonable idle RPM, the engine wants to stop.
 
Ed Haywood said:
That is the gist of my argument. A lot of pilots ground lean because they heard they should, and because they don't understand the function of the idle circuit or how to check it. They believe they are at full rich because the red knob is in.

The reality is that if your aircraft is set up correctly, you are already very close to peak, close enough that spending the time to lean further is not going to have a significant effect on fouling.

If your aircraft is NOT set up correctly, then yes, you need to ground lean.

So how many members of the church of ground leaning check their idle mixture adjustment? How many CFIs teach their students how and why to do this?
Click to expand...
Ed I agree with you on most risk stuff, but not this one. I don't lean on the ground because someone told me to. I do it because I didn't used to, had fouled plugs on a run-up in a Cherokee, had the CFI say "oh yeah, it'll do that if you don't lean right", and that activated the voice in my head to say "don't make that mistake again, or it could kill you." I say that not as an exaggeration, I fly out of a relatively short field with obstructions. If I have a significant power loss 50' in the air, I have a problem.

So I'd suggest not assuming that the idle setting is OK if it's your airplane, even if you set it yourself last week, and very much not assuming that a rental or club plane is setup perfectly, and leaning the engine for taxi. And even with that said, I don't know that idling above idle stop or taxiing an engine designed to run on 80/87 that's burning 100LL instead is not going to foul w/o leaning. Why would anyone take that chance? There are a handful of things that need to be set properly to safely take off in most airplanes, and trying to reduce that number of things to check by 1 while increasing the risk of a problem on takeoff doesn't make any sense at all. I'm sorry. Flaps? Primer locked? Fuel pump? Carb heat off? Prop? Mixture? It's not a lot of things in a typical single engine piston aircraft.

I don't bring this up to start or continue any argument, but because I believe the risk of not leaning for taxi is significant. Other's views and experiences may differ.

Oh, and one other thought, related to short field operations. I know some don't, but 100% of the time in a PA-28 on short field I'm holding the brakes and applying full throttle before rolling forward. I don't do that because of the distance. I do it because I want to hear and feel what that engine is doing at full throttle before I take off. If it's rough or weird or "stumbley", I'm going to pull off and sort it out. If your brakes don't hold in up to a -235 in that aircraft, to me it means your brakes don't work.
 
Albany Tom said:
So I'd suggest not assuming that the idle setting is OK if it's your airplane
Click to expand...

You don't have to assume. You can check it every time you shut down. Do you? Did that CFI show you how? I'll bet not.

There are really 2 sets of ground leaning advocates. The first, largest, and loudest group are those who have no idea their engine is already doing it for them.

The second group are those who understand what idle adjustment does, but don't trust it for various reasons. I don't agree with that reasoning in most cases, but it is an informed opinion.

I'll concede that hopping from plane to plane in a rental fleet may be a good reason not to trust it.

My comments are directed mainly at the first group. And some new arrivals to the second group who can't handle having their strong beliefs challenged.
 
Last edited:
Ed Haywood said:
You don't have to assume. You can check it every time you shut down. Do you? Did that CFI show you how? I'll bet not.

There are really 2 subsets of ground leaning advocates. The first, largest, and loudest group are those who have no idea their engine is already doing it for them.

The second group are those who understand what the idle adjustment does, but don't trust it for various reasons. I don't agree with that reasoning in most cases, but I think it is a reasonable option.

My comments are directed mainly at the first group.
Click to expand...

You can count me in the second group with the addendum that I’m not certain the idle adjustment, even if set correctly, is adequate for preventing fouling with the higher lead content of 100LL, and that I’m most often not at idle on the ground but typically around 1000-1200 rpm. Given those considerations, I believe the least risky approach is to lean very aggressively, so that the engine will stumble before it’s much above 1200 rpm.

That method removes any concerns about fouling during ground ops and any concerns about forgetting to enrichen for takeoff.
 
Half Fast said:
I’m not certain the idle adjustment, even if set correctly, is adequate for preventing fouling with the higher lead content of 100LL,
Click to expand...
I'm good with that, but will note that Lycoming says it is adequate. Higher taxi RPMs and 100LL are not new things to them.
 
Ed Haywood said:
I'm ok with that, but will note that Lycoming says it is adequate. Higher taxi RPMs and 100LL are not new things to them.
Click to expand...

Can’t look it up right now. Was Lycoming’s guidance written when octane lower than 100 was still available?
 
And while we are on not reading the operator's manual, let's talk about all these fouled plugs on runup. How many of them were actual fouling, versus excessive RPM drop due to prop loading at too rich a mixture? How many pilots know what Lycoming says to do if your RPM drop on mag check is excessive?
 
Half Fast said:
Can’t look it up right now. Was Lycoming’s guidance written when octane lower than 100 was still available?
Click to expand...
Operators Manual is dated 2005 for the 360 series. About half the models covered by it specify 100LL as minimum octane.
 
mandm said:
Oh I didn’t know that, I just put it about half way. I think I read in the POH that you should increase the MP for extended ground ops. Any thoughts on that?
Click to expand...

My CFI always had me lean the mixture “a knuckles width” while taxiing and go full rich on the runup. This was in a carbureted C172N.
 
Ed Haywood said:
And while we are on not reading the operator's manual, let's talk about all these fouled plugs on runup. How many of them were actual fouling, versus excessive RPM drop due to prop loading at too rich a mixture? How many pilots know what Lycoming says to do if your RPM drop on mag check is excessive?
Click to expand...
Can't speak for anyone else's experience, and I've only had it happen the one time. It wasn't just an RPM drop, it ran like hell on one of the mags. The surprise to me at the time was the fix - back to both mags, increase the RPM to about 2100-2200 or so, lean it to really rough, rich until pretty smooth and let it sit for a couple of minutes. That was a new procedure for me, and I wondered if it was harmful. Now I know it's not. Anyway, that cleared it up and it ran fine on both mags after that. Also after that, I always leaned after start. Never had a problem after that.

Now, if I hadn't done a mag check on run-up, would I have caught it? Probably not. Would it have climbed out just fine, with maybe a 50 RPM drop, and I wouldn't have noticed that? Maybe so. If another plug or two acted up and it got worse, would I have notice it on full power before takeoff? Probably. But why add an extra risk?
 
Ed Haywood said:
There are really 2 sets of ground leaning advocates. The first, largest, and loudest group are those who have no idea their engine is already doing it for them.
Click to expand...
Ed, 12:1 (probably closer to 11:1) air:fuel ratio isn't lean. That idle mixture adjustment isn't set anywhere near lean. If it was, you wouldn't get an RPM increase when you pull the mixture knob.
Half Fast said:
I’m not certain the idle adjustment, even if set correctly, is adequate for preventing fouling with the higher lead content of 100LL, and that I’m most often not at idle on the ground but typically around 1000-1200 rpm.
Click to expand...
It's not. The idle mixture is in the rich side of things. And the combustion temperatures are low enough to prevent the lead from being scavenged.
Ed Haywood said:
I'm good with that, but will note that Lycoming says it is adequate.
Click to expand...
Adequate for what? For preventing plug fouling? Or for keeping the engine running at idle, no matter how cold or how hot the engine is, at a rather large range of altitudes? These two can be conflicting goals.
 
Sac Arrow said:
I'm not sure I get the point of idle mixture control in a fuel injected engine.
Click to expand...
The injection servo has a throttle plate and a fuel metering valve. The two are connected via a link between their two levers. As the throttle plate opens, that link opens the fuel valve to match the fuel flow to the air flow. When the throttle lever is against its idle stop, the fuel valve is open just a tiny bit to let the engine idle, and if the link is not properly adjusted the engine will get too much or too little fuel at idle. Once the throttle is opened for takeoff or runup, the idle adjustment doesn't matter so much, just like in the carb.

Overall, the fuel control valve is not precise enough. Continental's engine-driven injection pump moves more fuel and increases fuel pressure as RPM increases, so it's part of the metering system. Lycoming's system measures the static and ram pressures of the airflow coming into the servo, moving several diaphragms to meter the fuel accurately to the airflow. In both cases the manual mixture control on the panel adjusts fuel flow for density altitude.
 
You must log in or register to reply here.
Back
Top