Hey y’all!

Judging by some videos and statements by pilots that I read this may be bordering on a religious discussion, so please be nice. Remember we are all friends here!

Ok, so here we go. This is both about flight sims and real life so GoGo real pilots!

As the title suggests this topic is about leaning. No, not you, the plane. The mixture of piston engine planes to be precise.
As y’all know I am not a real pilot so I never had to worry about fouled spark plugs or overheated cylinders.
I do know the basics of leaning and how it relates to power (based on density altitude and so on) but lately I remembered that I don’t really know how to properly do it, especially in a variable pitch propeller plane. I do it by ear in most planes.
So I looked up the topic, and while I did learn some things I also got a tad confused.

In short: Lean of peak (LOP) or rich of peak (ROP), and why and when?

I go by what the POH says, but I’m just a sometimes recreational pilot. IME it’s going to be ROP, since you are usually starting rich and then lean toward a peak temp. Or failing that, just lean until you get the most rpm while keeping the CHT in the green.


So… are there tables in the book about which setting to use at which (density) altitudes, in cruise and while climbing? Sounds like a lot of paper unless they manage to put that down somewhat concisely.
Or is it some kind of formula?

Not sure about the big boys, but it’s not that complicated. In my revised post above, you are adjusting mixture to get the most power without getting the temps too high.

The lawyer is gonna say operate according to the POH.

My first planes had no EGT or CHT gauges…so I was taught to lean until roughness, then one or two turns on the vernier toward rich. It was a real science :rofl:


My procedure for fixed props is: check RPM and sound. Pull the lever until it goes down, then push it in a tiny bit.
I admit I never looked at an EGT gauge on a prop plane… :sweat_smile:

Edit: and of course that technique doesn’t work with constant speed props.

The thing that made me wonder is this: richer mixture is supposed to cool the engine. Which should be good since hot engine can be bad. But fouled plugs and so on is also bad. Also rich means more fuel.
And then I saw that chart in which they show that on high settings ROP is actually worse, temperature wise. In some planes at least.

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Mike Busch is in my opinion the authority on engines. Years ago he wrote a somewhat controversial article (AOPA I believe) where he made a convincing case that the most unhealthy mixture setting for an engine is 50 degrees rich of peak (see @Aginor’s chart above) . I run 75 ROP on the rare occasions where I fly the PItts X-C. Why? Because Mike assumes an engine monitor where the CHTs* of all cylinders can be checked. I’d like to think that my single probe is showing me the hottest cylinder but I really don’t know. During acro I go WAY over 400F (Lycoming limit is 450. Mike’s limit matches Continental: 400.) So I want to run cool to and from the abuse AND I want to match how others have been operating aerobatic engines well past TBO for 2 generations now. So even though I agree with everything Mike writes, I do it my way mainly out of superstition. That said I have stretched contest cross countries by flying up at altitude where the air is cool at 50 LOP. Both the EGT and CHT are relatively cool up there and I know that I am doing no harm. Do try to find his article on the subject. He also has a good one on the old “MP/RPM squared” nonsense.

*Yes we are leaning for EGT. But according the Mike, CHT is a better metric for engine damage than EGT.

PS. When I am lazy I lean @BeachAV8R’s way with a modification. I switch to a single mag and lean slowly to roughness, then back to smooth. That always brings me to exactly the same fuel flow that 75 ROP would yield.


Thanks a lot for that insightful answer!

Why the single mag?
(That’s one of the other topics I haven’t really read into. I just turn both on and off and that’s it)

The single mag just makes the process more conservative. With just one plug sparking the low fuel/air mixture will more quickly result in misfiring.


Ah OK, that makes sense!

I’m linking this rather than copy/paste to avoid copyright issues, but here is the C172S POH. If you search for the section “LEANING USING EXHAUST GAS TEMPERATURE (EGT)”, section 4-36, you will find some pertinent information.

Executive summary:
Recommended lean is 50°F Rich of Peak EGT. Best economy is Peak EGT. If engine runs rough at Peak EGT, increase mixture to Recommended setting. It may take some time for EGT to respond to mixture changes. Adjust as necessary after change in altitude.

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So how does the cooling work in a continental engine you find in cessna’s? Does it have a dedicated coolant loop? Does it use a fuel-oil heat exchanger or an air-oil heat exchanger? is the oil used in a coolant loop?

I flipped through the document and I must say I am surprised how little information it contains about engine parameters. I had hoped to find a chart similar to the one I posted above, to find out if maybe the cylinder temps of the Cessna engine have a different curve which would explain the decision for ROP.

Anyway, the guys who wrote the POH are obviously the authorities on it. It just isn’t satisfactory to me since I would have loved to understand the mechanics behind the LOP vs. ROP debate.

My disclaimer first: I am not a pilot nor an aviation engineer, and nothing I say should make you do anything other than what is recommended in the POH.

That said,

As I understand it the debate isn’t really a debate, it’s a question of knowledge and risk management:

‘Traditional’ avgas burning aero engines are air-cooled and carbureted (yes I know this is a generalisation but bear with me). When such engines operate at full power with a perfect air/fuel mixture the cylinder heads and pistons can reach temperatures that can cause accelerated wear. By ‘perfect air/fuel mixture’ I mean that there is just enough air in the cylinder for the fuel to combust completely but no excess air.

The temperature at which the cylinders and pistons operate can be reduced in 3 ways:

  • reduce power - this reduces the total energy released through combustion
  • run richer - the extra fuel displaces some of the air resulting in incomplete combustion, increased pollution, and reduced power, but the unburned fuel also carries away some of the heat
  • run leaner - the extra air displaces some of the fuel resulting in reduced power, but the extra air also carries away some heat.

What are the risks?

  • Running at peak temperatures for an extended duration increases the risk of catastrophic failure of the engine, or at the very least reduced TBO.
  • Running rich can increase the chance of lead fouling as the spark plugs are exposed to unburned fuel over more of the stroke period
  • Running too lean risks having the fuel not combust. This can lead to fuel detonation in the exhaust leading to catastrophic failure of the engine.

These engines can be run LOP safely, but it needs more care to mitigate the risk of detonation and ensure sufficient cooling. Two more points to consider:

  • peak EGT does not correspond to peak cylinder temperature - EGT generally hits peak whilst the cylinders are still rich (I think?)
  • there will be variation in the behaviour of each cylinder due to differences in cooling and fuel/air flow from the carburettor.

In order to safely run LOP you really need temp sensors in each cylinder to ensure the richest one is lean enough to adequately cool, and you need to minimise the variation between the cylinders to ensure the cleanest one does not enter conditions conducive to detonation.

So you can see that when a manufacturer tasks someone to write the POH the run-rich scenario presents the lowest risk profile. Also, a single EGT sensor is cheaper and easier to maintain and operate. So the majority of POH’s in the private sector have recommended running ROP for this reason, and as a result generations of pilots have had it drilled into them to stay ROP.

If I’m wrong with any of this, please feel free to correct me, as always :grin:

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Judging by that picture I posted above (which I took from a YT video of a dude who took it from some paper) that seems to be correct, at least for that type of engine.

Thanks for your post!
While I cannot say whether you are right or not it does make a lot of sense to me.

Think less: “Airplane”
Think more: “circa 1950 British mortorcycle”

Now that your head is in the right place, cooling is primarily done by air entering the intakes and being directed by the cowling and engine baffles straight down around the cylinders and out the bottom of the cowl. Oil is cooled by an air-cooled oil cooler that is usually located in one of the intake inlets.

That’s a fair assumption but I think it is optimistic. They get their data from the Lycoming and Continental maintenance manuals which are themselves not very in-depth. These engines have been pretty much unchanged since the war. They are simple as dirt and as dependable as a sunrise. There is little reason to confuse the pilot with a lot of fiddly engine parameters. Just…

  • Don’t overheat it,
  • Don’t overrev it.

Make it more complicated than that and the lawyers double their billable hours after each accident. That is at least until the 90’s when very sophisticated engine monitors became available. They were expensive but they came with the promise that the owner could now safely maximize power and minimize fuel burn, quickly paying for the cost of the technology. But this sophistication was a bit much for us ignorant pilots and soon cylinders and exhaust pipes began to crack with increasing frequency. To bring us up to speed, pilot/mechanics like Mike Busch reached out to fill the vast gaps in our knowledge with pilot-friendly articles that have probably saved millions in maintenance to those who took the time to get educated. Today we are (FINALLY!) starting to add electronic ignition to the mix which will greatly increase the benefits of all this sophisticated leaning and monitoring. But electronic ignition is still pretty rare in certified machines. So the current situation with our 1920’s-technology magnetos is that ignition happens at a fixed crankshaft position (around 25deg btdc) regardless of power-setting or atmospheric conditions. All this tweaking with the prop and mixture is a little like using a mainframe to aim a snowball down an incline.

The good news for most of us is that a Lycoming or Continental carbureted or injected 4-cylinder motor is basically bullet proof. Run it however you want to run it. More harm is arguably done by sitting on the ramp on a hot day idling while you program the Garmin 1000. Or taxiing in after a bunch of heat-inducing touch and goes and shutting down immediately without letting the cylinders cool below 300.

Mike Busch and others are really speaking to the pilots flying tightly cowled IO-540s in high-performance singles and light twins. These engines cost $25,000 apiece to overhaul. So proper care is especially important.

I’ll end by repeating the point that Busch and others have made: 50 degrees ROP often results in the highest CHT. So for those people who were taught (by ME!) that this was the most conservative setting, he made the case that we were flat wrong. Were our students slowly destroying their engines? No. But nor were they doing the great good that they probably thought they were doing. 50 LOP for most motors is probably cooler. And if the engine is running smoothly, it is probably better. The only way to know for sure is to get a nice engine monitor and pull the plugs out for inspection between annuals.

One simple thing I forgot to add. Even though I said “carbureted” I am really talking fuel-injected only. I haven’t flown a carbureted motor since the 90’s. This is a total WAG on my part but if I were flying with a carburetor I would avoid LOP ops.


A laptop, a cheap 5 euro boroscope you can drop in and a few new plugs every now and then tell you so much about engine condition!

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Wow. Another great post. Thank you so much!

Sounds like those engines are indeed more kinda like my brother’s old VW Beetle engine (or indeed an old motorcycle engine) than anything else.
It does make sense though, since 100 years old technology is proven, we know its limits. Which is pretty important in aviation.

Question: if they have that kind of…uhmmm… simple ignition mechanism…
…don’t they misfire/knock a lot?
Edit: you mentioned misfiring in your post above, with one magnet, which of course makes it worse. My question is more about regular operation.

I am not an expert for engines but as far as I understood that’s what all the electronic ignition stuff is for.

They shouldn’t knock. That is part of the reason we still use leaded gas. Also, the compression is lower than your car motor. In fact, that is one sales pitches for electronic ignition in GA planes. It will allow for higher compression ratios. But they do pop and skip at idle. And when hot they are a bear to start. I’ve got a technique that works but I’ve seen a few exhaust fuel fires at aerobatic contests. This is more common in tightly cowled Extras with 11-1 IO-540’s. They need a lot of priming to start hot. The only way most of them will start is with full throttle and the mixture at idle-cutoff. Once it hits, you have to reverse that in about half a second or start all over with more priming and a heightened risk of gas pouring down a hot exhaust.


Oh wow. Sounds like fun. Not. Thanks again for the insights! I am flying the C172 in FS2020 right now, playing around with the mixture to put my new knowledge to the test. :slight_smile:

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