Mixture Control
 

Hello all.

Whilst reading over old threads I have encountered a few regarding different methods of mixture control. In my training (which is yet to finish, as I had to take a long life sabbatical until just recently) I was taught to lean the mixture either above 5000ft or on long cross country trips.

Now, I have just stumbled across a few threads about ground leaning, for instance. This is news to me. I'm fascinated and, as usual, want to know about it, what you do and other tips you may have.

Disclaimer: I know there have been a few threads about this over the years, but most are buried and I figured it's been long enough to do a RAFT check. (New acronym I've made up for "Re-animate a forum thread")

Read the Flight Manual (operators Manual, or whatever they call it nowadays ) for your type. It's all in there.

I figured this would be amongst the first answers. I have read it a few times. Nothing mentioned about ground leaning, however, and there are seemingly plenty out there that were taught this.

So, I'm interested in hearing from you folks too.

Last weekend, we just finished the APS seminar in Kortrijk.
John Deakin was one of the teachers. You can search for his columns on engine management under 'Pelican's Perch'. There is very good information there, even for advanced pilots.
If you lean on the ground (highly recommended) you should lean far enough that there is no chance the engine will develop any more power without stumbling. There will be no chance that you take-off with just a little bit leaning (detrimental). You can lean at any altitude in cruise. At high power and when taking-off be sure to be rich enough.

Problem with trying to lean the mixture correctly is usually a lack of information displayed in the cockpit.
An EGT gauge is very usefull.
However in older trainers they're either not installed or placarded inoperative. If working it's usually only hooked up to one exhaust stack and it may not be the hottest one.
It could have been installed on a random cylinder rather then the leanest one.
Yes, surprise, with carbureted engines they rarely run equal.
This has to do with intake plenum variations, carburetor design and so on.
An aftermarket engine monitor gadget should show you all cylinders.

Leaning for ground ops is a little easier.
For the usual 160-180 hp 4 cylinder : : after engine start run up to 1200-1300 rpm and start leaning.
Watch for initial rpm rise then a drop and a stumble. Lean for the rise.
You now have the correct setting for the max power setting you should need for taxiing. Unless you're operating off a potato field that is.
Also when going full power ( in the runup area!) the engine should stumble and die before reaching full power.
Problem with just arbitrarily pulling it back a little in the taxi is when you forget about it you may overheat a cylinder in the climb as the lean is not aggressive enough to prevent full power.
In the USA you're always taught to lean on the ground also.

Leaning on the ground is not so much a fuel saving issue - as the fuel flow is low during near-idle the savings are really marginal - but more an issue of keeping the engine clean. An excessively rich mixture (full rich) leads to incomplete combustion and leaves residu (soot) on the spark plugs. If the cylinder temps are low this soot won't burn off and thus accumulates. This may cause the plugs to "foul": Not produce a spark. Not good. It may also lead to quicker oil contamination and maybe some other adverse effects.

This effect is increased the richer the mixture becomes. When the OAT is high and/or when the DA is high, this means the mass of air entering the cylinders is reduced, but the amount of fuel stays the same. Thus the mixture becomes even richer. As the UK rarely experiences high temperatures or high density altitudes, leaning on the ground is not all that important there. But in hot or high environments (I learned to fly in Florida) it's good practice to lean a bit. How much is a bit of guesswork: We just pulled the mixture knob back one inch (PA28) but there may be more sophisticated methods.

In any case, during your runup make sure you select full rich (unless the DA is so high that you need to takeoff with a reduced mixture for max performance) and make sure the RPM drop L/R is within the limits stated in the POH.

There's nothing I could really add that's already been said here, most manufacturers recommend leaning above a certain density altitude (note not indicated altitude!) although this is often to prevent ham-fisted pilots from running the engine too lean and causing damage. 3000' or 5000' is usually the quoted figure in my experience.


I was lucky and was taught leaning, just as well as a PA38 I used to fly years ago (which I learned on) used to have regular problems with plug fouling. If it was encountered usually running the engine at 1800-2000rpm then aggressively leaning the engine to the onset of rough running fro a few seconds was enough to clear the fouled plug. Not a good idea to do it too often though, 2 or 3 attempts and then shutdown and a call to the engineer to have a look. That did happen a few times, much to the chagrin of the CFI. Otherwise it wouldn't have been too good for the engine's life I'd imagine.


The aircraft I flew (and most others since) have only had EGT guages that were u/s anyway so I was taught to lean to pear RPM then enrichen slightly. Not the most accurate but enough to avoid damage with the limited info available. There was one PA28 that had a serviceable EGT but I was so used to the RPM method that I just used that anyway.

Rick Durden writes about this quite a bit in "The Thinking Pilot's Flight Manual" (Vol. 1) and says that when the POH says this it is (almost?*) always referring to the takeoff or climb section of the checklist. The cruise section gives figures for a leaned engine and (again, almost?) always does not give a minimum altitude.

So, in the cruise, lean at any altitude unless the POH explicitly says otherwise, and be sure that you're using the POH material that you've read in the intended flight regime.

Durden's is a good book - I am just about to start on the companion volume.


*I've written "(almost?)" in brackets since, as Durden says, it's what is in the POH that counts - not what is in any other document, and generic statements are often wrong for your particular situation.

Its this (key) point that many don't understand (well said DaveW). The problem comes when trying to run the engine at high power settings (i.e. above cruise power) with a leaned mixture. If done correctly at cruise power or lower power settings there will be no issue at whatever density altitude.


I think many operators operate a "lean for taxi" procedure to avoid plug fouling (albeit on hard-standing). When taxiing on hard standing the engine's practically at idle in most cases so leaned almost to ICO mostly presents no problem.


But many (not all) FIs seem to teach PPLs to not touch the Big Red Lever unless starting or shutting down. Its a vital engine control and its usage should be fully taught and understood...


Edited to add - I only speak for NA engines with fixed-pitch props. I have no experience on Turbos nor with CS Props. From what I gather turbo engines are a rather different breed that require very careful handling procedures can differ significantly.

Temperatures will make or break a turbo piston engine.
Turbo Inlet Temperature is important as well as avoiding rapid temperature changes. EGT is controlled by the mixture control so turbo inlet temperature is controlled by the mixture controller.
With large piston engines and CS propellers careful throttle and mixture control is required to attempt to make it to TBO.

In flight, at 65% power or below, you cannot harm the engine with the mixture control. If you have an engine monitor, learn to use it.
If you have a carburetted engine, use of a bit of carb heat may even out the mixture enough to make it run lean of peak smoothly. If you have a fuel-injected engine, it may take some injector swapping (with the help of competent people) after doing Gami lean tests. My engine now runs smoothly LOP, the last straw was the change of spark plugs from Champion fine wire to Tempest.
The engine management and in particular the use of the mixture knob at PPL level is vastly underteached.

Interesting footnote that Charles Lindbergh taught the US Navy 'lean of peak' operations to extend the range of their carrier borne fighters.

This subject is dear to my heart as most pilots are scared of the Red Knob. Most likely because they were taught by a 300 hour pilot who was taught by a 250 hour pilot, etc.

This all leads/has led to all the old wives tales pilots have been told. So far in this thread there has (as usual) been some good advice and some bad advice.

My suggestion is visit Pelican's Perch https://www.avweb.com/news/pelican/182146-1.html Articles 8, 18 and 19 are compulsory followed by 15 and 16 and any/all the rest John Deakin has written.

If you can find articles written by Walter Atkinson of https://www.advancedpilot.com/ then study them. You will be better informed listening to these two gentlemen than the OWT from young instructors.

If you are going to lean on the ground, then lean aggressively. This will assist in keeping the combustion temperature up and prevent full power being applied. One post mentioned the PA38. Most 4 cylinder Lycomings have their mags wired left mag to left top, right bottom and right mag to right top, left bottom. (Plugs, that is...) The O-235 in the PA38 had one mag wired to top plugs and the other to the bottom plugs. The bottom plugs would always foul. Lycoming brought out an SB to cure this; the problem is lead accumulation and not soot.

I have owned and RV-6 (IO-360) and RV-10 (IO-540) and would gladly demonstrate full throttle at sea level and 150F LOP. (91% power)

Tin hat, flak jacket, foxhole....

I found this link on PPRune http://www.pprune.org/aviation-histo...-increase.html

Radial engines have perfect fuel and air distribution. Auto lean in big radial engines is 50 LOP.

Lycoming recommends cruise for best economy AT peak EGT, provided 75% power or less.

Below 75% power, no matter what you do with the red knob; you cannot damage the engine. So leaning for taxi, cannot damage the engine.

Air cooled engines have idle mixture set rich. This is so that it will idle when cold and accelerate after a glide approach. Leaning for taxi is advisable as long as you lean aggressively. You really do not want to go full throttle and be LOP as the engine might run nice and smoothly, but will not produce the power you need.

Chuck,

I would recommend 65% as the limit below which you cannot damage the engine with the mixture knob. I fly both a NA Bonanza and a TN Bonanza, always LOP except for takeoff and climb.

I've never understood the "don't lean below x000 feet" thing. In my TR182 I lean for taxi, otherwise the plugs foul. The only time I fly full rich is for take off and the initial climb to 1000 feet or so. After that I lean to CHTs of 400 or less - maybe up to 410 on the hottest cylinders. Much easier with a full engine monitor. Flying full rich (22 gph) would be pretty catastrophic for both my bank account and range, as compared to 14.5 which is what I can normally do without problems.

Thanks for the replies, all.

I'm bounding through Deakins' material with Pelican's Perch. Really fascinating stuff. He speaks highly of his GAMIjectors, which I've since read up on, as a segwayed note.

This is a practice I will encourage my instructors to teach, as deep as they dare. I want to get this in on the ground level, rather than discovering this some hundred hours into my license. Whilst I'm not afraid of that red lever (which isn't to say I don't respect it!) I also don't fully understand the art of it. Hence my post here.

Unfortunately, however, my trainer isn't equipped with EGT or CHT gauges, so I will be relying on the RPM and engine note to tell me all about leaning, for now. Thinking ahead, then, would it be wise of me to seek further dual training when I have access to an aircraft that does?

I lean out when on the deck, and as soon as I leave the pattern, depending on what I'm planning on doing once I get to pattern altitude will determine weather I'm leaning for best power or best economy. The only time I'm full rich is on run-up and take off. The new plane has a four cylinder EGT monitor which is an excellent tool.

Not full rich for landing? For go-arounds?

Assume you are returning to land after a long flight at altitude; richen as necessary during descent to ensure smooth engine operation, then for approach and landing leave the mixture where it was because you should only be using low power. On short finals richen as required for a possible go around. (At sea level that would be full rich.)

With a basic trainer if you have less than 2500 RPM and less than full throttle, lean till engine runs rough then richen ONLY ENOUGH for smooth operation. If you have a fancy engine monitor then use this with Lean Assist.

You really cannot cause detonation in a low compression aircraft engine (8.5:1 or less) any rough running is unbalanced power pulses so smooth running is good.

Deakin will tell you; detonation can only happen in a very narrow band of power/fuel/air/BMEP and either a richer OR a leaner mixture will stop it.

Keep it safe out there:)

At the risk of stating something obvious, and probably why a lot of organisations teach, and attempt to push people towards rich operations...

There was a historical "myth" that running LoP (lean of peak) would damage engines. This is probably why many POH of old planes often prohibit this. I have put myth in quotes, because, although LoP if done right and accurately, with accurate engine instrumentation has been proved to actually be better for the engine, than running ROP (or over-rich for that matter). Doing it wrong however, could cause both short and long term problems with the engine.

Most spam-cans (pa28/c172/152) that I flew during my training were often 40+ years old, with engine instrumentation somewhat basic and old. Temps being drawn from only one CHT/EGT probe. (whose accuracy I could only guess was roughly working as the needle was moving...) If anyone has ever flown with a digital engine management system with each cylinders displayed, they will know that there is a range of temperatures, especially in old(er) engines. So flying without each cylinders, would be leaving you with 3 unknown temps which may be causing damage without you having the faintest idea.

If you cannot accurately manage the engine temps, you do run the risk of making things worse for yourself. Running on the rich side, with the exception of fouling the plugs (rare above 1000rpm), you don't really have that much risk, and therefore a more cautious option for most of us flying crappy, old and tired planes.

If however, you do have a fancy digital engine management with proper calibration, and installation - you are better (so data tells me) running LoP, but this is often a continual adjustment to ensure proper operations. You can't get complacent about it - saying "oh I did that about 30 minutes ago in the cruise" because any slight difference (OAT, ALT, Pressure) will cause fluctuation and as you are running on the edge, you do need to manage it actively to ensure proper operation. (which is why I tend to prefer ROP operations - and enjoy the scenery a little more)

On a side note, a number of Mooney acclaim and ovations have been reported to have had shortened engine lifespan as a direct result of running over-lean, (so people tell each other) but I can only attribute that to poor engine management (despite their fancy kit).

Back to the main question however, leaning on the ground.... Do you ever remember your instructor telling you to maintain at least 1000rpm whilst taxying, and that you had to close the throttle, and use the brakes to slow down before returning to at least 1000rpm? Well that is so as to not foul the plugs at low rpm. An alternative to this, is to reduce the amount of excess fuel being fed to the engine, hence the leaning on the ground. The main reason why instructors that I know are not so keen on teaching to lean on the ground is because if you are a newly qualified PPL, with 3 friends in the plane, all excited and distracting you, the likelihood of you forgetting something is increased, and they believe (quite rightly in my mind) that forgetting the mixture would not be a great idea on takeoff!

With regards to being on final, I was always taught to go full rich as part of my downwind checks. I don't think it is a good idea to leave it till the go-around, or that moment when you suddenly need full power during final due to severe downdrafts / windshear, because chances are you'll need it full rich, and not have the power you need, when you need it, as a result.

Piper experience the same problems with the Malibu with recommended cruise power setting 50F LOP. The Malibu had rather poor cooling and when investigating the reason for the engine failures (I think it was GAMI/Atkinson) it was found that most pilots leaned to 50 LOP and then became nervous. Remembering the advice of their instructors, they went slightly ROP.

Slightly ROP is the worst place to be (25 - 50F ROP) because the fuel burns the fastest then and with a fixed advance ignition the cylinder head reaches the maximum temperature and pressures within the combustion chamber (BMEP) is at the highest. That is why training aircraft suffer many cylinder head failures/cracking.

I will try to post the Lycoming graph which you will find in almost all POH's for the 'general training aircraft.' (EGT / CHT / Percentage Power / Specific Fuel Consumption.) When leaning the mixture, the CHT peaks first (slightly ROP) if you continue leaning to PEAK EGT; the CHT DECREASES. It is safer to run at peak EGT than 'slightly ROP.' (Providing less than 75% power)

Follow the graph and check CHT at 50F LOP and look to the right to find the ROP temperature that gives the same CHT and you will find it to be off scale on the rich side. IE full rich will result in a hotter CHT than 50F LOP.

Looking at the fuel consumption graph, it is flat from peak EGT to 50F LOP then becomes worse. (GAMI et al have found that there is a very slight improvement.) 50F LOP is not for better economy as HP drops off rapidly, it is for cooler CHT as that also drops off rapidly.

Lycoming Graph.

Hopefully bigger this time...

For those that haven't found it yet, here is the John Deakin article about leaning. It was written nearly twenty years ago and remains valid today. Expect to spend several hours to absorb what is in there.

https://tinyurl.com/ybjoov4e

Restrictions on running LoP were not so much a "myth" as a certain limitation to specific types, I might be wrong but I think this was specifically related to higher-powered engines in Ye Olde Worlde Days when lower-octane fuel was in common usage. Higher-powered engines are more sensitive to detonation and for correct operation higher-octane fuels are a necessity. Just why we've ended up with having to run 180hp 4-cylinder engines on 100-octane fuel that requires toxic additive is a mystery to me (like many things in aviation, "Cos that's the way we've always done it") however we have what we have. Running LoP on certain engines or general cack-handed operation of the mixture control was a sure-fire way of inducing detonation which causes catastrophic engine damage in the medium to long term no mattery what type. Hence we ended up with people who were terrified of attempting to run LoP or even trying to lean the mixture at all.


Most engines in common GA aircraft today will quite happily run LoP (if done correctly in accordance with the POH and manufacturer's data, as alluded to in the previous posts with) without any detriment to the engine operation or lifespan if done correctly.


Often in horizontally-opposed engines different cylinders will run slightly leaner or richer than the others (and at a different temp), so unless you have a fancy engine analyser leaning can often be a bit of a compromise as one or two cylinders may start rough running as they're running a bit leaner than the other cylinders.


P.S. I was always taught that closing the throttle completely on taxiing before applying brakes was to save wear on the brakes, as even at 1000rpm there's enough thrust there to work against the brakes - saves Mx costs!

Ah - sorry maybe I didn't explain my point well enough. The running at above 1000rpm was to not foul the plugs, despite often giving you too much thrust at paved airports. As you pick up speed you need to slow down, by closing the throttle before applying the brakes for your very reason - yes! However, if leaned you could happily taxi at much lower revs without leaning, hence not need to keep opening / closing the throttle, and keeping a safe taxi speed without use of the brakes at all.

Interesting Chuck! I am quite surprised to hear your thoughts on this! So you're suggesting that they may have been running slightly ROP and that caused damage? That is surprising indeed! I may be reading what you linked wrong, but I don't see that significant a temp change between LOP and ROP. I was always under the impression that ROP operations would always enable a slight element of cooling from excess fuel going in. Interesting!

Alex you have the message, no confusion at all. Look at the graph again CHT peaks before EGT; so, if you lean till peak EGT then richen the mixture slightly 'so the extra fuel can cool the valves or cylinder heads' (my parentheses, not yours) then you are achieving the exact opposite.

The reason why the CHT peaks first is the fuel burns the fastest at that fuel/air (Stoicometric ratio sp?) and because of the fixed ignition most of the heat goes into the cylinder head and not into pushing the piston down. (Ineffective crank angle plays a part here, transferring linear motion to rotating motion.) Peak EGT slows the combustion and most of the heat/energy is used to force the piston down the cylinder. The cylinder head has to absorb less heat and thus runs cooler.

Below 75% power, if you are scared of LOP operation, then do your engine a favour and stay at peak EGT.

Never quite understood what ROP and LOP meant. Which Peak are you talking about?
If it is the peak of propeller rpm, then that is not really 'peaky' at all. It increases by about 50rpm then drops considerably as you go from Rich to ICO.
.

The 'Peak' referred to is peak EGT.

Rich/Lean of Peak refers to EGT.


Peak EGT is to the rich side of the best economy range IIRC - RPM wise I think that is when you set cruise power (e.g. 2200rpm), lean to peak RPM then keep leaning until RPM falls back to the original set value... however I could be wrong as I've never used that method in practice. ISTR that Best Power is lean to peak RPM then nudge it slightly rich, however my memory is somewhat hazy after several nights without sleep (teething toddler & pregnant wife about to pop No2 - not good :bored:) - happy to be corrected where wrong.


(As said previously, all for NA fixed-pitch props)

Tels, been there, done that, have the tee-shirts re: children. ;) Peak EGT, is peak EGT. Period. The idea of setting '2200rpm' then leaning past max power (read more RPM) to (less power) the same RPM means you are at the same point in the power delivered arc in the Lycoming graph above. Nothing else.

You might or might not have gone past peak EGT, you might or might not be ROP. The method proves nothing and is just another OWT. By your own admission, you have heard about it but not tried it. Now you need not try it nor use it.

In a carbureted GA 'trainer' in cruise pull the red knob back till the engine runs rough, then richen only enough for smooth operation. If you have an EGT gauge then use it to set max EGT and leave it there provided it runs smooth. (Mr Lycoming's advice, not mine.) The reason for insisting on smooth running should be obvious, torsional vibration from uneven power pulses is bad.

Mr Lycoming says max power happens at 100 to 150F ROP, GAMI/Atkinson et al have found this figure is actually about 75 to 90F ROP so unless you have a fancy engine monitor, rather run 100 to 150 ROP if in a race.

Long story short, the bottom line is that the Mixture control is as much and engine control as the Throttle is.
( and the prop control if so equipped)
Use it as such.

I appreciate this doesn't answer any questions, but during my PPL not once was I ever shown or told how to lean, I was told that seeing as the price we pay per hour is "wet" then to leave it fully rich as the school are cautious of damage to the engine.

I have to be honest since finishing my ppl I've been mainly focusing on ATPL exams but in the little bit of hour building I've done I've tried leaning it a few times whilst in the cruise. I know that I still leave it on the rich side but without having been taught what to do I feel it's probably better to be on the safe side as I'd be worried of over doing it

Stefan, did you ever read the POH for the aircraft you're flying? It's a good habit to get into for any type or sort of aircraft you're flying now, or are going to fly in the future.

When you follow the advice in there, you really can't go wrong. Plus, the manufacturer has taken into account the type of engine (fixed or variable pitch, injection or not) and the instruments fitted (RPM, EGT, CHT or not) and tailored its advice particularly for that situation. So you don't have to read through two pages of how to lean for Peak EGT or ROP or LOP when you don't even have an EGT fitted.

And if anybody questions your leaning practices, you can simply point to the POH and tell them that you're flying the aircraft according to the manufacturers advice. That'll shut the majority of critics up.

A word of caution here. You need to either do as you're taught, or you need to follow the manufacturers advice - assuming those two are not the same. But if you're just going to be making up your own compromises you may actually be putting yourself in danger. This doesn't apply so much to leaning, although there may be a few corner cases where leaning "just a bit" may be more damaging to the engine than not leaning at all or leaning all the way. But it applies more to flying in general. If you make up your own compromises which are not taught or described anywhere, you are in fact a test pilot. (A far more extreme example would be an outside loop down. You either don't do it at all, or you need to do it with full gusto, headache be damned. But if you do it halfheartedly you'll find yourself exceeding Vne really quickly, with no easy way out. And the cows getting bigger awfully fast.)

Fully agree. Best performance for the lowest fuel burn requires careful management of MAP, RPM and Mixture. However, it also requires a fair portion of your attention, so you should also know when to compromise because your attention is required elsewhere. That's why, somewhere during the approach, we shove both RPM and mixture all the way forward. Two less things to worry about in case of a possible go-around.

@BNI: If you want to stay happy as a Pistonpilot, learn and understand what the mixture knob does! I am very surprised you have training without intense teaching on how to manage fuel. Am I right, you have not gone through that part of the training were you scrub 15 traffic circles in 45 minutes?

Yes, it is indeed correct to point you to the POH, but it may be misleading as well. If you do training in an aircraft which was designed i.e. for 87 octane use and now fly on 100LL, the POH is not written for that and at latest, your FI should teach you what to do with the excess lead. If you drive an old 172 or 28 with TCM or Lycosaurus, you will get into trouble over time when not leaning on the ground -> thy collect lead on the sparks pretty quickly.

Thanks for the advice. I have looked through them but clearly I need to take another look through them and will do before my next flight!

I always fly with the mixture rich at any altitude below 5000 feet and set the prop set to "over square".
That's right...isn't it?

OH NO, the horrid over-square debate again. ;)

On your next flight set your power as you usually do, then note CHT. Next lean till peak EGT and note CHT will decrease. (Happier, cooler, cleaner combustion.) Not rocket science, it is all in the Lycoming graph posted above.

Thank you Chucky... that post was in jest!!