Questions about mixture leaning

LongRoadBob

LongRoadBob

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All flying I have done so far has been full mixture for takeoffs, landing, and maneuvers (most of the time) at or under 3000’. Really the only time flying and leaning the mixture is cruising to the air works area and back. That so far has been only set rpm (2200) and lean.

So I wondered, on cross country trips, mostly cruising, after initially choosing a power setting, and leaning the mixture, it’s not clear to me if that lean setting is valid also if you say add power to climb or lessen power to descend. Also not sure how you best adjust if it is needed. I can imagine it might depend on how long you will climb or descend.

And if you do need to readjust with power changes, do you first set mixture full rich and then go from there or adjust slightly from where it sits already?

wondering too if during descent, if you want to have it lean to avoid too much engine cooling?

The whole leaning thing, something seems off to me on how I’m being taught. Our checklist on startup has us lean the mixture just after engine start during warmup. Then taxi, and stop, full rich for run up checklist. The club checklist never tells us to lean again for taxi. I read in the POH that one should lean for ALL ground operations, but when I went to do that, the instructor said not to do that. Also, he has mentioned that if I lean at 1200 (actually in summer he has me doing this at 1000 rpm) when I then give more throttle as we taxi he said once that it then wasn’t properly leaned because of throttle being adjusted.

I need to ask him next time, as of now no student flights possible because of restrictions here because of corona, and also my health has been problematic lately, but I don’t feel like I have sufficient info on leaning, when, how, etc. other than set and forget during the short cruise we do to training area.

The Cessna is fuel injection, not sure if that makes a difference. Again, just wondering about your experiences and if many adjust mixture after making small or medium throttle changes?
 
You are probably flying the 172 R model (2000-ish), I remember a instructor telling me that there is something with the engine that on a cold engine the plugs foule pretty quick. After a flight I never leaned the engine for taxi back to the ramp. Never had problems with that.

About the mixture and power setting, when I want to climb I do add a little more mixture, this by looking at the EGT gauge. I find that my tempratures will be getting too high when I climb a lot on my 'old' mixture setting. There are some nice Youtube videos about leaning as well.
 
Aircraft engines and operations are right much different than the automotive engines most of us are used to. A book by Kas Thomas, "Aircraft Engine Operation Guide", is a good read with all kinds of useful information, including leaning and starting procedures. This book explains the principles of why we lean in more detail and the problems associated with improper leaning. I have owned this book a long time and refer back to it occasionally to refresh my understanding of correct operating procedures. The book goes into aircraft engine design in such a way as to explain why these engines behave differently than any others.
 
http://www.alfako.be/SAFETY DOCS/lycoming---flyer_key_reprints.pdf

Read the section on leaning. Your instructor is wrong about leaning for taxi.

My rule for descents, after leaning for cruise, is to leave the red knob alone at first. As you descend into denser air the mixture will become leaner (more air, same fuel) and the engine will let you know by momentarily stumbling or missing....push the red knob in until the engine runs smoothly and continue descent. Soon the engine will begin complaining again because of the mix being too lean, so push the red knob in for smooth operation again. Repeat as necessary.
 
LongRoadBob said:
The club checklist never tells us to lean again for taxi. I read in the POH that one should lean for ALL ground operations, but when I went to do that, the instructor said not to do that. Also, he has mentioned that if I lean at 1200 (actually in summer he has me doing this at 1000 rpm) when I then give more throttle as we taxi he said once that it then wasn’t properly leaned because of throttle being adjusted.
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Your instructor may have other virtues, but it seems mixture management isn't one of his strengths.

The misinformation, old wife's tales, etc. is unfortunately very persistent when it comes to this topic. It's not really complicated, though. A very good read, if you'd like to understand this topic better is John Deakin's "Pelican's Perch" articles on AvWeb: https://www.avweb.com/features/avweb-classics/pelicans-perch/pelicans-perch-index/
Maybe start with #18 from the series, and then see what interests you from there.

To address your two specific questions, on decent, as your percentage of power increases - assuming it does, because airplanes descend really well without much power - you will have to keep the mixture away from the area where pressures inside the cylinder and the cylinder head temperature (CHT) get too high. The high airspeed in the descent may help cool the CHTs, but doesn't do anything for the pressures inside the cylinder. So mixture control is important even in the descent. Look up the term "red box" (or "red fin"), it's the range of mixture settings around max power, and as a general rule, you want to stay out of this red box whenever your airplane is making a lot of power. You can stay out of the red box by being very rich, or very lean (as in "lean of peak"), but in between is a large zone your engine does not like.

The other case, right after startup: yes, lean the mixture after startup. Make it just rich enough so it runs smoothly. It will do wonders to prevent fouling of spark plugs.

- Martin
 
Mixture simply regulates how much fuel goes into the engine for a given amount of air (basically). If you aren't changing altitude (or rather, the air density isn't changing), the only real reason you'd have to adjust mixture when changing power is to manage engine temps

For climb, I usually leave it full rich until around 3K-5K and start to lean for better performance, careful to watch temps
For descent, I usually leave the red knob alone for a while until the engine seems rough.. I always go full rich though by around 3K in the club Archer and Skyhawk though

PS, I know pilots (myself included) tend to be control freaks, but honestly, given how poor most CFIs are at teaching engine management it really would be nice to have the plane regulate its own mixture. This, by the way, doesn't have to be fancy electronic technology.. the Twin Bonanza mechanically managed its own mixture as does the Antonov AN-2 (yes, the big Soviet biplane), and I'm sure many others. How many engines have been damaged or killed by incorrect mixture technique. Just let an engineer figure it out and have the engine manage itself
 
Good luck figuring it out, as you can see from the posts above everyone has their theories and bibles that they use. I started out in a simple 100 hp trainer that had a mixture lever and a fixed prop. That airplane was lean on the ground until run up then full rich full throttle until you got to altitude, set the prop to 2500 rpm, lean until the engine stumbled, then enrich until it smooths out. I'm flying an SR 22 now that I rent. In that airplane I start it up, then lean immediately after the engine smooths out by pulling the mixture until the rpm rises. It stays there until runup at which point it goes to full rich. I do the runup and generally leave it full rich for take off. If I'm number five in line or I'm told it will be 5 minutes before I can go, I'll pull the mixture again until the rpms rise and leave it there until just before takeoff.

After takeoff, I'll generally leave it full rich until 5,000 feet, then start leaning per the fuel flow indicator marking. Once at altitude, I'll stay full throttle until airspeed stabilizes, then pull back to my desired power setting. If I'm running at 75% I'll run 75 degrees rich of peak, if I want to stretch out fuel I'll run 65% or less power at 50 degrees lean of peak. During climb in the 22 you have to watch the CHTs on a hot day, cooler days are not a problem.

Now there are a lot of experts on the Cirrus and they all have their tricks. Cirrus recommends that you not operate the NA 22 above 65% lean of peak. People run them higher at lean of peak and claim it is fine. Who knows.

I fly an airplane someone else owns, this is how I have been told to run the airplane so I respect that request. If you don't own the airplane, then I suggest you operate it the way the owners want it operated too.
 
Tantalum said:
Mixture simply regulates how much fuel goes into the engine for a given amount of air (basically). If you aren't changing altitude (or rather, the air density isn't changing), the only real reason you'd have to adjust mixture when changing power is to manage engine temps

For climb, I usually leave it full rich until around 3K-5K and start to lean for better performance, careful to watch temps
For descent, I usually leave the red knob alone for a while until the engine seems rough.. I always go full rich though by around 3K in the club Archer and Skyhawk though

PS, I know pilots (myself included) tend to be control freaks, but honestly, given how poor most CFIs are at teaching engine management it really would be nice to have the plane regulate its own mixture. This, by the way, doesn't have to be fancy electronic technology.. the Twin Bonanza mechanically managed its own mixture as does the Antonov AN-2 (yes, the big Soviet biplane), and I'm sure many others. How many engines have been damaged or killed by incorrect mixture technique. Just let an engineer figure it out and have the engine manage itself
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I agree with having the engine regulate mixture itself and keep the pilot out of the picture. Cars have oxygen sensors that are the main tool used to control the mixture. We need lead in our engines that poisons the oxygen sensor very quickly. If they ever develop a usable unleaded fuel for our engines I think you would see the mixture knob disappear quickly.
 
PaulS said:
Good luck figuring it out, as you can see from the posts above everyone has their theories and bibles that they use.
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That is so true. The good news is physics is physics, and the data and information we need to understand how mixture affects our piston engine has been out there since WWII. I encourage everyone to seek a better understanding of how mixture relates to power, temperatures (EGT and CHT) and pressures, and the whole mystery goes away very quickly. All you need to get that understanding is out there - maybe a good learning assignment during the COVID-19 days of being stuck at home.

- Martin
 
LRB, here are two guidelines that will keep your engine happy.

1) Below 65% power, you cannot hurt your engine with the red knob. Adjust rich until the engine runs rough, then lean it until it runs smooth.

“Lean to rough, rich to smooth” is a mantra you will hear frequently until you fly behind a fully equipped engine monitor.

2) Above 65% power, full rich.

This is the equivalent of bringing a nuclear weapon to a street fight, but it works.

I fully expect others to speak up about how crude these guidelines are, and yes, they are crude. But it’ll get you going until you find the right instructor to refine your knowledge. -Skip
 
Do whatever your instructor wants you to do regarding leaning. First, you need to make him/her happy in order to succeed in attaining your certificate. Second, it's not your plane, and its health is his/her responsibility.

When you own your own plane readdress the leaning issue. It may vary from plane to plane and engine to engine. In my normally aspirated Piper Warrior, I lean aggressively on the ground. So much so, that if the throttle is advanced the engine will stumble. This is as insurance against taking off too lean. Generally, I take off full rich, then lean above 3000 MSL/DA (ish). At cruise, I lean as the POH suggests -- lean to roughness, then enrichen to smoothness.
 
Skip Miller said:
LRB, here are two guidelines that will keep your engine happy.

1) Below 65% power, you cannot hurt your engine with the red knob. Adjust rich until the engine runs rough, then lean it until it runs smooth.

“Lean to rough, rich to smooth” is a mantra you will hear frequently until you fly behind a fully equipped engine monitor.

2) Above 65% power, full rich.

This is the equivalent of bringing a nuclear weapon to a street fight, but it works.

I fully expect others to speak up about how crude these guidelines are, and yes, they are crude. But it’ll get you going until you find the right instructor to refine your knowledge. -Skip
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Sitting on a hot taxiway with no wind at idle, you're way below 65% and can certainly harm your engine by improper leaning.
 
Salty said:
Sitting on a hot taxiway with no wind at idle, you're way below 65% and can certainly harm your engine by improper leaning.
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Hmmm, I think you might get some pushback on the leaning part, but I'm not giving it to you. I have however sat on a hot taxiway watching that cht rise unchecked though, can be stressful.

Cirrus recommends that if you are cruising lean of peak and the chts are rising that you further lean to bring them down, never really have had to do it, but the recommendation is there.
 
Skip Miller said:
LRB, here are two guidelines that will keep your engine happy.

1) Below 65% power, you cannot hurt your engine with the red knob. Adjust rich until the engine runs rough, then lean it until it runs smooth.

“Lean to rough, rich to smooth” is a mantra you will hear frequently until you fly behind a fully equipped engine monitor.

2) Above 65% power, full rich.

This is the equivalent of bringing a nuclear weapon to a street fight, but it works.

I fully expect others to speak up about how crude these guidelines are, and yes, they are crude. But it’ll get you going until you find the right instructor to refine your knowledge. -Skip
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I agree with the folks who indicate you won't burn a valve by running too lean of peak below about 60%-65% rpm.

However, an engine monitor is on the short list of things I want to add, in part because the above "lean until it runs rough, then enrich enough to make it smooth again" process isn't as straight forward on my fuel injected 7KCAB. With the carburetor equipped Supercub and Super Cruiser I've had in the past, the lean until it stumbles and then enrich until it's smooth process worked fine and the demarcation between "smooth" and "stumble" was very clear. Old school and crude but it worked.

However, on my fuel injected IO-320-E2A, leaning slowly from full rich at 2200 rpm will eventually show a slight 50 rpm increase to 2250 rpm, then a slow drop of 50 rpm back to 2200, (lean of peak I presume), then a larger than I'd like area of no change in RPM followed by very slow initially but progressively greater loss of RPM but with a still smooth sounding engine. So power loss but no rough running "stumble". It's a little odd, and it almost always leaves me second guessing myself as to exactly where I am in terms so being safely lean of peak and below the so called "red zone" around peak. I normally lean until I get noticeable drop in RPM below 2200 rpm (assuming its the smooth version of "stumble") and then increase slightly back to the point just before it started to drop below 2200 rpm. I'll then monitor the oil temp over time to ensure it doesn't increase.

Total fuel burns indicate it is leaned about right, but the whole thing is pretty crude with no CHT to monitor temps and no EGT to use to set the mixture lean of peak. I've got a non functional clock/timer in a 2 1/4" instrument hole, so I'm going to put a JPI EDM-700 in it and take the uncertainty out of the process.
 
I lean aggressively on the ground. Run up around here usually also requires some leaning, due to altitude — something you should be aware of if you go to high airports. Full rich for takeoff here will rob you of power, but you need it rich enough to avoid detonation and bad juju in non turbos. Turbos, it’s full rich still of course.

Descent, if I planned it right being “too lean” on the way down usually isn’t a problem because power is reduced but with that full rich thing being too rich at the surface here, going full rich ends up blubbery at reduced power settings. Like someone else said, I go “rich enough for smooth operation” and lean toward the slightly over rich side of that in case of a later go-around. On the go-around itself I’ll often still go richer just in case, since it’ll only affect climb rate if too rich.

Sounds touchy feely it kinda is. Most folks teaching up here teach it as book procedure plus “lean for best engine operation and performance” and a longer discussion during density altitude lesson(s), where a nice hot day can provide examples of poor performance.
 
PaulS said:
I think you would see the mixture knob disappear quickly
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That would be fantastic

For what it's worth the method on the AN-2, at least as my ex Ukraine AF pilot friend told me, uses a sealed balloon which contracts or expands with density changes (much like an altimeter works) which is then mechanically linked to a mixture control. There *is* a mixture knob, but you are instructed not to touch it. And this is tech dating back almost 100 yrs.
 
PS - some CFIs teach full rich on the ground because I think they're worried a student may attempt a take off with it still leaned for taxi and either experience (at best) a very hot engine or (at worst) very poor engine performance. Some people think that a fouled plug is a smaller risk then someone taking off leaned out

I don't advocate this position.. just what I've heard
 
I guess I’m with the majority, lean aggressive on the ground, cruise at just under 65% while leaning till roughness and then slightly enrich. As I descend, will keep power just under 65%and not touch mixture till touchdown, and then lean aggressively. I try to use this power setting all the time to get a good handle on fuel consumption, one tank is for take off, cruise, and landing tank, the other tank only for cruise.
 
Tantalum said:
PS - some CFIs teach full rich on the ground because I think they're worried a student may attempt a take off with it still leaned for taxi and either experience (at best) a very hot engine or (at worst) very poor engine performance. Some people think that a fouled plug is a smaller risk then someone taking off leaned out

I don't advocate this position.. just what I've heard
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If only there was a checklist to make sure they don’t do this...
 
Skip Miller said:
@Salty That is why I want a reference or other data. I haven't seen CHTs that high at idle power...
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Ok, probably not while at idle, but eventually, presumably you are going to power up, and if you're already running 300+ degrees at idle, you're going to get hot quick.
 
PaulS said:
If they ever develop a usable unleaded fuel for our engines I think you would see the mixture knob disappear quickly.
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I doubt it. Unless every TCd airplane currently in existence gets an STC for a new fuel system that has automatic mixture adjustment and people incorporate it, you’re likely going to be in the same situation you’re currently in for the duration of both our flying careers regardless of what gasoline gets used.
 
Tantalum said:
PS - some CFIs teach full rich on the ground because I think they're worried a student may attempt a take off with it still leaned for taxi and either experience (at best) a very hot engine or (at worst) very poor engine performance. Some people think that a fouled plug is a smaller risk then someone taking off leaned out

I don't advocate this position.. just what I've heard
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"Some CFIs" should have their heads examined if they do not teach their students to check that the power controls are set properly before adding takeoff power. Sheesh!!

Bob Gardner
 
mondtster said:
I doubt it. Unless every TCd airplane currently in existence gets an STC for a new fuel system that has automatic mixture adjustment and people incorporate it, you’re likely going to be in the same situation you’re currently in for the duration of both our flying careers regardless of what gasoline gets used.
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Agreed, I was looking forward, you are looking back, but good point.
 
Tantalum said:
PS - some CFIs teach full rich on the ground because I think they're worried a student may attempt a take off with it still leaned for taxi and either experience (at best) a very hot engine or (at worst) very poor engine performance. Some people think that a fouled plug is a smaller risk then someone taking off leaned out...
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I think you’re probably right about this. Having said that, if learned properly for ground/taxi ops, applying full throttle for takeoff should cause the engine to stumble and quit if left leaned.
 
Low Level Flyer said:
I think you’re probably right about this. Having said that, if learned properly for ground/taxi ops, applying full throttle for takeoff should cause the engine to stumble and quit if left leaned.
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Agreed. The big word in your statement is “if”. I’ve seen countless pilots lean ‘a little’ on the ground. Lukewarm to the point of not really doing much good (stop fouling) yet lean enough to possibly ‘redbox’ the engine upon applying takeoff power.


Sent from my iPhone using Tapatalk
 
bobmrg said:
"Some CFIs" should have their heads examined if they do not teach their students to check that the power controls are set properly before adding takeoff power. Sheesh!!

Bob Gardner
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For sure.. it's sloppy. Especially when you hear "it's club policy" or some garbage like that you'll often see posted on line by a user.. or like in the case of the OP's post, and his subsequent question on the logic

By the way, as @Low Level Flyer mentioned that's exactly how I was taught to lean when I got back into flying a second time. The CFI noticed I was doing the half A$$ "pull it out a little" trick but then he gave it a monster lean and we really couldn't get much above 1,500 RPM (at all) with it.. I do the same from now on, and get the same surprised looks by people

A) - you certainly won't foul the plug
B) - you certainly won't be able to take off if you ignore the checklist, or don't actually comprehend it (just blindly read and repeat it)
C) - aren't most people taught to just push everything forward? Blue, Red, Black kbons? Your palm should just jam all of it up
 
jordane93 said:
If only there was a checklist to make sure they don’t do this
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The way I see many people read checklists I'm convinced you could put a line in there that says something absurd like "get out of the airplane and wave to the control tower then defecate on the taxiway" and people would blindly read/glaze over it and move on to the next item with zero comprehension being given to what was just read
 
Tantalum said:
PS - some CFIs teach full rich on the ground because I think they're worried a student may attempt a take off with it still leaned for taxi and either experience (at best) a very hot engine or (at worst) very poor engine performance. Some people think that a fouled plug is a smaller risk then someone taking off leaned out

I don't advocate this position.. just what I've heard
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I've heard that excuse before, but the concern is easily and very safely mitigated by leaning aggressively such that the engine just barely runs smoothly at idle. If from there you advance the throttle for take-off without first moving the mixture to rich, the engine will quit. That should get the student's attention. :)

I suspect the real reason why some CFIs teach that is they lack a good understanding of what the fuel/air mixture really does.

- Martin
 
Martin Pauly said:
I suspect the real reason why some CFIs teach that is they lack a good understanding of what the fuel/air mixture really does.

- Martin
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Yup. And their CFI taught them the red knob is only for killing the engine at the end. Which is how -their- CFI taught them...


Sent from my iPhone using Tapatalk
 
The general sage advise on taxi operations is to lean the thing to the point of idle cutoff.
There are two reasons for this. First, on most of our engines, if you don't lean this far at idle settings you're not LEANING. The idle loop keeps the mixture rich through most of the red knob's travel.
Second, if it's landed that much, once you get it beyond idle, the engine will stumble enough for you to notice you've not enrichened it for takeoff.
 
Tantalum said:
PS - some CFIs teach full rich on the ground because I think they're worried a student may attempt a take off with it still leaned for taxi and either experience (at best) a very hot engine or (at worst) very poor engine performance. Some people think that a fouled plug is a smaller risk then someone taking off leaned out

I don't advocate this position.. just what I've heard
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In our club we use checklists. Major point in it is full rich.
 
LongRoadBob said:
In our club we use checklists. Major point in it is full rich.
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No high-density altitude takeoffs in Norway, Bob? The club's checklist is just dandy at or near sea level density altitude. Start adding zeroes to the density altitude number and it becomes dangerously misleading.

Bob Gardner
 
bobmrg said:
No high-density altitude takeoffs in Norway, Bob? The club's checklist is just dandy at or near sea level density altitude. Start adding zeroes to the density altitude number and it becomes dangerously misleading.

Bob Gardner
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That's a good point. Checklists in the POH many times say "...at high density altitudes do...". Typical club checklists that I've seen are written strictly for the airport where the plane is based. And that's usually good enough. The problem is that those students don't get taught the "why" behind everything on that checklist. I can't remember how it happened, but I realized shortly after my first lesson that one of the things I really enjoyed about learning to fly is that there is a reason for everything. If a CFI says, "do it this way", it's perfectly valid to ask why, and then go out on your own and research it yourself.
 
I always made my own checklists (based off of the POH/AFM) for the airplanes I fly. I'd move things around to match how my brain works and change things like "Mixture - RICH" to "Mixture - Set for DA/Takeoff" (or whatever). Or add things like "Towbar - REMOVED". Don't ask...
 
That’s why on some of our checklists it says “as req’d.” For example engine anti ice. Under certain conditions it will be on and other times it will be off. Some items are not always required.

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