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5X8B
04-12-2010, 05:23
Hello....

Just had a question about leaning. The c172 poh says to lean above 3000.

How important is it to lean at 3-4000, can you get away with not doing it?

I understand higher you will have to.

Thanks to all.

Brad1711
04-12-2010, 07:15
You should lean for cruise at any altitude. Full rich is too rich for cruise even at a density altitude of 0 MSL. At higher power settings, we typically run richer because cooling sucks in piston airplanes and the extra gas going through the engine (and NOT being burnt--just going out the tailpipe) helps to take a lot of heat with it. At cruise power, though, that extra bit of cooling is not needed and running rich is just wasting gas.

3000 feet is a rule of thumb altitude, below which most people generally keep it full rich since the correctly-leaned mixture would be "pretty close" to full rich anyway, and below 3000 feet you're usually in the pattern or doing maneuvers. Leaning below 3000 feet, you won't directly notice any engine performance gains, but your fuel flow will be cut significantly.

At or above 3000 feet, especially getting on up to 5000 or 6000 feet, you'll definitely notice reduced engine performance if you don't lean. Additionally, your fuel burn will be 150% or more compared to what it should be if you are leaning properly.

You should also lean while taxiing or idling on the ground; running full rich on the ground can lead to spark plug fouling and subsequently a failed mag check or developing only partial power when you go to take off. However, going to full rich should be part of your Before Takeoff check because if ever there were a time when the engine's working hard, takeoff is it.

Bottom line: Leaning is not optional.

hm
04-12-2010, 07:39
Running too rich can lead to fouled spark plugs. It definitely leads to wasted fuel. It definitely results in a fuel burn rate that appears nowhere in the POH (so how are you doing your flight planning?)

You _can_ lean any time you're below 75% power. In most circumstances, if you _can_ lean, then you should lean, for the above reasons. The reason that 3000' is mentioned in the POH is because at a density altitude above that, the engine isn't developing full power, and it's okay to run it full throttle while leaned (so it's not that you only have to lean above 3000', it's that there are some circumstances where, below 3000', you should not lean).

So I lean on the ground, and lean in cruise at any altitude. But full throttle maneuvers (takeoffs and climbs) get full rich below 3000', and I go full rich below those altitudes in the pattern before landing, in preparation for the possibility of a go-around.

If you're doing a series of high-power maneuvers, like power-on stalls or slow flight, then you might just leave the mixture rich for the whole series, but most training maneuvering can be done leaned. And if it can be done leaned, then it should be done leaned. There's no convincing reason to try to "get away with" not leaning. If it's beyond a very early student pilot to manage that, then his CFI should be taking care of it.
-harry

jerryp
04-12-2010, 08:05
Leaning is particularly important when making long trips. If you look at the POH cruise performance charts, you'll see "recommended leaning as per Lycoming instructions" on the chart. This means that if you don't lean, then the fuel burn numbers on that chart are useless. You will be burning >10 gph, and if you planned for the 7-8 gph from the charts, you'll be short on fuel.

Most training 172s don't have either an EGT gauge or fuel flow indication, so you'll need to use the "scare the passenger" method of leaning. After you get to altitude and settled in on cruise speed and power, slowly lean until the engine RPM drops or the engine starts to run rough, enrich until it is smooth again then another 1/2 inch rich.

UHBlackhawk
04-12-2010, 14:55
As Harry already pointed out, it's full rich below 3000' DA. Not AGL, not MSL.
I also lean for taxi. As a matter of fact when by myself I will adjust my RPM for about 1200, then lean my mixture and adjust my taxi speed with mixture rather than throttle. Not something I would recommend on a check ride, but you won't hurt anything and I've never had the plugs in a -360 series engine get fouled since doing this.

AFmech97
04-12-2010, 17:12
So this makes me wonder why the school I have been going through has me keeping the mixture full rich all the time. I just did my first solo cross country that way. The engine is fuel injected but there is still a mixture adjustment.

soulie13
04-12-2010, 17:20
So this makes me wonder why the school I have been going through has me keeping the mixture full rich all the time. I just did my first solo cross country that way. The engine is fuel injected but there is still a mixture adjustment.

Most likely because all of your training happens at 3000' or below, and it's just easier that way. And honestly, in the training environment, you are usually in the air for 1.5 hrs with a whole bunch of fuel. Wondering if having your flight last 30 minutes longer never registers that, gee, maybe we're going to run out of go-juice.

And here is where a big gripe about this type of training comes in - neither our OP nor this reply seem to understand *why* you lean. For example, the fact that you have a fuel-injected engine has zippo to do with leaning(you are probably thinking of FADEC). Even on your training cross countries, you go 55 miles or so each way. You probably left with fuel sloshing up along the top of the tanks.

I've always considered this one of the areas where the phrase "Licencse to Learn" is particularly appropiate. In theory, you've talked about leaning, but in the training environment, it just hasn't been something you come face to face with.

So, our intrepid folks here should sit down with their CFIs and learn, I mean, really learn, not just how you learn and some rule of thumbs, but the ins and outs of *why* you are leaning.

God, I'm starting to sound like HighWING.

KeithSmith
04-12-2010, 17:21
So this makes me wonder why the school I have been going through has me keeping the mixture full rich all the time. I just did my first solo cross country that way. The engine is fuel injected but there is still a mixture adjustment.I'm just guessing at their logic, but they may feel there is more danger in students running the engine too lean and they are willing to accept the disadvantages of running too rich rather than risk the disadvantage of running too lean. It is certainly a reasonable question to ask your flight instructor.

Warever
04-12-2010, 17:26
So this makes me wonder why the school I have been going through has me keeping the mixture full rich all the time. I just did my first solo cross country that way. The engine is fuel injected but there is still a mixture adjustment.

If you're too rich for the conditions (altitude and temp), you'll be burning more fuel than you may expect, you'll possibly foul spark plugs, you'll have less than optimal power for any particular throttle setting, and you MAY actually (in theory - I've never heard of it actually happening to anyone) stall the engine (that's engine stall as in a car stalling - NOT an aerodynamic stall). But you're not likely to do MAJOR engine damage.

If you run too lean, however, you can cause detonation which CAN cause serious engine damage. Plus stalling the engine, of course.

A not too uncommon problem is that someone who remembers to lean on the way up may simply forget to lean on the way down - and wind up running too lean. Which, of course, is why the "M" in GUMPS... to remind you to set the mixture properly. If you came down from 12,000 feet, though, and you're now at sea level, you should already have richened the mixture long before the GUMPS check, though.

Perhaps they're afraid that they may have engine damage if students don't properly lean. Pretty dumb in my opinion, though - they're probably far better off just ensuring that students are taught HOW to lean and when.

One interesting side note... one of my instructors (two, actually) always had me lean out during the runup BEFORE taking off in a fuel-injected 172S (though their procedure was to set to a specific setting on the gauge rather than just the "lean till it stumbles" in the air). Their theory was that it often gets pretty hot there so during summer months, you could easily be well above 3,000' DA. That or perhaps their mechanic just always set the "all in" setting WAY too rich.

AFmech97
04-12-2010, 17:33
"And here is where a big gripe about this type of training comes in - neither our OP nor this reply seem to understand *why* you lean. For example, the fact that you have a fuel-injected engine has zippo to do with leaning(you are probably thinking of FADEC). Even on your training cross countries, you go 55 miles or so each way. You probably left with fuel sloshing up along the top of the tanks."





I understand why you would lean the fuel mixture. Being an avionics tech/aircraft mech for 13 years has given me some understanding. (Ha Ha) I was just wondering why a school would teach this way. I was being sarcastic with the fuel injected/mixture adj. comment. I apologize, I know sarcasm doesn't translate well into text.

5X8B
04-12-2010, 18:21
Quote: "And here is where a big gripe about this type of training comes in - neither our OP nor this reply seem to understand *why* you lean. For example, the fact that you have a fuel-injected engine has zippo to do with leaning(you are probably thinking of FADEC). Even on your training cross countries, you go 55 miles or so each way. You probably left with fuel sloshing up along the top of the tanks."
_____________________

I do understand "leaning and why". I was just curious about 30-4000, and if it was that important at those alt's.

Heck, being an engine builder/racer, I probably have leaning down to a science than most; although I lean alky motors, talk about a bomb waiting to go of...

thx

Warever
04-12-2010, 19:24
I do understand "leaning and why". I was just curious about 30-4000, and if it was that important at those alt's.

Rule of thumb.

Technically, the air density (and amount of oxygen in it) is going to vary at every altitude as you climb (or descend). So ideally, you'd be adjusting mixture all the time.

But obviously, that isn't really practical. If it takes you 20 seconds to properly lean, you're not going to be doing it every 30 seconds.

AFAIK, convention is that a change of roughly 3,000' is often enough not to cause major problems, but not so often as to be impractical. If you're climbing 1,000 fpm (good luck in a 172), that's every 3 minutes. Climb 500 fpm and it is every 6.

I've been known to do it more often if the engine is starting to feel like a dog, and certainly, remember that it is DENSITY altitude. You could be flying straight and level but going from a hot valley to over a cool ocean (or vice-versa), and the temps will change, making the density altitude change.

It doesn't HURT to properly lean more often (and by lean, I mean lean to the proper mixture, not necessarily make it leaner) except that it takes away from you doing something else.

shdw
04-12-2010, 19:58
I have not read the thread, this is just my procedure:

I lean above pattern altitude as long as there is a landing field available and workload pending.

Side topic: I'd request every pilot check for a landing site before adjusting the little red guy (mixture).

~Shdw

UHBlackhawk
04-12-2010, 20:20
Probable reasons you have not been shown how to lean:
1. CFIs were not taught how to lean so they don't know the proper procedure.
2. The type of training. Too often training concentrates on going to the training area, doing some maneuvers then return for touch and goes. No problem with doing things like slow flight with your mixture leaned if DA calls for it.
I would recommend some of Mike Busch's Savy Aviator series on mixture and leaning.

bobmrg
04-12-2010, 21:08
Get the information from the horse's mouth: Google Lycoming and drill down to Key Lycoming Reprints. There is quite a discussion about "leaning the direct-drive Lycoming engine." The key is not altitude but percentage of power you are asking the engine to provide. No normally aspirated engine delivers 100 percent, so you are ahead of the game right away. Lycoming doesn't want you to lean when you are asking the engine for more than 75 percent, and you lose power continually as you climb. They have chosen 5000 feet density altitude as the point at which almost all normally aspirated engines can only deliver 75 percent, so you can't go wrong following that advice. However, if you are flying at a lower density altitude and back off on the throttle to less than 75 percent you are home free...lean to your heart's content. It is especially important to lean when taxiing....you are sure not developing much power then.

As noted by others, failing to lean means running an over-rich mixture (duh) which results in plug fouling at a minimum, screws up your flight planning fuel consumption figures, and makes you look like a dummy when you move up to more sophisticated airplanes....but it's not your fault, it is the school's fault.

Get to know a couple of mechanics who work on the school's airplanes and ask them about not leaning below 5000 feet.

My 172P book says "To achieve the recommended lean mixture fuel consumption figures shown in Section 5, the mixture should be leaned until engine RPM peaks and then leaned further until it drops 25-50 RPM. At lower powers it may be necessary to enrich the mixture slightly to attain smooth operation. Should it be necessary to cruise at higher than 75 percent power, the mixture should not be leaned more than is required to provide peak RPM."

I'll admit that my book is for a carbureted engine, but the general rule is valid. In the long run you will do more harm by not leaning than you will be overleaning...because the engine will let you know loud and clear when it does not have the proper mixture.

Bob Gardner

UHBlackhawk
04-12-2010, 23:22
I almost forgot the Lycoming reprints. I would also recommend getting a copy of their engine operations manual. Can get them at esscoaircraft.com for $14.
http://www.esscoaircraft.com/product_p/7022.htm

EppyGA
04-13-2010, 07:39
The later model 172's have it in the checklist after you start to set 1000 RPM and then lean for max RPM before taxi.