How much to lean the mixture?

[JHW]

True for landings. Not for go-arounds. You must enrich the mixture before cramming the throttle (or removing the carb heat) or you risk an engine stall at the worst possible time.

wow, that sounds bad. I should check to see if I'm able to reach the mixture from my normal seated position. Thank you for the information.

 

[Clark1961]

I'm surprised at the number of full power run-up advocates in the thread. We don't see folks typically doing full power run-ups here in the city area much, only up in the hills. Unless they're loaded to max gross or something. Kinda hard on props, sucking crap into them.

Most schools here will lean for peak RPM at normal run up RPM which leaves you a tad rich at full throttle once you go there and then they'll have you twist two-three 1/2 turns in from there... for a normal run-up anyway. They'll recommend the full power run-up for short runways, heavy loads, or any other reason you think you'll need max performance.

We did full power run-ups at Western Air (BJC). No ifs, ands, or buts....

 

[poadeleted20]

I'm surprised at the number of full power run-up advocates in the thread. We don't see folks typically doing full power run-ups here in the city area much, only up in the hills. Unless they're loaded to max gross or something. Kinda hard on props, sucking crap into them.

Not a problem if you're selective about where you run up. Most airports have plenty of places where you can do that.

Most schools here will lean for peak RPM at normal run up RPM which leaves you a tad rich at full throttle once you go there and then they'll have you twist two-three 1/2 turns in from there...

That's not a very precise way to do it, and leaves you subject to the wide variations in carb adjustment I've seen.

And of course, Tom and others are right too... you can adjust after takeoff.

Well, yes, but that will leave you short of best power on the takeoff rule, and I don't know why anyone would want to do that.

Another pro tip from the mechanic long ago that stuck... Carbureted airplane, if you know about where that red knob usually ends up at, after doing that leaning procedure, pull the darn knob out to roughly that point (a couple of inches back usually) for engine START. Makes starting go a hell of a lot easier on you and your engine. ;-)

Never heard that one before, and don't understand the logic. Back in the day, we pulled the choke for start and opened it up once running. Still do that with lawn mowers and generators. Forget to choke it, and you wear out your arm.

On the ground, I lean aggressively enough that the engine will be just about to stumble and will stumble if throttle is added. "Learned" that habit from a Cessna 172RG that LOVED to foul plugs up here. Heck, it loved to foul plugs at any altitude.

Good advice, once running. See that SL 185B mentioned above for more on how to do that and why it's important.

 

[Palmpilot]

With a fixed pitch prop, peak RPM is the only thing that tells you for sure you're leaned for best power, and the only instrumentation you need for that is a tach. EGT is only an approximation, and the amount rich of peak EGT for best power is not a fixed value. What is more useful when leaning at low altitudes is CHT, as that will tell you if you're going to cook the engine, but above 5000 DA that can't happen when leaned for peak RPM unless you have some other problem.

According to this article, CHT alone is not enough to tell you whether the engine is in danger of being damaged:

http://www.avweb.com/news/pelican/pelicans_perch_84_mixture_cht_194816-1.html

 

[MAKG1]

Never heard that one before, and don't understand the logic. Back in the day, we pulled the choke for start and opened it up once running. Still do that with lawn mowers and generators. Forget to choke it, and you wear out your arm.

Especially, hot starts at high density altitudes don't work very well full rich. I tried it (oops) at South Lake Tahoe last summer, 8000 foot DA. It actually started once I pulled the red knob out some 1.5 inches, and opened the throttle to WOT for a few minutes before trying again (flooded).

There really isn't any reason to use full rich at density altitudes above 5000 feet, ever.

 

[denverpilot]

Not a problem if you're selective about where you run up. Most airports have plenty of places where you can do that.

Depends on the airport and their maintenance. Plenty of bad asphalt coming apart around here lately if you're away from the big city airports.

That's not a very precise way to do it, and leaves you subject to the wide variations in carb adjustment I've seen.

What's your recommendation in a carbureted aircraft with factory instrumentation? Remember I'm talking about a "normal" day here (not a hot one) with a 10,000' runway. A Skyhawk can do that at BJC (or APA) with a big yawn factor.

Most folks are far more freaked out about the groundspeed than they are about climb rate after liftoff. (Because they don't run the numbers. That probably should have been point number one by everyone, because people get lazy.)

Well, yes, but that will leave you short of best power on the takeoff rule, and I don't know why anyone would want to do that.

Because the difference between best power from a full power run-up and this technique is really marginal. And I'm discussing a normal day, not heavily loaded or limited by anything else. If you've done your takeoff numbers (see freaked out about groundspeed and ground roll distance above...) the 10% difference or so isn't even worth writing home about unless you're pushing margins. Most airports up here have monster runways for the jet kids... Climbing out of BJC or APA in a Skyhawk, you're usually halfway to pattern altitude before the runway ends, it's just that the runway is 10,000' long.

Never heard that one before, and don't understand the logic. Back in the day, we pulled the choke for start and opened it up once running. Still do that with lawn mowers and generators. Forget to choke it, and you wear out your arm.

It works. Try it. Three blades full rich, one blade set to the right mixture for conditions. I wouldn't have believed it either if I hadn't watched my mechanic start my airplane. Of course, he was the previous owner's mechanic too, so he's probably started our engine more times that each of us has individually, too.

I have no problem telling folks new to high DA ops from the flatland to do a full-power run up, just pick a good spot. Sucking rocks into props isn't smart.

If you don't need 100% performance, being the tiny bit rich a normal run up RPM lean will get you, isn't a problem. Taking off full rich will be a much bigger problem.

I was surprised to see Clark say Western Air requires it. They're the only ones I know in the area who do. Certainly not Aspen, IA, or any of the APA FBOs I've flown with. Only in marginal aircraft or on days when conditions dictated it be done.

They do however require TOLD sheets at Aspen and I believe over at IA. You hand in hard paper copy that shows you did your takeoff and landing calculations. (I suspect with their iPad mania and multiple WINGS courses on same at IA, an e-mail of the data would suffice.)

 

[poadeleted20]

According to this article, CHT alone is not enough to tell you whether the engine is in danger of being damaged:

http://www.avweb.com/news/pelican/pelicans_perch_84_mixture_cht_194816-1.html

I won't argue with that, but with this engine/plane, you won't damage the engine as long as you stick to the Lycoming recommendations and lean for takeoff/climb solely by RPM. Further, neither EGT nor CHT will help you get more power.

 

[poadeleted20]

What's your recommendation in a carbureted aircraft with factory instrumentation?

Just what I said up front -- full throttle, lean to peak RPM, and leave it there for takeoff.

Remember I'm talking about a "normal" day here (not a hot one) with a 10,000' runway. A Skyhawk can do that at BJC (or APA) with a big yawn factor.

...until the engine quits due to plug fouling from the overrich mixture or runs out of steam due to an overlean mixture. Nothing based on some number of turns of a vernier mixture or clicks on a "regular" mixture is really reliable. Full throttle/lean to peak RPM is always reliable and the most accurate method, so why use anything less?

Most folks are far more freaked out about the groundspeed than they are about climb rate after liftoff. (Because they don't run the numbers. That probably should have been point number one by everyone, because people get lazy.)

That's a training issue.

 

[denverpilot]

...until the engine quits due to plug fouling from the overrich mixture or runs out of steam due to an overlean mixture. Nothing based on some number of turns of a vernier mixture or clicks on a "regular" mixture is really reliable. Full throttle/lean to peak RPM is always reliable and the most accurate method, so why use anything less?

So you're willing to put the engine squarely in the red box on a cold day here, and can also ignore the auto-lean effect, if you have to do an early power reduction that takes the WOT enrichening feature of most carbs out of the loop?

You also trust the induction system is so even to all cylinders, without an engine monitor, that they're all operating at peak lean or richer, by setting peak RPM?

I'm not quite that trusting, especially on an O-470... Which I understand isn't the engine the OP will be flying behind on a Skyhawk.

A hard fast rule to be at peak RPM has the potential to cause engine damage on a cold day here, since we're often in that zone of higher power being produced that leads to "red box" performance numbers.

On a hot day, I can live with your technique. Not going to produce enough power to hurt anything no matter where you put the red knob. It'll quit before you hurt it.

Colder weather, it's getting a tweak toward the rich side, if I'm paying for the overhaul. Full rental power on a cold day here is fine, if you're not buying the engine parts.

IMHO, peak RPM isn't always where you want to be. Gotta think about power level that's going to be produced and be smart about it. I doubt it's more than 100' of runway different, if anything at all that's consistently reproducible. I'd be willing to gather data if it's hot out, but since we're well into colder weather, it'll have to wait until next summer unless someone wants to go flog someone else's engine and report back.

 

[JHW]

So you're willing to put the engine squarely in the red box on a cold day here, and can also ignore the auto-lean effect, if you have to do an early power reduction that takes the WOT enrichening feature of most carbs out of the loop?

Do you really think a low-powered engine with a fixed pitch prop operating at a high enough airport where this is a concern, has anything resembling a "red box" ?

 

[Tom-D]

Do you really think a low-powered engine with a fixed pitch prop operating at a high enough airport where this is a concern, has anything resembling a "red box" ?

" THE RED BOX"

Is an operating area of the engine that will produce high CHTs, that does the damage.

how much MAP do you need to get HIGH CHTs.?

 

[Henning]

So you're willing to put the engine squarely in the red box on a cold day here, and can also ignore the auto-lean effect, if you have to do an early power reduction that takes the WOT enrichening feature of most carbs out of the loop?

You also trust the induction system is so even to all cylinders, without an engine monitor, that they're all operating at peak lean or richer, by setting peak RPM?

I'm not quite that trusting, especially on an O-470... Which I understand isn't the engine the OP will be flying behind on a Skyhawk.

A hard fast rule to be at peak RPM has the potential to cause engine damage on a cold day here, since we're often in that zone of higher power being produced that leads to "red box" performance numbers.

On a hot day, I can live with your technique. Not going to produce enough power to hurt anything no matter where you put the red knob. It'll quit before you hurt it.

Colder weather, it's getting a tweak toward the rich side, if I'm paying for the overhaul. Full rental power on a cold day here is fine, if you're not buying the engine parts.

IMHO, peak RPM isn't always where you want to be. Gotta think about power level that's going to be produced and be smart about it. I doubt it's more than 100' of runway different, if anything at all that's consistently reproducible. I'd be willing to gather data if it's hot out, but since we're well into colder weather, it'll have to wait until next summer unless someone wants to go flog someone else's engine and report back.

There is no 'Red Box' above a 5000' DA with a Naturally Aspirated aircraft engine. DA takes temp into account.

 

[denverpilot]

Do you really think a low-powered engine with a fixed pitch prop operating at a high enough airport where this is a concern, has anything resembling a "red box" ?

It's not really the high airports, it's the ones between 3000-6000' DA that are above the manufacturer's 3000' breakpoint in the POH for full-rich vs leaned.

We do see days below 6000' DA here in the winter, and the OP was asking about BJC. He also didn't give a time of year.

 

[poadeleted20]

It's not really the high airports, it's the ones between 3000-6000' DA that are above the manufacturer's 3000' breakpoint in the POH for full-rich vs leaned.

Which manufacturer is that? For all 172's built after the 1967 model year (other than the XP's with the 195 HP Continental engine), that's Lycoming, and they give a 5000 DA breakpoint for leaning for takeoff and climb.

 

[Palmpilot]

Colder weather, it's getting a tweak toward the rich side, if I'm paying for the overhaul. Full rental power on a cold day here is fine, if you're not buying the engine parts.

And since the peak in the horsepower vs. mixture curve is rather broad, I'm thinking that your tweak toward the rich side would not result in a significant reduction in power, so I don't see any significant downside in doing it.

 

[Pilawt]

Which manufacturer is that? For all 172's built after the 1967 model year (other than the XP's with the 195 HP Continental engine), that's Lycoming, and they give a 5000 DA breakpoint for leaning for takeoff and climb.

Not the 172N (Lyc. O-320-H2AD). From the 1977 C-172N POH, page 4-15:

The POH for the 1975 C-177B (Lyc. O-360) says the same thing, and the AFM for the current C-172S says for enroute climb, "Mixture Control - RICH (above 3000 feet pressure altitude, lean for maximum RPM)."

 

[poadeleted20]

Not the 172N (Lyc. O-320-H2AD). From the 1977 C-172N POH, page 4-15:

The POH for the 1975 C-177B (Lyc. O-360) says the same thing, and the AFM for the current C-172S says for enroute climb, "Mixture Control - RICH (above 3000 feet pressure altitude, lean for maximum RPM)."

It may say that in the Cessna POH, but Lycoming built the engines, and if you read the Lycoming O-320/360 Operator's Manuals, it says otherwise.

 

[Dan Thomas]

It may say that in the Cessna POH, but Lycoming built the engines, and if you read the Lycoming O-320/360 Operator's Manuals, it says otherwise.

As I understand it, the POH trumps the engine maker's recommendations.

From Lycoming's Service Instruction SI1094D, a couple of excerpts:

First, on normally-aspirated engines:

"5. If manual leaning of the mixture is permitted at take-off, climb power, or high performance cruise, it will be specified in the POH/AFM and will list required ranges for fuel flow, power settings, and temperature limitations."

On turbocharged engines:

"2. If manual leaning of the mixture is permitted at climb power, it will be specified in the POH/ AFM and will list required ranges for fuel flow, power settings, and temperature limitations."

http://www.lycoming.com/support/publications/service-instructions/pdfs/SI1094D.pdf

Dan

 

[Pilawt]

It may say that in the Cessna POH, but Lycoming built the engines, and if you read the Lycoming O-320/360 Operator's Manuals, it says otherwise.

Where do the Lycoming operators' manuals say otherwise? I've looked through the Lycoming manuals for both the O-320 and O-360 series and can't find any reference to a specific altitude below which leaning in climb should not take place.

The Lycoming manuals state, "during [...] climb, roughness or loss of power may result from over-richness. In such a case adjust mixture control only enough to obtain smooth operation -- not for economy. Observe instruments for temperature rise."

The only reference to 5,000 feet is in the next sentence, "Rough operation due to over-rich fuel air mixture is most likely to be encountered in carbureted engines at altitude above 5,000 feet." That's not the same as saying that leaning should not occur below 5,000, and is not inconsistent with the Cessna manuals over the past forty years. I'm inclined to give Cessna engineers credit for a pretty good understanding of how to operate these engines.

For these same engines, Grumman-American manuals read: "The mixture should be full rich during takeoff and climb at altitudes below 5000 ft MSL. However, during takeoff or climb from high-altitude airports, the engine should be leaned to achieve best power (maximum RPM)."

Piper manuals: "The mixture should be leaned during cruising operation above 5000 feet altitude and at pilot's discretion at lower altitudes when 75% power or less is being used. If any doubt exists as to the amount of power being used, the mixture should be in the FULL RICH position for all operations under 5000 feet."

So who is right? Which is most consistent with Lycoming's manual? More importantly, which is most consistent with best and safest operation of these engines?

 

[jesse]

Agreed.

Agreed +2

To a point. I increase the mixture to the point needed to be able to successfully do a go-around generally when turning final.

 

[Tom-D]

It may say that in the Cessna POH, but Lycoming built the engines, and if you read the Lycoming O-320/360 Operator's Manuals, it says otherwise.

which take presidents the flight manual/operators manual or the Lycoming engine manual?

 

[Henning]

I lean at all altitudes including 25'MSL, I just pull it back and cruise. If heading for altitude I will typically cruise climb LOP.

 

[denverpilot]

I lean at all altitudes including 25'MSL, I just pull it back and cruise. If heading for altitude I will typically cruise climb LOP.

Yes we know, we all do. We're talking about takeoff power. Are you? Do you ground lean for takeoff at 25' DA?

 

[denverpilot]

P.S. I'm utterly floored that I knew a minute detail of Cessna POH's that Ron wasn't aware of. I think hell just froze over.

Granted, it just comes from having flown up here near 6000' MSL since I was 19, and having been taught by every single instructor that leaning during run-up to peak lean, then a couple of turns to the rich side, is the place to be to make our engines happy unless you're runway length constrained, that whole time. (Like I said, Clark's comment that they're teaching full-power run-ups at one FBO at BJC these days was a surprise.)

Cessna says 3000'. Typically unhelpful, they don't say 3000' DA, they use MSL which may be why they set this leaning breakpoint in their checklists, so low. I've always noticed the number of glaring discrepancies between Cessna and the engine manufacturers for high-DA ops and the training provided here. Most of the real operational info seems to be passed along from CFI to CFI.

The engine management info from the various places like GAMI and others that started coming out in the 90s has always had more published science behind it (well, that we pilots get to see anyway) than anything ever published by Cessna, that's for sure. I've always appreciated all their work, even if there's some agenda bias behind it all.

 

[Henning]

Yes we know, we all do. We're talking about takeoff power. Are you? Do you ground lean for takeoff at 25' DA?

Not under normal circumstances.

 

[poadeleted20]

Greg was correct -- it wasn't in the O-360 Operator's Manual. It was in the Lycoming Key Reprints:

2. For 5,000 feet density altitude and above, or high ambient temperatures, roughness or reduction of power may occur at full rich mixture. The mixture may be adjusted to obtain smooth engine operation. For fixed-pitch propellers, lean to maximum RPM at full throttle prior to takeoff where airports are at 5,000-feet density altitude or higher. Limit operation at full throttle on the ground to a minimum. For direct-drive and for normally aspirated engines with a prop governor, but without fuel flow or EGT, set throttle at full power and lean mixture at maximum RPM with smooth operation of the engine as a deciding factor.

 

[Henning]

I don't see a single thing limiting leaning below 5000', this reads like at 5000' you should no longer use the option to not lean.

 

[denverpilot]

What if he's flying a Continental O-300 model?

 

[Tom-D]

What if he's flying a Continental O-300 model?

In position and hold, full throttle, mixture pull to best RPM, release brakes, airspeed to best climb (70mph), and tweak mixture to best vertical speed.

 

[Tom-D]

How many ways can the fuel leave the float bowl in a Marvel MA3-SPA carb?

 

[denverpilot]

In position and hold, full throttle, mixture pull to best RPM, release brakes, airspeed to best climb (70mph), and tweak mixture to best vertical speed.

Heh. That's not what I was tweaking. I was tweaking Ron's Lycoming reprints.

Whats's the Continental equivalent?

Do they contradict Cessna's POH like the Lycoming docs do?

"Inconsistency breeds contempt."

 

[Tom-D]

Heh. That's not what I was tweaking. I was tweaking Ron's Lycoming reprints.

Whats's the Continental equivalent?

Do they contradict Cessna's POH like the Lycoming docs do?

"Inconsistency breeds contempt."

both manufacturers have publications on how to best operate their products. both are pretty close to the same idea, both manufacturers have verbiage to say power is controlled by mixture.

the 5000' rule is an old wives tale, promoted by the early Cessna POHs, but has been proven wrong many times over by the LOP guys.

 

[Henning]

How many ways can the fuel leave the float bowl in a Marvel MA3-SPA carb?

An intact one working properly? I'm guessing 2 jet circuits, the vent, the inlet, and I'm pretty sure that carb has an accelerator pump as well, so there is that nozzle. Anything else? It's been 20 years since I had one apart in my hands cleaning it I just don't remember the metering circuits in it.

 

[denverpilot]

both manufacturers have publications on how to best operate their products. both are pretty close to the same idea, both manufacturers have verbiage to say power is controlled by mixture.

the 5000' rule is an old wives tale, promoted by the early Cessna POHs, but has been proven wrong many times over by the LOP guys.

Did you miss that Cessna says 3000' and its Lycoming that says 5000'?

If 5000' is wrong, it blows your statement that the manufacturers have publications on how best to operate their products. Lycoming's "reprints" mentioned by Ron above, are the source of the 5000' number, not Cessna.

This is the inconsistency that I dislike in most engine operation data. The manufacturer of the aircraft and the engine manufacturers never seem to agree until you get to turbines.

 

[Henning]

Did you miss that Cessna says 3000' and its Lycoming that says 5000'?

If 5000' is wrong, it blows your statement that the manufacturers have publications on how best to operate their products. Lycoming's "reprints" mentioned by Ron above, are the source of the 5000' number, not Cessna.

This is the inconsistency that I dislike in most engine operation data. The manufacturer of the aircraft and the engine manufacturers never seem to agree until you get to turbines.

Sigh, fuel mixture cares not one iota as to manufacture of engine or what product it's in. The real problem with what gets taught for mixture management has nothing to do with achieving best effect, but rather to 'pilot proof' their legal standing in case of catastrophic failure.

 

[poadeleted20]

Heh. That's not what I was tweaking. I was tweaking Ron's Lycoming reprints.

Whats's the Continental equivalent?

Do they contradict Cessna's POH like the Lycoming docs do?

Lycoming built the engine, not Cessna, so Cessna's POH is the one in contradiction, not Lycoming's.:wink2:

 

[Henning]

Lycoming built the engine, not Cessna, so Cessna's POH is the one in contradiction, not Lycoming's.:wink2:

Which is governing though? The reality is both are woefully insufficient and aimed at being read by idiots who have absolutely no clue as to how engines operate. They are written in such a way to avoid any hint at liability. Remember the primary function of an operating manual is to limit liability in court.

 

[JHW]

even down here near sea level, on a 100deg day with the pawnee full of water, I'm leaning to my best guess for best power before starting to roll. There's no point in debating CHT's if you don't clear the trees.

 

[poadeleted20]

Which is governing though? The reality is both are woefully insufficient and aimed at being read by idiots who have absolutely no clue as to how engines operate. They are written in such a way to avoid any hint at liability. Remember the primary function of an operating manual is to limit liability in court.

When there is conflict like that, I'll stick with the more conservative approach, in this case, waiting until above 5000 DA.

 

[jesse]

When there is conflict like that, I'll stick with the more conservative approach, in this case, waiting until above 5000 DA.

But is that really the more conservative approach?