lean for taxi ?

[minitour]

I think bobbysamd has introduced leaning for take off, which of course you should do above 3,000 ft and there is a procedure for this, its not done at idle.

Lets not confuse leaning for taxi and leaning for takeoff.

That procedure would be....?

I'm guessing apply the breaks, takeoff power, lean to max RPM, then you're set?

-mini

 

[uwochris]

minitour,

According to the Cessna POH, if you are planning to lean on T/O, you should perform a full-power static run-up and lean for peak RPM.

Again, this procedure applies to the 152 I fly. Many a/c will specify different procedures to lean on T/O- in fact, some may even forbid it (I read that if your engine is turbocharged, you should keep the mixture full rich on T/O).

Some POHs also often state that the mixture must NOT be leaned below a certain % power (example: 75%). If your taking-off from a high elevation airport, and your engine puts out more than 75% power at max power, you should still keep the mixture rich, despite the fact that you may be above 3000 feet. Once you throttle back below 75%, you should start leaning.

 

[minitour]

minitour,

According to the Cessna POH, if you are planning to lean on T/O, you should perform a full-power static run-up and lean for peak RPM.

Again, this procedure applies to the 152 I fly...

I knew I saw that somewhere

-mini

 

[midlifeflyer]

AvBug,

I understood what you said. I still don't understand how it works. I currently fly out of Centennial (5883) and lean for taxi every single time. Depending on the density altitude that day, sometimes I need to lean more and sometimes I need to lean less. If what you are saying is correct, every single piston powered airplane I have flown, regardless or make, model, N-number, age, FBO, maintenance shop used, etc, has been set wrong.

That's certainly possible, but I need to understand it. So, let me take a scenario step by step so you can correct me at the source of the error.

My base is Leadville (9927 msl), so my idle mixture is set for that airport.

Now, maybe I am misunderstanding you, but it sounds like what you said is that, with the manual mixture control at full rich, the airplane is already leaned properly for idle operation at Leadville. Therefore it is unnecessary to lean for taxi at Leadville.

1: I don't understand why variations in density altitude don't require leaning? At what temperature is the initial setting based? I don't see how a mixture set, say for standard temperature is proper for a nice balmy 20°C day when the density altitude on the ground is over 12,500'

2: I take off from Leadville and fly to Centennial (5883 msl) just for the day.

As I see it, if the idle mixture is already leaned for the ground at Leadville, it is =way= too lean for Centennial. And there is no way to enrichen the mixture. So the engine starves sometime on approach to Centennial.

 

[uwochris]

I think what avbug is pointing at is that you do not need to lean the mixture while taxiing IF the throttle is at idle. If not, then leaning will have an affect on fuel flow and temps.

This has to do with the construction of the carburetor. I liked the article on "howstuffworks.com." (http://auto.howstuffworks.com/question377.htm.) Note the "idle jet," which is required because there is not enough airflow through the venturi to create a low enough pressure to draw fuel from the nozzle. The idle jet allows fuel flow to the cylinders, and this cannot be controlled by the pilot. It is set by the mechanics.

I hope I'm not off base on your interpretation avbug. If I am, I apologize for causing more confusions!

Chris.

 

[JeffSKDTW]

We took it off our checklists.

Jeff, as a fellow Michigander, I've done just the opposite--I've added it to my checklist after flying in Colorado with Midlifeflyer. In Colorado, I leaned for taxi because I had to (we flew from Denver to Aspen, Leadville (9927'), etc.). However, I do it here in Michigan primarily due to plug fouling. Since doing so, I have not had a single fouled plug.

 

[A Squared]

And there is no way to enrichen the mixture. So the engine starves sometime on approach to Centennial.

Midlife, this statement suggests a misunderstanding of idle mixture. When your engine is idling, meaning the throttle plate is very close to being closed, the fuel is supplied to the engine through an idle jet, little or no fuel is supplied by the main jet in the venturi. The fuel air ratio during idle is controlled by a needle valve which is ground adjustable.

At higher power settings (ie: in flight) the adjustment of the needle valve in the idle circuit has no signifcant effect on the fuel air mixture. it is too small, and because of the throttle position, there is no vacumn present to draw fuel from the idle jet. In flight the fuel air-ratio is controlled by the main metering jet and the position of the mixture control valve. Simply put, the adjustment (or misadjustment)of your idle mixture valve will not result in being unable to enrichen your mixture sufficiently for flight at low altitude. Your airplane will not quit running in the air due to insufficient fuel, while on this hypothetical flight from leadville to denver. You may however, find upon landing at denver that your engine is running quite lean while idling, perhaps too lean to run smoothly. Your only recourse at this point is to have a mechanic run out and adjust your idle mixture valve.

I think that perhaps you are having difficulty sorting out the facts from opinion here. Avbugs facts are, as usual, dead on target. His opinions are have merit also (as they usually do) but you may be blurring the distinction between the two which is hampering your understanding.

I don't understand why variations in density altitude don't require leaning?

whether it *requires* leaning is a matter of opinion. As a *factual* matter, the density altitude *will* affect the idle mixture (notice that avbug states that idle mixture needs to be adjusted as average temp varies seasonally, so we concur on that fact)

At what temperature is the initial setting based?

At the existing ambient temperature on the day it was adjusted. If you had a mechanic adjust your mixture on a morning when the temp was 60 degrees, and a week later you go flying in midafternoon when the temp is 90 F, your idle mixture will be richer. As factual matter it will be approximately the same amount richer as it would be if you had gained 2000' in altitude. Does this require additional leaning? That is a matter of *opinion*

Will the mixture control affect the idle mixture? yes, it will, that is a fact. *Should* you use it to adjust mixture? That is a matter of opinion.

It is true that the mixture control is designed to most effectively regulate fuel flows much greater than idle fuel flows. It is true that it only regulates the idle fuel flow at the extreme end of it's operating range. It is true that if you pull the mixture control out halfway, you haven't affected the idle mixture one iota. It is true that adjusting the mixture while idling requires a careful touch. These are facts. Does this mean that one should not use the mixture to lean on the ground? That is a matter of opinion.

As a factual matter, a mechanic following the prescribed procedures will set the idle mixture rich. (an RPM rise on shutdown indicates a slightly rich mixture) That's what the maintence manuals call for. Whether or not this is the best idle mixture is a matter of opinion. Some may prefer to accept this slightly rich idle mixture setting, others may prefer to adjust it using the mixture control. If you fall in this second group, be aware that moving the mixture control out *a bit* doesn't do anything at all. If you do not move the mixture out until there is a change in RPM you haven't accomplished anything at all.

What do *I* do? While taxiing in my personal airplane, I lean until I get an RPM drop. Why? It won't hurt the engine and it makes me happy. It *may* reduce spark plug fouling. As a factual matter, an engine which is idling very excessively rich will foul sparkplugs. On the other hand, one which is running quite lean will not. So, will an engine idling with the mixture lean of peak foul fewer plugs than it will at it's properly set, slightly rich idle mixture? I like to believe that is true, but I don't have any factual data to support that.

 

[bobbysamd]

Leaning for taxi

I think bobbysamd has introduced leaning for take off, which of course you should do above 3,000 ft and there is a procedure for this, its not done at idle.

Lets not confuse leaning for taxi and leaning for takeoff.

No confusion. In Denver and at Riddle we always arbitrarily leaned the mixture about one-half inch or so for taxi. For takeoff, during runup, power to 1700, lean to peak (or EGT), then back in about two turns (or a couple of notches if a ratchet-type mixture control was present), check mags, etc.

 

[A Squared]

In Denver and at Riddle we always arbitrarily leaned the mixture about one-half inch or so for taxi.

Arbitrary, and completely ineffectual. As stated by several before, pulling back "a half inch" or "half travel" or some other arbitrary amount has no effect. Unless you pull the mixture control back until you get an RPM rise or change in engine sound, you might as well leave it full forward. As several have said, the mixture control valve is a fairly high flow valve, it has no effect on idle mixture until it is almost completely closed (idle cut-off)

Here's an analogy: Open your sink faucet just a little, just enough for a small trickle to flow. Now, go down into the basement and turn the main water service valve halfway toward closed. Go back up to the kitchen, and take a look at the sink. You'll find that the faucet is still running at the same rate. This is exactly what happens to your idle mixture when you pull the mixture knob back halfway, or half an inch: Nothing.

 

[minitour]

Here's an analogy: Open your sink faucet just a little, just enough for a small trickle to flow. Now, go down into the basement and turn the main water service valve halfway toward closed. Go back up to the kitchen, and take a look at the sink. You'll find that the faucet is still running at the same rate. This is exactly what happens to your idle mixture when you pull the mixture knob back halfway, or half an inch: Nothing.

That is a really good way to explain it...wow...you dont mind if I add that to my notes do you?

-mini

 

[A Squared]

That is a really good way to explain it...wow...you dont mind if I add that to my notes do you?

-mini

No, go right ahead ... but if you make any money off it, you gotta cut me in for a share. <g>

 

[ROB-x38]

Some good aircraft/engine systems discussions here.

From http://www.lycoming.textron.com/main.jsp?bodyPage=/support/publications/keyReprints/operation/sparkPlugFouling.html

At engine speeds from 1000 to 1200 RPM, the spark plug core temperatures are hot enough to activate the lead scavenging agents contained in the fuel which retards the formation of the lead salt deposits on the spark plugs and exhaust valve stems.

The above reference suggests spark plug fouling is unlikely at enigne speeds > 1000 RPM.

So taking the case where we are taxiing an aircraft with a throttle setting producing slightly less than 1000 RPM, presumably fuel will be being metered through the main jet. In this case wouldn't a leaner mixture prove beneficial when trying to reduce the likelihood of spark plug fouling?

On the other hand, playing devil's adovate, it's interesting to note that in the above article Lycoming doesn't once mention to lean the aircraft on the ground to avoid fouled plugs. Hmmm...

Thoughts?

 

[avbug]

I've removed and cleaned a lot of fouled plugs that fouled at RPM settings much higher than 1,000, and a at power settings much higher than idle. The reference you quoted doesn't suggest in the least that fouling is unlikely, only that scavenging properties are better at higher power settings.

Temperature has nothing to do with engine RPM, but everything to do with engine power being produced, and mixture setting for any given power setting.

Weather fuel is being metered through the main jet or not depends on the idle setting of the engine. As stated earlier, don't confuse the RPM of the engine with the mixture, liklihood for fouling, temperature, or idle.

 

[midlifeflyer]

Midlife, this statement suggests a misunderstanding of idle mixture.

No doubt.

Thank you. Your explanation was very helpful.

Let's take my hypothetical flight one step more. I'm flying my piston airplane from my home in Leadville to Florida. If I am understanding what you are saying properly, there is really no issue with respect to the mixture at higher than idle power settings.

But might I want to have that idle jet adjusted somewhere along my route? Or, using your slow faucet analogy, is the difference still pretty insignificant?

 

[avbug]

A Squared fauced anology is an excellent one, and works for the cockpit mixture control with respect to idle. The mixture control will have an effect right at the cutoff point, but you aren't adjusting mixture, you're shutting off fuel. You could try the same thing by shutting it off at the selector valve, too.

If you set your idle mixture in leadville, then you'll want to reset it for lower elevations. If were me, and I knew I'd be moving my base, I'd make an enrichening adjustment prior to leaving. Then fine tune it on arrival.

Or, you could always stop on the way. The decision, as always, would be governed by where has the best food selection.

 

[SLagonia]

My Cardinal POH calls for "aggresive Leaning during taxi" -

 

[JeffSKDTW]

Gentlemen -

This discussion has been very enlightening. Thanks.

 

[A Squared]

The decision, as always, would be governed by where has the best food selection.

See, it always comes down to BBQ.

 

[tathepilot]

Gentlemen -

This discussion has been very enlightening. Thanks.

yes, very much so !