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Thread: Leaning on Taxi

  1. #1

    Leaning on Taxi

    Just received the email below from out local FAAST Team representative concerning ground operations.

    "During the discussion a couple of operating habits came to light..... Leaning on taxiing out for takeoff : a carbureted engine has a fixed idle jet. That means a fixed amount of fuel goes to the engine. If you are below 1400 rpm the mixture control does nothing. You should be below 1400 RPM taxiing. Fuel not only cools but lubricates. Unless there is specific information in the POH/AFM that tells you to lean on the ground...it is not a good technique!


    The last issue is plug fouling. This is fairly rare yet I hear about it all the time. Clearing a plug should be done at no higher than run-up RPM and should only occur for less than 30 seconds. Repeated plug fouling or lack of clearing per the manufacturers recommendations means you have a mechanical problem. Get an AMT to look at it. It might be the wrong plug, too cold of a plug, poor timing, etc.

    YOU ONLY HAVE ONE ENGINE AND YOUR LIFE DEPENDS ON YOUR KNOWLEDGE OF HOW TO CARE FOR IT. "

    At our club we insist that our members lean during taxi - I am a little surprised that they are also suggesting that the idle jet is set to 1400 rpm. Should this not be set to 600 -650 rpm ? Since this is from a source of some authority just wanted to check if I'm missing something ?

  2. #2
    Hiperbiper's Avatar
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    Quote Originally Posted by firstflight View Post
    Just received the email below from out local FAAST Team representative concerning ground operations.

    "During the discussion a couple of operating habits came to light..... Leaning on taxiing out for takeoff : a carbureted engine has a fixed idle jet. That means a fixed amount of fuel goes to the engine. If you are below 1400 rpm the mixture control does nothing. You should be below 1400 RPM taxiing. Fuel not only cools but lubricates. Unless there is specific information in the POH/AFM that tells you to lean on the ground...it is not a good technique!
    So if the mixture control doesn't do anything below 1400 how does this guy stop his motor at the end of the flight? A log in the prop? The fixed idle jet (orfice) is goverened by two things: flow thru the venturi section of the carb and fuel avialable to be pulled from the idle jet. The mixture control limits the amount of fuel available to be pulled thru the orffice. Hence you can (and should) lean on the ground.
    He is correct in one reguard; a rich mixture DOES cool the motor but it is at the top end of the RPM range not the idle/taxi portion of the flight!


    The last issue is plug fouling. This is fairly rare yet I hear about it all the time. Clearing a plug should be done at no higher than run-up RPM and should only occur for less than 30 seconds. Repeated plug fouling or lack of clearing per the manufacturers recommendations means you have a mechanical problem. Get an AMT to look at it. It might be the wrong plug, too cold of a plug, poor timing, etc.

    ...fairly rare yet I hear about it all the time. Which is it?? Someone should tell this Boob that forced use of 100LL in a low compression motor meant to use much less lead will foul the plugs in short order. Leaning on the ground (which bozo doesn't think is possible) TCP and lean runups post-flight help to keep the plugs clean. Unless you are using oil AND fouling the plugs an A&P ain't gonna do anything but lighten your wallet.


    YOU ONLY HAVE ONE ENGINE AND YOUR LIFE DEPENDS ON YOUR KNOWLEDGE OF HOW TO CARE FOR IT. "
    Then this guy is a dead man walking...


    At our club we insist that our members lean during taxi - I am a little surprised that they are also suggesting that the idle jet is set to 1400 rpm. Should this not be set to 600 -650 rpm ? Since this is from a source of some authority just wanted to check if I'm missing something ?
    The idle jet as well as the mid- and high range jets are controlled by the throttle (butterfly position/vaccum signal) and the mixture control (availible fuel thru each jet). You set the idle by limiting the air flowing thru the carb venturi...less air passing the butterfly the less fuel is picked up by the airstream and the less RPM the motor achieves.



    Chris
    Last edited by Hiperbiper; 11-08-2012 at 11:59 PM.
    You Tube only proves that more airplanes have crashed due to Video Camaras than any other single reason...

  3. #3

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    The official advice sort of simplifies everything to the point of uselessness. But if you do not want a detailed explanation and are happy with a dumbed down operating rule, stop reading now.

    First, the "jets" are not cockpit adjustable. In fact, they are fixed. You can control airflow and fuel flow from the cockpit. The typical aircraft carburetor has an idle circuit that includes a jet, and a mid-range circuit with another jet. The airplane idle speed and fuel mixture is set using two knobs on the carburetor that are not connected to any cockpit controls. With the cowling off you set the engine idle speed and the idle fuel flow using those on-carburetor controls. Best done in the winter (high air density) at sea level if you want to tweak it to the lowest fuel flow where the engine runs. Otherwise you might fly from Colorado to Delaware in January and find that the engine shuts down when you pull the throttle to idle to land. The idle circuit is the main contributor to fuel flow up through 1400 RPM. Then its contribution to fuel flow becomes smaller than what the mid-range circuit provides. Note: One does not turn off and another turn on. They all keep contributing fuel and it is the proportion that changes.

    The main control that pilots have for engine power is the throttle control. As noted in a post above, the cockpit throttle control is connected to the carburetor butterfly that allows more or less air mass into the engine. The air flow through the carburetor pulls fuel through the jets into that air flow. The on-carburetor adjustable idle controls and the cockpit connected mixture control set the amount of fuel that is available to mix into the air mass in the carburetor throat.

    As stated above, the operation of the idle circuit and the mid-range circuit overlap. So even at idle RPM's, the mid-range circuit is providing a little fuel. Once the RPM gets to around 1400RPM, the mid-range circuit starts providing most of the fuel, but the idle circuit is still contributing its fuel.

    So when you are taxiing at 900RPM, and you pull the mixture out, you are not reducing the fuel provided by the carburetor idle circuit, but you ARE reducing the fuel provided by the carburetor circuits that provide fuel flow at higher throttle settings (butterfly more open). This reduction in fuel flow, and slightly higher combustion temps, can help keep you plugs from building more lead in them and therefore fouling.

    So I will offer the opinion that leaning while taxiing with a carbureted engine certainly won't hurt and will likely help.

    So why do plugs foul? I will offer the opinion that this is often the result of the pilot over-priming and simply washing some dirt into a lower plug to short it out. Every engine model seems to need a different amount of prime on a cold day and I think that I see way too many pilots put way too much prime into their engines. Too much gas is worse than not enough. Read the manual. One of my pet peeves is watching a guy who is used to flying behind a Continental try to prime a Lyco the same way. Or vice versa. They are different animals at start time.

    On the topic of clearing a fouled spark plug, my observation is that the traditional technique of running the RPM up to 2000 and leaning for 30 secs or so is marginally effective. My personal experience is that what clears a fouled spark plug is increased heat and pressure in the combustion chamber. So my personal technique is to point the airplane in a direction where the prop wash will not blow anyone or anything away, pull the control stick or yoke full aft, stand on the brakes, and run the rpm up to 2200 or 2300. If you have an engine monitor that shows you what is happening in all of your cylinders, you will see the problem plug start running and can immediately reduce the throttle back to mag check RPM. If you are not comfortable with this, then taxi back to the hangar, pull and clean plugs, and try to execute a better, cleaner, restart.

    Every airplane is a little different so the best thing that you can do is read your manual and look under the cowling to see how it works.

    Hope this info is helpful,

    Wes
    N78PS
    Last edited by WLIU; 11-09-2012 at 07:23 AM.

  4. #4

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    Doesn't a lot of this depend on the airport itself?

    At our little airport very little time is spent on taxi operations....maybe two minutes from hangar area to threshold, in the walking-slow taxi of a Champ to the typical end for take-off.

    By the time one adjusted the mixture for taxi it would be time to put it back for run-up and take-off (non-towered, usually vacant).
    The opinions and statements of this poster are largely based on facts and portray a possible version of the actual events.

  5. #5
    Thanks to Chris & Wes for illuminating the overlap between idle and mid range circuit and for the clear discussion over fixed on-carb settings V's pilot controllable settings. Typically rpm speed at taxi is around 1,100 rpm - so well into the range where the higher fuel flow settings are contributing.


    I have always understood that the richer the idle fuel mixture is, the more deposits on the spark plug. Whilst there may be other reasons for not leaning whilst taxiing, Frank points out one - I found the blanket statement that " it is just not good practice" alarming.


    The point about over priming is well noted and actually this would have been a much more helpful operational point to make - As Wes says It is easier to put more fuel in than take it out. The aim again being to reduce the amount of combustion deposits created by excess fuel.

  6. #6

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    FirstFlight,

    I disagree with the Fast info.

    I can't recall all of it but here is what I know for sure:
    In my Bonanza, which is a 1988 B 36 TC with a Cont TSIO 520, leaning at low rpm is good. If you prime as the manual says and start at full rich, the engine will start ok, but soon run sluggishly and "heavy", not really clean ,showing that it is too rich. As soon as you start, and idle at perhaps 700 rpm, I begin to lean the mixture. The engine will run cleaner, speed up several hundred rpm and smooth out. To get peak idle speed and smoothness, the mixture control will come back about 2/3 to 3/4 travel, and I can also see the combustion better and warmer on my Gem cylinder indicator.

    So his claim that leaning the mixture doesn't affect it below 1400 rpm is definetly false, at least for my plane.

    Now this is a fuel injected engine and I usually am starting at around 5000 feet to 8000 feet airport elevation, but I think it will also be true for many carb type engines like a Cessna 172.
    !72s are also prone to fouling plugs in the 4 cylinder Lycomings if too rich at idle.

    I think what Wes writes is much more of the truth than the Faast stuff.
    Last edited by Bill Greenwood; 11-12-2012 at 05:49 PM.

  7. #7
    Hiperbiper's Avatar
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    "So when you are taxiing at 900RPM, and you pull the mixture out, you are not reducing the fuel provided by the carburetor idle circuit, but you ARE reducing the fuel provided by the carburetor circuits that provide fuel flow at higher throttle settings (butterfly more open). This reduction in fuel flow, and slightly higher combustion temps, can help keep you plugs from building more lead in them and therefore fouling."

    I disagree. If you taxi @ 900 RPM and pull the mixture back what happens? The motor quits! You just reduced the available fuel for ALL the jets (fixed runs/orfices) in the carb/FI unit including the idle circuit.
    The mixture knob controls ALL fuel flow to the carb/FI unit low, mid and high. The same knob that leans the mixture for taxi on the idle jet allows you the lean for best mixture thru the main jet(s) in flight.
    Try this for yourself: Start the motor. Allow it to warm up. Now pull the mixture back to the point the motor starts to quit at idle and then give it just enough mixture to make the motor run smooth. You are now idling at you leanest idle mixture you can (highest EGT).
    Now; without touching the mixture knob, try and advance the throttle. The motor will die. What you have just found is the mixture knob DOES control the fuel that is availible to ALL the jets in the carb/fuel servo including the idle circuit.

    Again; the throttle butterfly is controlled by the throttle knob and thus control the amount of AIR going to the motor.
    The mixture control controls ALL orfices available to put fuel into the venturi. High and low it's the same control.


    You can kill the motor at any throttle setting using the mixture control.


    Chris
    You Tube only proves that more airplanes have crashed due to Video Camaras than any other single reason...

  8. #8
    Quote Originally Posted by Hiperbiper View Post
    You can kill the motor at any throttle setting using the mixture control.
    Not with a Stromberg. Mine won't kill the engine at idle, and, in fact, you can't even tell that the mixture is out until the RPM goes past a thousand or so. A lot of Strombergs just have the mixture control wired full rich.

    But I do lean my carb when taxiing...pull it out all the way, in fact. I figure it's gotta be doing *something*, when I'm running 100LL.

    I haven't had any plug fouling problems yet. Don't know if its the leaning, the Marvel Mystery Oil, or the sacred Paul Poberezy talisman on the altar in my hangar.

    Ron Wanttaja

  9. #9

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    "Now; without touching the mixture knob, try and advance the throttle. The motor will die. What you have just found is the mixture knob DOES control the fuel that is availible to ALL the jets in the carb/fuel servo including the idle circuit."

    You are incorrect in your understanding of how the carburetor idle circuit works. What you example has done is to shut down the mid-range circuit, then supply more air than the idle circuit can provide fuel for. The mixture downstream of the carburetor is now too lean to support combustion so the engine gets quiet. You can kill the engine by providing too much air or too much fuel. The example provided too much air for the amount of fuel that the idle circuit can provide.

    Strombergs - My reccollection is that the small engine carbs in Champs and Cubs do not have a mixture control. So you cannot lean when taxiing making the distance to the runway the largest factor in the build up of lead in the plugs when running 100LL fuel. Another reason to find a grass airport where you do not have to wait for takeoff behind a bunch of IFR departures.

    "I haven't had any plug fouling problems yet. Don't know if its the leaning, the Marvel Mystery Oil, or the sacred Paul Poberezy talisman on the altar in my hangar." I vote for the statue of "Pope" Paul.

    Best of luck,

    Wes
    N78PS
    Last edited by WLIU; 11-13-2012 at 06:56 AM.

  10. #10

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    Quote Originally Posted by firstflight View Post
    Unless there is specific information in the POH/AFM that tells you to lean on the ground...it is not a good technique!
    If it's such a bad idea, wonder why the FAA has a publications that advise to lean during taxi? The FAA continues with it's long history of contradicting itself. What else is new? Like a Kansas wheat stalk, just "lean" whichever way the wind blows.....

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