lean for taxi ?

[minitour]

Above what RPM are you usually out of the idle setting?

I would assume the POH would have that number somewhere in either the performance or normal operations (normal checklist) sectioins...

Like for the Cessnas (152/172) after engine start it recommends 1000 RPM (and after runup). I would just assume that 1000 RPM is the "idle" position.

Just a guess though....been wrong before

-mini

*edit*
then again that might not make sense since they don't build the engine...

 

[DGdaPilot]

So you've stated why it's pointless to lean a carb-equipped engine at idle, but what about a fuel injected one? They don't have primaries and secondaries like carbs.

 

[minitour]

So you've stated why it's pointless to lean a carb-equipped engine at idle, but what about a fuel injected one? They don't have primaries and secondaries like carbs.

Doesn't a fuel injector already regulate how much fuel is going into the cylendars (I don't have much experience with injectors). I realize that you do have to lean them out at altitude, but wouldn't the injection system already be putting the right amount of gas in at idle?

*curious*

-mini

 

[avbug]

Trying to think in terms of RPM for idle and idle mixture is incorrect. Most light piston recips produce between 600 and 800 rpm at idle, however, idle, and the idle mixture, occurs when the throttle plate is closed. Airflow around the throttle plate is served fuel by a jet in the passage wall, rather than centered in the passage (venturi) like the main jet(s).

Fuel injected systems use one of several different types of fuel distribution and control methods. Fuel delivery is generally somewhat erratic at best at very low power settings and low fuel flows, and improves in accuracy and delivery, as well as general mixture and combustion in the cylinder, at higher power settings, and fuel flows. Burning tends to be a little more uneven and sporadic at lower power settings, and the disparity between cylinders so far as mixture is greatest when operating at these lower power settings.

 

[bobbysamd]

Leaning for taxi

In Oklahoma City, field elevation 1,299 feet, and in Florida, sea level, we did not lean for taxi. In Prescott, Arizona, field elevation 5,045 feet, ERAU had us lean for taxi and, of course, for takeoff. Same in Denver, with a field elevation of 5,883 feet. Of course, the objective is to avoid fouled plugs.

Think of it this way. Although you are firmly on the ground, the airplane thinks it is at altitude. I realize there are many schools of thought about which altitude to lean, but I learned to lean for cruise at 3,000 and above.

 

[avbug]

Again, regardless of density altitude, if the idle mixture setting is adjusted properly, there is no need to lean. The problem is that too many owners, schools, and rental facilities see idle mixture adjustment as something that's done when the carburetor is installed or reinstalled, and that's it.

If the base elevation of the aircraft is changed, or the average temperature for a given elevation is changed (such as occurs seasonally), then the idle mixture should be readjusted as a maintenance item.

You can check the idle mixture by yourself, and should always do so as part of your post flight engine checks (just prior to shutdown). An idle check, and an idle mixture check should be part of the shutdown proceedure (and if you'll read the engine manufacturer guide, you'll probably find that for the aircraft you're flying, it is).

Run the engine up to runup RPM (typically about 1700, though it may be governed by manifold pressure instead; some engine manufacturers and aircraft manufacturers dictate using barometric pressure as the guide, rather than engine RPM...this is especially true of larger recip engines), and lean it to ensure it's clear and not loaded. Run it like that for a minute or so, leaned out, and then retard the power to idle, Return the mixture control to rich. Note the resting RPM with the throttle closed. Your airplane will have a range within which it should be idling, generally 600-800 rpm.

Check to see that it does idle, and that it isn't too erratic. Begin moving the mixture slowly to cutoff. At some point, you should get a RPM rise of no more than 25-50 RPM (check your maintenance manual for specifics), before the engine dies. As the engine begins to die, return the mixture to rich and try it again; you should get the same result.

You've just checked the idle mixture. As you moved the mixture to cutoff, if the idle mixture was adjusted properly, you saw a very slight increase in the idle rpm, often almost imperceptible. If there was no increase, the idle mixture was already too lean. If there was any more than about a 25 rpm increase, it was too rich.

Note that the point on the mixture travel where this occurs is the only place that the mixture control will have any effect on an idling engine. You can move the control where ever you like the rest of the time, to no effect. Try it next time you fly, and you'll quickly see that there is only a very small point on the mixture control, a couple of milimeters of travel often times, that has any effect on that idle mixture.

Bear in mind that other things can cause loading up during taxi. A leaking primer can do so, and can cause a number of other problems, including engine failure. A plunger type primer with a bad packing/o-ring can lead to the engine drawing fuel, or air, and can alter the mixture at the individual cylinder. This is sometimes a little harder to detect, as usually only a few cylinders receive primer; the effect isn't as perceptible.

Before looking at ground leaning practices (which may be important), look at the maintenance the airplane is receiving in the first place. That's always a good place to start.

 

[midlifeflyer]

Again, regardless of density altitude, if the idle mixture setting is adjusted properly, there is no need to lean.

I'm sorry, but that doesn't make sense to me. If the mixture were set for idle at a specific field elevation, wouldn't that cause problems as soon as you landed at another airport?

You may well be right, but if my airplane id set for idle already leaned at Leadville (9927 msl) wouldn't that mean I would be guaranties to starve the engine before I ever landed at a sea level airport?

 

[avbug]

Therein lies the problem with quotations taken partially, or out of context. I addressed this previously by stating:

If the base elevation of the aircraft is changed, or the average temperature for a given elevation is changed (such as occurs seasonally), then the idle mixture should be readjusted as a maintenance item.

You questioned the statement that if the idle mixture is adjusted properly, there is no need to lean.

I stated several times now that the idle mixture must be adjusted for the base elevation. If you change your base, or operate out of a different location, the idle mixture is no longer appropriate and may need adjustment.

If you've adjusted to a high altitude, your idle mixture may be too lean for lower elevations. You may need to open the throttle enough to be operating off your main jets. You can't enrichen it.

Part of the purpose of properly adjusting your idle mixture is to ensure it's slightly rich, as will be noted in the procedure I previously outlined. Allowance is then made for descent to a lesser elevation.

As noted, there is almost no ability to lean the idle mixture using the mixture control; the only leaning capability you have is rotating the mixture to the cutoff position, and using the actual cutoff to modulate what little fuel flow remains. That's a little like trying to regulate fuel flow in flight by partially closing your fuel selector valve...it's not the way the system was designed, and makes very little sense.

If you're constantly flying from leadville, CO, then adjusting your idle mixture for that elevation makes a lot of sense. If you're only going to visit there, then adjusting the idle mixture for that elevation is foolishness.

Leaning is a very misunderstood area among pilots, even experienced ones. Pilots often lean for takeoff at runup RPM, which also accomplishes nothing. Pilots seldom lean properly for landing, or perform post-flight runups as most manufacturers dictate. Tell them aboutit, and the response is usually that they've not heard of it, and therefore it's not the correct way to do business.

Many pilots are afraid of and don't understand lean of peak operations. Many pilots don't understand EGT and CHT...or even the principles of their fuel and induction system. Understanding these things for some is an anoyance, but for others it can become a matter of life and death. Try introducing the subject of carburetor icing, and see how many have absolutely no clue when it comes to formation, recognition, prevention, or removal. None. I've seen folks deadstick the airplane and have no idea that it was carburetor ice...and then be amazed when I've climbed into the airplane and fired it right up for them. Idle mixture is no different.

Idle mixture cannot (should not) be adjusted on the road by the pilot (it's possible, but not legal, nor recommended). Adjust it per the manufacturer recommendations, and do it regularly, and it's not a problem. It is possible to affect the idle mixture by using the cutoff feature of the main jet mixture (the mixture control in the cockpit), but only in a rough way, and not very effectively.

 

[BoDEAN]

Whats the deal with this.
Some people do it. Is it necessary in ny? 82msl

We took it off our checklists.

 

[Vik]

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.