To lean, or not to lean?

[crog27]

Question for anyone....I IS IT OK OR ARE YOU SUPPOSE TO LEAN THE MIXTURE OUT WHEN ON THE GROUND? In most of my flight training it has been mixed with most saying you should go lean on the ground. At Riddle is was in the checklist to lean the mixture before taxiing. I've had a couple of Examiners tell me it's bad for the engine to do that. Most of the people telling me not to are people that have a great deal of experience. Just wanted to see what the thoughts are out there

 

[bobbysamd]

Leaning

If you're a Prescott Riddler, it makes sense to lean while taxiing. You don't want to foul the plugs. After all, you're 5000 feet above sea level. By the same token, you should lean before takeoff. Here again, without getting off the ground, you're already 5000 feet above sea level. The manual would tell you to lean, probably above 3000 feet MSL.

Follow your checklist. Somebody wrote it up the way it is for a reason.

Now, if you're a Daytona Riddler and you were told to lean while taxiing, well, I dunno what to tell ya . . . . . . .

 

[Speedtree]

Well, I am not an expert or anything but I have instructed in the Houston area for a few years. We always lean for taxi, usually just an inch or two of mixture travel. I don't know of any way it would harm the engine unless you lean too far. I don't believe you could lean for taxi like you do for takeoff during the run-up because during taxi the rpms are lower. (this also means you need less fuel) so trying to get a specific mixture setting would be difficult. I have experienced spark plug fouling when we forgot to lean or didn't lean enough. It happens more in the higher performance lycomings like the 182s but occasionally in a 152. Maybe there is a mechanical adjustment to fix it or it could be the humidity I don't know and I don't have much control over either one.

Just my thoughts.

Also, I have found that everyone in General aviation has a trick to tell you or a different opinion about something and if you tried to follow everyone's advice it would take you 3-4 hours to get airborne and drive you crazy.

Learn as much as you can, test everything you hear and always be ready to learn more.

 

[starchkr]

Just a quick experience of why you should lean on the ground...

I was ferrying a Beech Sierra from Dallas to Las Vegas with the new owner and Albequerque (sp) was our refueling point. We got to ABQ just fine, but after landing noticed a really rough running engine as we taxied in... note the mixture was full forward just as the old Dallas owners checklist had told us to do. After a short fuel stop and conversation with some firefighters coming off a 73 after fighting forest fires we jumped in and tried to start up for our final leg to LAS. First sign of bad things for this guys "new" airplane... the prop wouldn't turn, come to find out the bendix spring was too loose, and with the help of a local mechanic we were started up and on our way in about ten minutes. (Ok, so now to why you lean on the ground) Because this was his new airplane we decided to do another runup to get him used to the procedure. We got the thing spun up to the required rpm's and everything seemed just fine, but then he brought it back to idle, and what do ya know, the darn thing stopped. Sitting there in ABQ with a Southwest jet behind us taxing and the engine stops leaving us helpless and in the way. We tried and tried and finally got the thing started again with a high power setting, but when we brought it back to idle, guess what... it stopped again. By this time we had let the tower know what was going on and they had begun to reroute the jets away from us... "the long way around to their departure runway," and believe me you could hear it in their voices that they were not happy. Ok, so we got it started one more time and decided to taxi back to the FBO to have it looked at, all the while riding the brakes because taxiing with that much power could have almost gotten us off the ground. So, to make a long story even longer now, we found out that we weren't taking the altitude into consideration when we were taxiing and doing our runup which fouled the plugs which caused them to lose their spark at low RPM's. We were very thankful we didn't get off the ground and find out during our approach into our destination that when you bring the plane to idle the engine quits, or what if something happened right after takeoff. So yes you do lean during taxi and takeoff at higher elevations.

One more little story... while training in Dallas we always brought the mixtures back to the "U" in mixture in the seminole for taxi, and then pushed it forward as required for takeoff at near sea level altitude.

 

[ILLINI]

When I used to instruct at American Flyers, we used to lean out the engine after every flight, as well in order to burn any excess carbon off the plugs. Just before pulling into our ramp we would bring the power up to 1500 RPM and lean out the mixture until we got a drop in RPM, then richen it back up to peak RPM. We would let the engine run in this leaned condition for about 10 seconds and then park and shut down. This made it easier for the next student on start-up. We did this on our 172s and 172RGs because they spent quite a bit of time at lower RPMs during stalls, full stop taxi backs, ground ref. maneuvers, etc. I don't think this would be as necessary if you owned the plane and just used it to go from point A to point B at normal cruise power settings and if you followed the POHs procedure for leaning at cruise.

 

[ShawnC]

I know at least my instructors had me lean on the ground also, thats here in Daytona and on the other side of the state.

For me it seems to run just as well one way or the other but I don't have to deal with the maitnece of the aircraft.

 

[FlyinBrian]

As an instructor in the Phoenix area for a couple summers, I can tell you we always leaned on the ground for taxi. If you didn't, you'd spend an extra five minutes in the run-up area clearing the plugs. If you didn't lean for taxi after landing, you would foul the plugs for the next guy before shutting down. We had many many many many (I can't even tell you how many) newer pilots bring planes back for "bad mags" becuase they'd taxi out, get to the runup area and find that when you switched to L or R it ran like crap. The truth is that they either didn't lean the mixture on their taxi, or the pilots before them may not have leaned it when they came in. Either way, it was very frustrating for our mechanics who would have to taxi the aircraft out, clear the plugs and bring it back. (When someone says they have a bad mag, you can't assume they are wrong.) It made the mechanics absolutely furious, and some poor pilot would then get a lecture about proper leaning technique.

For what it's worth, I found that Cessna's seemed far more prone to low idle plug fouling than Pipers. I couldn't tell you why though.

 

[Wiggums]

I've had a couple of Examiners tell me it's bad for the engine to do that. Most of the people telling me not to are people that have a great deal of experience. Just wanted to see what the thoughts are out there

No matter what you do with the mixture you can't hurt the engine at idle power. Like FlyinBrian said, if you don't lean it you stand a good chance of fouling the plugs. Also, if you have an O-235 (C-152, Tomahawk), then keeping the engine at 1200 RPM on the ground will help prevent fouling also.

 

[Pilotadjuster]

Leaning

Salt lake is 4227 MSL and we always lean prior to takeoff; no way to develop full takeoff power at that altitude without proper leaning prior to takeoff run. Also lean "slightly" for taxi to the runway, as fouled plugs are a possibility (and a pain) with full rich at this altitude. Also--no "full rich" for landing either; have to be ready for the go-around. On a x-country where you've been at altitude, I would lean again when close to the airport and short of the traffic pattern as well.

I have found Cessnas to be more prone to fouling as well; same Lycomings as the Pipers Ive flown. No reason I can see for that either.

PA

 

[low&slow]

Question for anyone....I IS IT OK OR ARE YOU SUPPOSE TO LEAN THE MIXTURE OUT WHEN ON THE GROUND? In most of my flight training it has been mixed with most saying you should go lean on the ground. At Riddle is was in the checklist to lean the mixture before taxiing. I've had a couple of Examiners tell me it's bad for the engine to do that. Most of the people telling me not to are people that have a great deal of experience. Just wanted to see what the thoughts are out there

When people get into these discussions, I always like to quote some type of published material, other then what people "think" or "tell" me to do, or what my instructor told me to do. This is an excerpt from an article that can be found on Avweb written by John Deakin (http://www.avweb.com/articles/pelperch/pelp0019.html). He discusses use of the engine controls (including mixture) during various phases of flight:

Leaning on the ground

Finally, you may find it beneficial to lean your engine after start, and for all ground operations. In theory, a properly set up engine will run at "taxi power" without fouling plugs, but the reality is that most general aviation piston engines are set up so rich (for easy cold-starting) they do often foul plugs. You can test this on your own engine by setting minimum idle RPM, then leaning until the engine quits. Watch the tachometer very closely for a small rise just before the engine quits. The more rise you see, the richer your idle mixture setting. On the big radials, this rise should be almost imperceptible, or "barely detectable," but some of the flat engines call for as much as 100 RPM rise. Again, this is mostly for optimum starting, not running. It's perfectly safe and often desirable to correct this with manual leaning, once the engine is running.

The downside of leaning on the ground is the very distinct possibility of attempting a takeoff that way, so if you lean on the ground, lean it brutally! You can't hurt the engine by leaning at "taxi power," but you sure can cause some heavy damage if you take off with the mixture partially leaned! If you attempt a takeoff while "brutally leaned," the engine will simply wheeze and die when you try to apply throttle. If you enrich at any time, for any reason, either go right to full rich and leave it there for takeoff, or re-lean it "brutally" once again.

According to him (and I consider him far more knowledgeable on the subject then I am) theoretically you shouldn't have to lean on the ground. But as many of you have experienced, if you don't lean on the ground, you will get fouled plugs (esp. during high density altitude). At idle power and a "brutally lean" mixture, the engine isn't developing enough heat to damage anything (assuming it is still running smoothly). The engines are often set too rich on purpose. As Mr. Deakin says, "It's perfectly safe and often desirable to correct this with manual leaning, once the engine is running." I would do it and not worry about it; at such a low power setting, a lean mixture can't damage the engine any more then a rough running engine due to fouled plugs...

Comments?

low&slow

 

[avbug]

If the idle mixture is correct in a carbureted engine, you needn't lean. Leaning has no effect at idle, other than taking the engine to cutoff. At any power setting above idle, adjusting the mixture in the cockpit does make a difference. You cannot adjust idle mixture from the cockpit; this is a maintenance function.

Idle mixture should be adjusted any time the aircraft is moved to a new base of regular operation, that involves a change in elevation, temperature, or climate. However, most operators don't do this. The result is a need to lean agressively.

You cannot hurt the engine at or near idle by leaning. You're best to lean almost to the point of cutoff when taxiing at idle. If you're at this point and inadvetantly attempt to take off or increase power, the engine will bark at you. It will remind you that you're far too lean. This is desirable. Taking off with the power at an intermediate point is a bad thing; if you leave the mixture at an arbitrary point, you can run into engine damage and detonation issues. Either be set for takeoff, or be agressively leaned, but nothing in-between during ground operations.

Most engine manufacturers also recommend a post-flight runup to clear the engine; I've always encouraged students and pilots to do this. Typically it involves a runup to 1,700 to 2,100 rpm just prior to shut down. (You should also routinely perform idle mixture checks and a grounding check at this time).

Performing idle mixture checks will give you an indication of how far off your idle mixture is. Run the engine up just prior to shut down, lean it out, then pull it back to idle. Run the mixture to full rich for a moment and let it idle, then slowly retard the mixture. Just prior to the engine dying, you should see a slight RPM increase of about 25 rpm or less (15-25 rpm is ideal). Less of a rise indicates a lean idle mixture, and much more of a rise indicates a rich idle mixture. If you see a rise of 100 rpm or more, your idle mixture is excessively rich, and needs to be adjusted; this accounts for your fouling problems.

Again, if the idle mixture is set properly, you can taxi all day with the mixture full rich at idle, and have no problems at all. How good is your maintenance?