Lean for taxi

asechrest

asechrest

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asechrest
For those who don't know, I'm a newly minted private pilot.

For most of my training, my flight school had us lean for taxi. For the later part of my training, school policy suddenly changed to no leaning for taxi. Following this change, I had my first instances of rough-running during mag checks. I asked why the change in policy, but wasn't given a clear answer.

My belief is that a lot of you lean for taxi to prevent spark plug fouling. Now that I'm on my own, I think I'm going to switch back to leaning for tax.

Are there pros/cons here that I should consider? A big con I assume is failing to go full rich for takeoff. But I've got that on my checklist and on my lights-camera-action check.
 
Both major engines in service will benefit from leaning as lean as it will operate at all throttle settings and altitudes.
 
Tom-D said:
Both major engines in service will benefit from leaning as lean as it will operate at all throttle settings and altitudes.
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Thanks. Thoughts on why my school suddenly changed policy? My instructor seemed perplexed about the change. Something as simple as they had students forgetting to go full rich for takeoff, maybe?
 
That seems really odd.

An effective way to keep from taking off lean is to lean aggressively for taxi. Cram the throttle for takeoff and it will really let you know.
 
MAKG1 said:
That seems really odd.
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I thought so. Are there any maintenance-related issues with aggressive leaning? My school was first a maintenance business before branching out into flight training, but I've not heard of any issues in that regard with leaning for taxi.
 
asechrest said:
For those who don't know, I'm a newly minted private pilot.

For most of my training, my flight school had us lean for taxi. For the later part of my training, school policy suddenly changed to no leaning for taxi. Following this change, I had my first instances of rough-running during mag checks. I asked why the change in policy, but wasn't given a clear answer.

My belief is that a lot of you lean for taxi to prevent spark plug fouling. Now that I'm on my own, I think I'm going to switch back to leaning for tax.

Are there pros/cons here that I should consider? A big con I assume is failing to go full rich for takeoff. But I've got that on my checklist and on my lights-camera-action check.
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When I was getting my PPL in North Carolina, I was Never taught to lean for taxi. When I moved to Wisconsin and got checked out in a 172, the chief instructor couldn't believe that I didn't lean for taxi.

It makes complete sense, and now it's automatic, but I'm thinking it might be a matter of how one was trained, or how often the CFI got yelled at by the school's A&P mechanic.
 
asechrest said:
Thanks. Thoughts on why my school suddenly changed policy? My instructor seemed perplexed about the change. Something as simple as they had students forgetting to go full rich for takeoff, maybe?
Click to expand...
If you lean per the engine manufacturers' recommendations, you'll be so lean that the engine won't accelerate past about 1500 RPM when you go to full throttle, choking and sputtering so bad only a complete bozo would not realize there was something wrong and abort the takeoff to figure it out. Maybe your school has some complete bozos flying their planes? :dunno:
 
For a school I can see them just teaching to leave at full rich. Perhaps a couple mistakes were made - close calls, whatever and they just decided the risk wasn't worth the reward. They have to do 100 hours on all those anyway...not a huge deal to pull 4 plugs and clean them out, I guess.

We fly a Cherokee 235 and our most senior partner is adamant about not leaning on the ground. He's been in the plane almost 15 years - they haven't done it and for the most part haven't had any significant issues with the plugs. I can't find anything from Lycoming (it's a O-540) to definitely say anything otherwise, so I don't argue with him. They are also a little squeamish because a previous partner in the plane leaned the hell out of it (mostly in the air) and burnt up a couple cylinders trying save $$ on gas I presume. So, there's that... He's a retired airline Captain with about 15k hours...and I'm a newbie PPL with 70. :dunno:

We did have one mag that was experiencing an out of tolerance drop and we thought it might be fouled plugs. So, we had our A&P swing by and look at them - one was oil fouled a little and a couple were lead fouled - nothing we couldn't clean with a pick and a brush and certainly nothing that looked troublesome. He tested compression, which was fine and we still ended up with the one dropping over 200rpm. Turns out the timing was off a little - he readujsted the timing on that mag and now it's a nice smooth drop - less than 75rpm for both.

I typically will pull it out an inch or so just from habit (my CFI was a A&P and worked on a fleet of Piper's - so I trusted his judgement). If it's not busy and I know I'll taxi - do a quick runup and go, I might not mess with it. If I know I'm going to be waiting or taxiing to the other end of the airport, I'll pull it a little.

Other thing that I have to be careful about in my plane is that it's a push button start with the impulse coupler on the left mag. So, we start with the key on the left mag and then once the engine fires up, turn the key to both. If you forget, you'll for sure foul plugs and get a rough mag during the run up.
 
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Ron Levy said:
If you lean per the engine manufacturers' recommendations, you'll be so lean that the engine won't accelerate past about 1500 RPM when you go to full throttle, choking and sputtering so bad only a complete bozo would not realize there was something wrong and abort the takeoff to figure it out. Maybe your school has some complete bozos flying their planes? :dunno:
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Yes, yes, that's right, and oh ya. :)
 
Cap'n Ron - I've seen both of those. Maybe I'm just missing something here or not 'reading between the lines' but, please direct me to the text that says lean it while taxiing and exactly how much?

I found new information (to me anyway the first time I went through these...) about running up to 1800 prior to shutdown, proper adjustment of the idle speed, operating the engine at 1000-1200 RPM to activate scavenging agents that burn up the lead and a few other interesting things.

However, I find no reference to anything that says yank the mixture out while your taxiing.

I mean, don't get me wrong - I'm not trying to be argumentative here...it makes total perfect sense to me. I have an old Porsche that I do all the engine work on - I'm no stranger to understanding air/fuel ratio's, proper plugs, etc...

All that turbine experience don't mean diddley when it comes to piston engines using leaded fuel.
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I'm pretty damn sure this guy forgot more about flying airplanes in the past week than I've learned thus far. He's a very sharp guy and been an owner in this plane for over 15 years - and we do all owner assist maintenance so it's not like we never touch it. I don't know how many of his hours are in pistons but he didn't start flying 777's the day they gave him his PPL...and whatever the number is...it's a helluva lot more than me.
 
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Ron Levy said:
All that turbine experience don't mean diddley when it comes to piston engines using leaded fuel.
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Yes it does, ask any Turbine pilot what happens when you remove fuel from the engine and they will tell you it looses power.
 
kenjr said:
Cap'n Ron - I've seen both of those. Maybe I'm just missing something here or not 'reading between the lines' but, please direct me to the text that says lean it while taxiing and exactly how much?
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Most POHs will say "lean for taxi" or "lean 1in for taxi". In other words, not a true leaned condition, but not full rich for low-power ops either.
 
As a guy who plays with Auto engines I can understand leaning on carb engines to avoid plug fouling, but what about Fuel Injected units, the injector flow cycle working in sync with the amount of throttle response should control the mixture without the need to manually pull the mixture control back ..Yes?
 
Xscaliber said:
As a guy who plays with Auto engines I can understand leaning on carb engines to avoid plug fouling, but what about Fuel Injected units, the injector flow cycle working in sync with the amount of throttle response should control the mixture without the need to manually pull the mixture control back ..Yes?
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No. Gotta lean on the FI engines also and for the same reason one leans with a carb. Most modern fuel injected engines (read "not certified aircraft engines") use a flow sensor to detect the mass of air moving into the engine and adjust the amount of fuel injected. On aircraft engines for the most part the pilot has to adjust the amount of fuel injected to match operating conditions.
 
Ron Levy said:
If you lean per the engine manufacturers' recommendations, you'll be so lean that the engine won't accelerate past about 1500 RPM when you go to full throttle, choking and sputtering so bad only a complete bozo would not realize there was something wrong and abort the takeoff to figure it out. Maybe your school has some complete bozos flying their planes? :dunno:
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Bingo! I lean aggressively on the ground and we have some inclines to climb on our airport. Sometimes I can't get enough power to climb them and have to give the knob a twist or two.

From what I've read Mooneys are tuned very rich at idle speeds and MUST be leaned to avoid plug fouling.
 
asechrest said:
I thought so. Are there any maintenance-related issues with aggressive leaning? My school was first a maintenance business before branching out into flight training, but I've not heard of any issues in that regard with leaning for taxi.
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None, only benefits. You cannot over lean on the ground, meaning you cannot hurt anything.
 
Perhaps the guy that made the recommendation gets a cut of the fuel profit but doesn't have to pay for aircraft maintenance?

Perhaps they figure after you have a few mag check failures during runup, the reason for leaning will be more permanently ingrained in your brain.
 
I'm sure there are idiosyncrasies with every engine. With a 172 it seems you can get away with almost anything. With my TIO-540 you have to lean aggressively on the ground, do a runup with less than full mixture, or you are sure to fail a mag check. Perhaps that is the turbo charging, regardless when I take the runway I like a lot of instant clean power, so I lean a lot.

For ground ops after the engine runs a little rich for a minute or two I lean until peak RPM. Then adjust throttle for taxi or whatever. Seems to work pretty for all the different altitudes and temps I operate in.
 
I was never taught to lean on the ground. However, when our club 172 started developing an intermittent rough condition, we all started leaning aggressively on the ground. That alleviated a lot of the problem (a carb rebuild finally fixed it completely).
 
Xscaliber said:
As a guy who plays with Auto engines I can understand leaning on carb engines to avoid plug fouling, but what about Fuel Injected units, the injector flow cycle working in sync with the amount of throttle response should control the mixture without the need to manually pull the mixture control back ..Yes?
Click to expand...

Fuel injected auto engines aren't the same. Not even the mechanical injection systems like VW CIS. All it means for aircraft is that fuel is forced through an orifice at each intake stroke, instead of through a venturi.

There is no lambda sensor, no air charge temp sensor, no air flow sensor, no manifold pressure sensor (sometimes, there is a gauge, but that's for the pilot, not the engine) and most importantly no PCM.

Really rich is still really rich, and plugs foul just the same.
 
Not only do I lean out during taxi, but I lean for max power during my run up, something no CFI ever taught me.
 
steingar said:
Not only do I lean out during taxi, but I lean for max power during my run up, something no CFI ever taught me.
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That's a high altitude procedure. You really should qualify that as it CAN damage things if you then take off like that at sea level.
 
steingar said:
Not only do I lean out during taxi, but I lean for max power during my run up, something no CFI ever taught me.
Click to expand...

Do you takeoff full power with it leaned? What elevation?

You could be hurting the engine. Somewhere above 75% power, you want to be full rich. The plugs won't foul at that power output. Remember as your altitude increases the % power the engine makes decreases at wide open throttle.

If your density altitude is above 5000 feet or so, then it is appropriate to take off leaned to max power. Otherwise, play it safe and leave it full rich for full power climbs at low altitudes.
 
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My guess is the change is for liability reasons. If it's not in the POH, don't do it. Therefore, if the POH is silent on leaning on the ground, then they don't lean on the ground, and just live with the added mx and fuel burn. Someone may have leaned on the ground, and had just enough mixture to get full throttle but not full RPM or rough running and they continued the take off.

I always lean aggressively at idle once the engine is started and leave it that way until ready for take off. I can usually but not always do the run up to 1500 with the mixture lean, then go full rich for take off and climb.

YMMV
 
Have always leaned on taxi ,as I was taught. Never had a problem with my 172,arrow,or Travelair. All engines went to TBO. Now I have fadec and the mixture is done by computer.
 
kenjr said:
Cap'n Ron - I've seen both of those. Maybe I'm just missing something here or not 'reading between the lines' but, please direct me to the text that says lean it while taxiing and exactly how much?
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Hmmm...it appears the language from SL 185A was removed when they went to 185B. What it used to say was to lean on the ground at low power. At 1200 RPM, pull the mixture back until the engine begins to run rough, then back in until the RPM peaks. Leave it there for all ground ops except run-up. If you have the instrumentation, you'll see this raises the EGT by around 150F, which makes a big difference in scavenging.

Also, note the information in 185B about setting ground idle speed. I notice flying a lot of different airplanes that RPM at the throttle idle stop is often in the 800-1000 range, rather than the recommended 600-650 range. In that situation, you have an excessively rich mixture, which increases the need to lean on the ground at low power.

If you need more ammunition, talk to one of the engine gurus like Charlie Melot at Zephyr Engines in Florida or Bill Scott at Precision Engine in Kentucky.
 
steingar said:
Not only do I lean out during taxi, but I lean for max power during my run up, something no CFI ever taught me.
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Be careful doing that below 5000 DA, as you can increase heat production to the point that the lower cooling flow at climb speed will not keep the CHT's within an acceptable range. I recommend against leaning below the manufacturer's recommended altitude (5000 in most planes) unless you have all-cylinder CHT to monitor this.
 
kenjr said:
Cap'n Ron - I've seen both of those. Maybe I'm just missing something here or not 'reading between the lines' but, please direct me to the text that says lean it while taxiing and exactly how much?

I found new information (to me anyway the first time I went through these...) about running up to 1800 prior to shutdown, proper adjustment of the idle speed, operating the engine at 1000-1200 RPM to activate scavenging agents that burn up the lead and a few other interesting things.

However, I find no reference to anything that says yank the mixture out while your taxiing.

I mean, don't get me wrong - I'm not trying to be argumentative here...it makes total perfect sense to me. I have an old Porsche that I do all the engine work on - I'm no stranger to understanding air/fuel ratio's, proper plugs, etc...



I'm pretty damn sure this guy forgot more about flying airplanes in the past week than I've learned thus far. He's a very sharp guy and been an owner in this plane for over 15 years - and we do all owner assist maintenance so it's not like we never touch it. I don't know how many of his hours are in pistons but he didn't start flying 777's the day they gave him his PPL...and whatever the number is...it's a helluva lot more than me.
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Lycoming does not make a declarative statement, but you can read between the lines: If you are told to lean when above 5000', that equates to about 65 percent power. Do you taxi using 65 percent power? I didn't think so. Lycoming does say to use full rich for takeoff and climb, which certainly implies a leaner setting prior to takeoff.

Bob Gardner
 
I was always told that taking off with too lean a mixture can cause damage to the engine from detonation. Not so lean that the engine doesn't produce power, but just leaner than it should be. I did some research and found an article on leaning the mixture in the Avweb website. Here's the conclusion to the article.

Now the bad news: you can do serious damage to the engine by taking-off with the mixture manually leaned. Possible engine damage includes preignition, detonation, and high engine temperature. You cannot, however, damage the engine by adjusting the idle mixture setting too lean.

Rather than manually leaning your engine for ground operations, it's better to have your mechanic adjust the idle mixture to a properly lean setting. If you lean manually on the ground, you need to understand the risks.

If you lean close to idle cutoff, the engine won't accelerate when you advance the throttle for takeoff; no takeoff and no engine damage occurs. If you lean just a little, then the mixture isn't lean enough to do any damage if you forget to push the mixture back in at takeoff. However, if you lean in a middle-of-the-road sort of way and forget to push the mixture in during takeoff, then you may damage the engine. For this reason, I'm hesitant to recommend leaning the engine at idle, unless specified in the POH.
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WakeNCAgent said:
I was always told that taking off with too lean a mixture can cause damage to the engine from detonation. Not so lean that the engine doesn't produce power, but just leaner than it should be. I did some research and found an article on leaning the mixture in the Avweb website. Here's the conclusion to the article.
Click to expand...

Damn! Just when I was deciding to aggressively lean for taxi, the article throws me for a loop!

It does, perhaps, help explain why my flight school decided to discontinue leaning for taxi. Maybe they adjusted the idle mixture to a properly lean setting for ground operations?
 
asechrest said:
Damn! Just when I was deciding to aggressively lean for taxi, the article throws me for a loop!

It does, perhaps, help explain why my flight school decided to discontinue leaning for taxi. Maybe they adjusted the idle mixture to a properly lean setting for ground operations?
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Let's repeat part of the article.

You can't over-lean at idle. If they did adjust the idle mixture in that manner, the worst thing you'll do if you lean for taxi is cut the engine and have to restart it.

It's terrible advice at high altitude. Fly to a sea level airport tuned for idle at high altitude, and the engine will die at idle. Could be inconvenient at short final, but more likely when trying to clear the runway after landing.
 
We lean for taxi.

Bigest problem and why your school changed is probably people forgetting to "un-lean" before runup and take off!
 
MAKG1 said:
Let's repeat part of the article.

You can't over-lean at idle. If they did adjust the idle mixture in that manner, the worst thing you'll do if you lean for taxi is cut the engine and have to restart it.

It's terrible advice at high altitude. Fly to a sea level airport tuned for idle at high altitude, and the engine will die at idle. Could be inconvenient at short final, but more likely when trying to clear the runway after landing.
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True. And the plug fouling I experienced after the change at my school would seem to indicate they didn't adjust the mixture, or didn't adjust enough.

Seems like my best strategy is: lean aggressively, near idle cutoff, for taxi. Can't damage engine by overleaning, and by doing so aggressively, the engine will fail to produce takeoff power if I forget to go full rich, also preventing engine damage by taking off too lean.

This turned into a nice discussion. Thanks everyone.
 
WakeNCAgent said:
I was always told that taking off with too lean a mixture can cause damage to the engine from detonation. Not so lean that the engine doesn't produce power, but just leaner than it should be. I did some research and found an article on leaning the mixture in the Avweb website. Here's the conclusion to the article.
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True, but if you lean as recommended for taxi, you cannot get anywhere near full power before the engine starts gasping for fuel. Taking off with the mixture set for taxi just isn't a mistake anyone with five operating senses can make.
 
Ron Levy said:
True, but if you lean as recommended for taxi, you cannot get anywhere near full power before the engine starts gasping for fuel. Taking off with the mixture set for taxi just isn't a mistake anyone with five operating senses can make.
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Further, if you don't lean enough that the engine stumbles at full power, you probably have done ZERO for leaning at idle. Idle mixture is pretty much unaffected by the red knob until you get near cut-off.
 
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