Fouling plugs during flight?

[RyanB]

So we know that leaning for taxi and ground ops is recommended to reduce plug fouling, but what about during flight? Obviously we lean for fuel economy and best power during climb and cruise flight, but if we’re richer than we need to be at altitude, can we foul plugs by not being lean enough?

 

[chemgeek]

If you run rich and cooler, less lead gets scavenged by combustion heat, increasing the risk of building up lead deposits on plugs and elsewhere. I don't think there is any demonstrable benefit of running excessively rich during cruise flight.

 

[RyanB]

I don't think there is any demonstrable benefit of running excessively rich during cruise flight.

To clarify, I don’t run excessively rich, I try to lean as best I can without an engine monitor, but there are times when I wonder if I’m too lean or too rich. It’s hard playing Mr FADEC at all moments of flight .

 

[bobmrg]

To clarify, I don’t run excessively rich, I try to lean as best I can without an engine monitor, but there are times when I wonder if I’m too lean or too rich. It’s hard playing Mr FADEC at all moments of flight .

If you run "too lean," the engine will let you know by running rough (or quitting.) Without instrumentation, lean until the engine begins to run rough and then enrichen until it smooths out.....and no more. Do a web search for "Key Reprints from the Lycoming Flyer" and then for "Leaning Textron Lycoming Engine." The information can be applied to any carbureted engine.

Bob Gardner

 

[Tom-D]

To properly lean, do this, obtain level flight. lean until you see the aircraft climb, trim to level flight.
The engine will now still have some cylinders too rich. with calibrated injection this is as good as it gets.

 

[chemgeek]

The Lycoming engine operation manual provides two methods to properly lean without engine monitor instrumentation:

• Best economy: lean until the onset of roughness, then enrich until the engine smooths out.
• Best power: lean to maximum rpm

For cruise, best economy is typically preferred. Best power might be used for maximizing climb rate above 5000 feet, or when you want maximum airpseed at the expense of extra fuel burn. When descending you can leave the mixture where it is, and push it in a bit when the engine starts to stumble as you descend into denser air. I don't go to full rich until doing the pre-landing checklist, typically 5-10 miles out VFR, or approaching the FAF for an IFR approach.

If you lean to best economy in cruise, you will be rewarded with the best possible cruise fuel economy, and squeaky clean plugs at annual time.

 

[Ravioli]

I don't think there is any demonstrable benefit of running excessively rich during cruise flight.

Not sure what "excessively rich" is meant to impart, but running richer will reduce operating temperature. Right?

 

[Tom-D]

Not sure what "excessively rich" is meant to impart, but running richer will reduce operating temperature. Right?

Maybe.
Without calibrated injection, you will not know what the mixture is, some cylinders may be too rich, and some will too lean.
and without a real EGT, you are still guessing. = real EGT is a pyrometer, not a bimetallic thermal couple.

 

[denverpilot]

I’ve only ever seen one plug foul in flight. Was a 172RG and a really long slow descent at very low power and way too rich.

Wasn’t noticed until after landing. Rough engine operation during taxi.

That engine probably had other things wrong with it also.

Rental and owner was clearly ticked when he heard. Clearly he’d been fighting with fouled plugs for a while by his reaction to the news.

 

[Capt. Geoffrey Thorpe]

Not sure what "excessively rich" is meant to impart, but running richer will reduce operating temperature. Right?

If you are already rich of peak, then more richer gets lower temperatures. If you have a big spread in mixture from cylinder to cylinder, then exactly where each cylinder is can be less than obvious so the typical advice is to be well rich or well lean to avoid yea olde peake. BTW, your car runs right around peak purd near all the time.

(Rich is to the right in this diagram)

 

[flyingron]

As far as Continental is concerned, if you're operating at recommended cruise or less, you can set the red knob anywhere you like from full rich to so lean it is misfiring.

 

[Getonit]

As far as Continental is concerned, if you're operating at recommended cruise or less, you can set the red knob anywhere you like from full rich to so lean it is misfiring.

i have been under the impression as long as less than 65% power the mixture doesn’t matter, red box and such.

 

[Tom-D]

i have been under the impression as long as less than 65% power the mixture doesn’t matter, red box and such.

Then tell us why many of us lean idle?

 

[Tom-D]

Then tell us why many of us lean idle?

you can save at any power setting.

 

[idahoflier]

As already mentioned, you can run rich to lower CHT. Yes, running rich may cause fouling, but it's a lot easier and cheaper to clean spark plugs than to replace cylinders. I used to lean shortly after takeoff for best power in climb and cruise. After getting an engine monitor installed at my overhaul I was shocked by the high CHT's I was seeing. Since then I have always climbed full rich (when I can) and when it's hot I still usually have to run a bit rich just to keep the CHT reasonable...

 

[flyingron]

i have been under the impression as long as less than 65% power the mixture doesn’t matter, red box and such.

Red box is a Lycoming thing.

 

[MooneyDriver78]

If you are already rich of peak, then more richer gets lower temperatures. If you have a big spread in mixture from cylinder to cylinder, then exactly where each cylinder is can be less than obvious so the typical advice is to be well rich or well lean to avoid yea olde peake. BTW, your car runs right around peak purd near all the time.

(Rich is to the right in this diagram)

Cars don’t use EGTs, but have an Oxygen sensor. So no O2, mixture is running leaner. I believe other than the first few seconds after cold starts, or when full power is required or knock is detected ; you’re running lean (there may be a timing change vs mixture change or both in these cases)
I’m not sure an O2 sensor would work at high altitudes or with leaded fuel.

 

[Capt. Geoffrey Thorpe]

Cars don’t use EGTs, but have an Oxygen sensor.

Correct. And, there is about a 15% chance that the vehicle you currently drive has an air/fuel control algorithm that I had a big hand in.

So no O2, mixture is running leaner.

The stoichometric air fuel ratio that is needed for the three way catalyst to work is very close to peak EGT. Running leaner would improve fuel economy, but dealing with the NOx is a problem. Big problem. So, gasoline vehicles are pretty much stuck running at stoich. (But not for a lack of trying.)

I believe other than the first few seconds after cold starts, or when full power is required or knock is detected ; you’re running lean (there may be a timing change vs mixture change or both in these cases)

You have to add a little fuel for cold starts. For extended full power operation, the engine may go rich for cooling. The company I worked for used an exhaust/catalyst temperature model to determine when to go rich (no temp sensor, as is typical). Knock pretty much just retards spark. (But, obviously, I can't speak for EVERY oem.)

I’m not sure an O2 sensor would work at high altitudes or with leaded fuel.

Works just fine at high altitudes - they are based on a Nernst cell that generates a voltage when there is an oxidation potential difference between the atmosphere and exhaust - when the exhaust is rich of stoichometric, oxygen ions migrate through the solid electolyte and generate a voltage. Wide band sensors are a bit more complicated, but still based on the Nernst cell.
Lead is bad juju when it comes to oxygen sensors.

 

[Salty]

when the exhaust is rich of stoichometric, oxygen ions migrate through the solid electolyte and generate a voltage.

I'm sure I"m missing a fundamental principle, but I would think if it were rich there would be less o2.

 

[Capt. Geoffrey Thorpe]

Red box is a Lycoming thing.

And, here I thought it was a grocery store thing. Silly me.

I'm sure I"m missing a fundamental principle, but I would think if it were rich there would be less o2.

Yes. When rich, the exhaust is O2 deficient so the Oxygen ions flow from the atmosphere side (sensors are typically "vented" through the wire harness) to the exhaust side where they react with the unburned hydrocarbons.

 

[PaulS]

So we know that leaning for taxi and ground ops is recommended to reduce plug fouling, but what about during flight? Obviously we lean for fuel economy and best power during climb and cruise flight, but if we’re richer than we need to be at altitude, can we foul plugs by not being lean enough?

I think it depends. If you are at 2,000 feet, then full rich is probably not an issue for fouling. But if you are 8000 feet, full rich is much more fuel than oxygen required and you could end up with issues if you do this regularly.


For the other poster on more rich lowers EGTs, yes if you are ROP. If you are LOP more rich will raise the EGTs (if you stay LOP). Cirrus recommends pulling more lean if CHTs are climbing when running LOP, generally though for a NA 22, my experience, when LOP, the cylinders run pretty cool.

 

[Salty]

And, here I thought it was a grocery store thing. Silly me.


Yes. When rich, the exhaust is O2 deficient so the Oxygen ions flow from the atmosphere side (sensors are typically "vented" through the wire harness) to the exhaust side where they react with the unburned hydrocarbons.

Okay, yeah, you're way over my head. So, I guess it's a "simple" matter of keeping the "flow" of ions to zero to achieve stoichiometric balance. If they're moving one way, add fuel, the other remove fuel...

 

[flyingron]

Yes. When rich, the exhaust is O2 deficient so the Oxygen ions flow from the atmosphere side (sensors are typically "vented" through the wire harness) to the exhaust side where they react with the unburned hydrocarbons.

My grandmother always lectured me on the importance of hydrocarbons in my diet.

 

[flyingron]

The issue with not being a the stoichiometric point isn't always that you're just going to shoot the excess O2 or hydrocarbon out the exhaust. You can get incomplete combustion given the ratios involved. At close to the stoichiometric point, you gett a little CO and H2 in the output. You lean too much and you start getting methane or even elemental carbon (i.e., carbon fouling).

 

[MooneyDriver78]

CMIIW, but doesn’t running lean mean no CO emissions?

A definite plus.

 

[WDD]

Yes you can foul plugs during cruise, especially when it's summer (hot and humid).

Reset your mixture when you're at cruise altitude.

I pull mix back until it stumbles, push it back in, twist a few times and it seems to work well to get to rich of peak. (At least that's what the rental/school folks as well as the club CFI's specify you do with their planes).

 

[Capt. Geoffrey Thorpe]

CMIIW, but doesn’t running lean mean no CO emissions?

A definite plus.

Peak EGT is pretty close to the green line.

 

[MauleSkinner]

i have been under the impression as long as less than 65% power the mixture doesn’t matter, red box and such.

Then tell us why many of us lean idle?

“Red box and such” indicates that he’s talking about not damaging the engine. Fuel economy and protecting the engine are separate discussion points from his post.

 

[sarangan]

So we know that leaning for taxi and ground ops is recommended to reduce plug fouling, but what about during flight? Obviously we lean for fuel economy and best power during climb and cruise flight, but if we’re richer than we need to be at altitude, can we foul plugs by not being lean enough?

Here is a hypothetical question. If the temperature is -30C, the density altitude at sea level will be -6000 ft. Under that circumstance, will the engine be too lean even with the mixture knob pushed all the way in?

 

[Capt. Geoffrey Thorpe]

Here is a hypothetical question. If the temperature is -30C, the density altitude at sea level will be -6000 ft. Under that circumstance, will the engine be too lean even with the mixture knob pushed all the way in?

Given just how pig rich aircraft engines are with the knob all the way in...

No.

 

[EdFred]

I've flown at almost -25C and had no issues.

 

[WannFly]

I pull mix back until it stumbles, push it back in, twist a few times and it seems to work well to get to rich of peak.

based on my engine monitor, that method puts you closes to the lean side, not the rich side. granted there is no way to twist the red knob in my archer

 

[Ed Haywood]

RTFI: https://www.lycoming.com/content/leaning-lycoming-engines

 

[Jeff767]

As already mentioned, you can run rich to lower CHT. Yes, running rich may cause fouling, but it's a lot easier and cheaper to clean spark plugs than to replace cylinders. I used to lean shortly after takeoff for best power in climb and cruise. After getting an engine monitor installed at my overhaul I was shocked by the high CHT's I was seeing. Since then I have always climbed full rich (when I can) and when it's hot I still usually have to run a bit rich just to keep the CHT reasonable...

You should lean based on CHT in climb not EGT. EGT values are somewhat irrelevant other than noting trends and change. EGT values can change dramatically just by where the holes are drilled in the exhaust. By climbing full rich you are reducing power and lengthening the time the engine is in climb with higher power and poor cooling. Another recommended technique is to note your EGT when climb power is set and lean as you climb to maintain that EGT. If you climb at full power note your EGT at 1000 feet. CHT should be monitored and adjustments made in climb speed or mixture to keep them reasonable. I like to keep mine below 385.

 

[Jeff767]

based on my engine monitor, that method puts you closes to the lean side, not the rich side. granted there is no way to twist the red knob in my archer

Peak or on the lean side is where you want to be. The worst possible place to operate your engine is 100ROP to about 20 ROP.

 

[flyingron]

You should lean based on CHT in climb not EGT.

Just about everybody disagrees with this (as do I).

EGT values are somewhat irrelevant other than noting trends and change. . EGT values can change dramatically just by where the holes are drilled in the exhaust.

Of course. The absolute value is relatively unimportant. What is important is that when you peak the EGT as you adjust the mixture, you know where you are on the stoichiometric point. This lets you know where you are for efficiency and detonation potential.

By climbing full rich you are reducing power and lengthening the time the engine is in climb with higher power and poor cooling.

So you want to climb with even worse cooling instead? Full rich might not be the right value for many engines (I don't peak the EGT on the big injected Continentals, but I do reduce the fuel flow).

. CHT should be monitored and adjustments made in climb speed or mixture to keep them reasonable.

Yes, CHT is an indicator about engine overheat, and you shouldn't let it get too high (by what ever means you have: less rich, less power, lower angle of attack, open cowl flaps). But it's far from being the primary leaning indicator.

Your "lean to maintain EGT" is bizarre. What is the justification for that?

 

[midlifeflyer]

based on my engine monitor, that method puts you closes to the lean side, not the rich side. granted there is no way to twist the red knob in my archer

It typically does exactly that. The old fashioned lean until it stumbles, then enrichen to smooth generally produces a LOP mixture in a piston engine.

 

[midlifeflyer]

Your "lean to maintain EGT" is bizarre. What is the justification for that?

Didn't read the comment to see if it was being stated correctly or not.

The "target EGT" technique, touted by Mike Busch and taught by the Advanced Pilot Seminars group, Deakin, Braly, and Atkinson, is a 2-step process. First, you check your EGT shortly after takeoff in a full power climb from sea level or a relatively standard day. That is the target which, if it is maintained in the climb, will ballpark best power at any altitude. Once you know what it is, you can use it anywhere. I guess the theory can apply even if not starting at sea level altitude. If you lean for takeoff at a high density altitude airport the old fashioned way, the resulting EGT should be the same. Either way, I see it as a ballpark, not necessarily a hard number.

For anyone interested, while the full course is a pay-for, there are a number of YouTube videos fro the group which do a decent job of summarizing both the science and the technique. A summary presentation pdf is available here

 

[idahoflier]

RTFI: https://www.lycoming.com/content/leaning-lycoming-engines

Generally good advice, but Lycoming also says CHT red line is 500°F and that you may lean for takeoff above 5,000' DA. If I'm not full rich at takeoff my CHT's climb well north of 400°.

 

[idahoflier]

You should lean based on CHT in climb not EGT. EGT values are somewhat irrelevant other than noting trends and change. EGT values can change dramatically just by where the holes are drilled in the exhaust. By climbing full rich you are reducing power and lengthening the time the engine is in climb with higher power and poor cooling. Another recommended technique is to note your EGT when climb power is set and lean as you climb to maintain that EGT. If you climb at full power note your EGT at 1000 feet. CHT should be monitored and adjustments made in climb speed or mixture to keep them reasonable. I like to keep mine below 385.

Where did I say anything about EGT? I agree climbing full rich reduces power and lengthens the time in climb, but given the choice of running cylinders at 400°F for 10 minutes or 430°F for 8 minutes, I'll take the full rich climb...