Questions about mixture leaning

[Larry Vrooman]

Sounds like a great example of why to brief your passengers prior to doing anything that will confuse or cause concern for them.

Absolutely, and the number of things to brief can be large and unexpected.

For example my wife's father used to own and regularly fly a Cessna 150, so I figured she was pretty familiar with flying in small planes. However, the first time I flew with my wife, I did what I always do in a light aircraft like a Cub, Champ, Citabria, etc at a small, uncontrolled field - I pulled the power all the way back abeam my intended landing point.

The approach and landing was perfectly normal. However, after the flight my wife commented that she had gotten really worried when the engine dropped to idle power, as she thought it was a mechanical problem. Apparently her father flew approaches with 1500 rpm or so and relied on excess drag from excess flaps to slow the aircraft down and control the approach angle.

After that, I started just narrating over the intercom to explain what I was doing and why for all new to me passengers.

 

[PaulS]

Sounds like a great example of why to brief your passengers prior to doing anything that will confuse or cause concern for them.

I brief my passengers not to touch anything unless I explicitly tell them they can. Part of the brief in the Cirrus is about the chute and how to activate it. I finish that one with the only time they put their hand on that is if I am dead, or unconscious and we are spiraling toward the ground.

 

[WannFly]

spiraling toward the ground.

if you are in a steep spiraling decent or in a fully developed spin because the pax did something they really shouldnt and you are unconscious, would the chute actually recover the aircraft from that attitude? dont know the answer, hence asking

 

[PaulS]

if you are in a steep spiraling decent or in a fully developed spin because the pax did something they really shouldnt and you are unconscious, would the chute actually recover the aircraft from that attitude? dont know the answer, hence asking

Yes it should, but the instruction is to push the blue button (level) pull the red knob (mixture) then pull the red handle (chute). Ideally the airplane would not get that out of control. Also the only approved recovery from a spin is to pull the chute. I was using hyperbole, although anything could happen.

 

[WannFly]

blue button (level)

ahh.. i forgot the envelop protection. thanks

 

[William Pete Hodges]

Hey everyone. I just did some research into Advanced Pilot Seminars, the group that discusses in depth the dynamics of running ROP and LOP. Right now they are offering their online course FREE !! I just signed up and started the course. I get 45 days to finish. They only asked for an email address. No Credit Card no nothing! I have already completed the initial fun test to determine how much I know or don't know. I just went to www.advancedpilot.com and clicked on ONLINE COURSE. Maybe I got lucky but I'll take it!! Petehdgs

 

[William Pete Hodges]

Hey everyone. I just did some research into Advanced Pilot Seminars, the group that discusses in depth the dynamics of running ROP and LOP. Right now they are offering their online course FREE !! I just signed up and started the course. I get 45 days to finish. They only asked for an email address. No Credit Card no nothing! I have already completed the initial fun test to determine how much I know or don't know. I just went to www.advancedpilot.com and clicked on ONLINE COURSE. Maybe I got lucky but I'll take it!! Petehdgs

It turns out the course is not free, but the first section is free and it gives you a preview of what is to come. I have made the decision to pay for it in part because what I learn will be insightful in my own business as a diesel and natural gas engine-gererator repair company. Anyway I didn't want my previous post to stand without a correction on my part. One Miracle at a time.

Have FUN! Fly SAFE! Petehdgs

 

[R Elves]

I used to make it a practice to kill the engine in cruise with the mixture to show the student that it was a non-event. I also shut off the master switch, to prove to non-believers that doing so would not, in fact, kill the engine. It always surprised me to learn how many students are afraid to touch something for fear that the engine will quit.

Bob Gardner

The old adage - “It depends” - Turn off the master in an aircraft with electronic ignition the engine will quit because you’ve robbed the ignition of power. To keep the engine running you must switch to the alternate power source, usually a backup battery, then can shut off the master and the engine will continue to run. If you have an primary electrically failure in flight, need to switch to the back up battery ASAP to continue to fly.

 

[William Pete Hodges]

I did complete the Advanced Pilot Seminar course, and I liked it. I learned a lot about Rich of Peak (ROP) and Lean of Peak (LOP) operations and have applied some of what I learned to flying my Mooney Super 21. I flew several XCs this summer and cruised at 10,500 feet, 140 kts TAS at 8 GPH. This fuel burn is outside of the RED FIN danger zone for continuous operation. 8 GPH cannot damage the engine no matter how rich ROP or how lean LOP the engine is running. This gave me a place to run that is efficient without damaging the engine.

So what is the RED FIN? It defines the area of higher stress operation based on engine load and fuel/air ratio (measured as a reference to exhaust gas temperature) that can be harmful to engine longevity if operated in that area continuously. The RED FIN gets smaller as engine load is reduced, and disappears completely below 61% Load. As load increases above 61% the RED FIN gets larger defining how rich ROP or how lean LOP the engine must be run to stay out of the danger zone and prolong engine life, and/or prevent premature engine failure. All of this is very well explained in the course, but the graphic material left out some important information can lead to a much better understanding of the principles involved.

I found out about the RED FIN elsewhere as a variation of the RED BOX, the idea that operation within the box will be harmful to the engine. The RED BOX varies in size as engine load changes, but how the size changes with load is undefined, a mystery. The RED FIN shows directly how the danger zone changes in size as the load changes on the engine. But when looking at the RED FIN graphs I found, it appeared to me that the fuel flows were not depicted correctly. It was implied that increasing the fuel flows from LOP to ROP the graph would move up vertically across the RED FIN. This is not the case. As the fuel mixture is richened from LOP the engine load also increases, thus moving the graph diagonally at first and then curving up more vertically the more ROP the settings go. This means that even where the RED FIN is small, the distance across it as the mixture is adjusted is longer than expected.

Attached to this post is my graph of the RED FIN vs constant GPH fuel flows and constant air flows where RPM and MP remain unchanged as mixture is adjusted. The graph is based on Lycoming engine perfomace data for the IO-360 rated 200 HP. The graph has my copyright on it and may be posted here and printed by individuals for personal use for free. Contact me if you have any questions.

Thank you for reading this post. Sorry it is so long.

 

[Domenick]

The Red Fin discussion is intriguing, but it's just a series of statements. What's the math (equations) behind the Red Fin? How was the Red Fin derived.

How does one go about adjusting the Red Fin for different HP engines?

Looks like 3 copies of the same attached image.

 

[William Pete Hodges]

So, how do you use the RED FIN in your flying? This was also a big question for me. Martin Pauly has an excellent video on this subject. It is really well done and is not boring at all, an impressive feat for such a complex subject. He explains the basic principles including ROP, LOP, the RED BOX and the BIG MIXTURE PULL. I have watched this video several times and was able to improve my understanding a little more each time. Thanks Martin for your excellent work.

In order to use the RED FIN chart effectively you need certain engine monitoring equipment, and you need to verify your engine can run smoothly LOP at moderate loads below 61%. If your engine isn't smooth on the lean side of Peak EGT, you will need to determine what is causing that and get it resolved if you can. A good engine monitor with multiple EGT and CHT readings is very valuable in identifying the potential problems and getting them resolved.

When your engine is running above 61% load, the Cylinder Head Temperature (CHT) is used to determine overall stress on the engine. This principle has been verified through advanced dynomometer testing where internal engine pressures were documented as loads and mixture settings were changed. This testing determined that various load and mixture settings can alter the internal stress on the engine, that the CHTs closely follow this stress, and that this stress can vary greatly between the cylinders, especially on the LOP side. Multiple CHT readings allow measuring the stress on each engine cylinder individually improving your awareness of how the engine is performing, good or bad.

Exhaust Gas Temperature (EGT) is used to determine the mixture setting and the Fuel/Air ratio. The attached chart shows how to the Fuel /Air ratio changes with EGT, CHT, Specific Fuel Consumption (SFC) and Power (Load). I used this chart's data to create the fuel and air curves on my RED FIN chart in the previous post. Like CHT readings in the previous paragraph, the EGTs will be different for each cylinder, the important part being where each cylinder reaches Peak EGT relative to each other. Ideally they should all reach Peak EGT at the same mixture setting during the leaning procedure, but that is not likely. It is important that the differences between them be small, that is what allows for smooth engine operation when running LOP at moderate loads. Multiple EGT readings allow you to see and test this for yourself, and can help you trouble shoot and resolve these issues if you have them.

One quick and dirty test you can do, even if you don't have multiple EGT readings is to fly along at 50-60% power and slowly lean the engine until it runs rough, if it does. My engine continues running smoothly until it reaches lean cutoff, and just cuts out abruptly. That tells me I can run practically anywhere I want as long as I am outside of the RED FIN.

More later...

 

[RoscoeT]

Sitting on a hot taxiway with no wind at idle, you're way below 65% and can certainly harm your engine by improper leaning.

Man where do you come up with this stuff

 

[David Megginson]

All flying I have done so far has been full mixture for takeoffs, landing, and maneuvers (most of the time) at or under 3000’. Really the only time flying and leaning the mixture is cruising to the air works area and back. That so far has been only set rpm (2200) and lean.

So I wondered, on cross country trips, mostly cruising, after initially choosing a power setting, and leaning the mixture, it’s not clear to me if that lean setting is valid also if you say add power to climb or lessen power to descend. Also not sure how you best adjust if it is needed. I can imagine it might depend on how long you will climb or descend.

And if you do need to readjust with power changes, do you first set mixture full rich and then go from there or adjust slightly from where it sits already?

wondering too if during descent, if you want to have it lean to avoid too much engine cooling?

The whole leaning thing, something seems off to me on how I’m being taught. Our checklist on startup has us lean the mixture just after engine start during warmup. Then taxi, and stop, full rich for run up checklist. The club checklist never tells us to lean again for taxi. I read in the POH that one should lean for ALL ground operations, but when I went to do that, the instructor said not to do that. Also, he has mentioned that if I lean at 1200 (actually in summer he has me doing this at 1000 rpm) when I then give more throttle as we taxi he said once that it then wasn’t properly leaned because of throttle being adjusted.

I need to ask him next time, as of now no student flights possible because of restrictions here because of corona, and also my health has been problematic lately, but I don’t feel like I have sufficient info on leaning, when, how, etc. other than set and forget during the short cruise we do to training area.

The Cessna is fuel injection, not sure if that makes a difference. Again, just wondering about your experiences and if many adjust mixture after making small or medium throttle changes?

Leaning is probably the biggest ongoing (usually friendly) debate in aviation. While you're working on your PPL, do whatever your instructor wants, especially if you're renting the plane wet. After you're flying for real, here are a few basics:

1. The thing we're actually worried about is internal cylinder pressure. If that pressure gets too high, then the fuel-air mixture might detonate without a spark (like in a diesel engine) and push the piston back at the wrong time, which is really bad for the engine (since the crankshaft isn't in the right place yet for that push). That's called "predetonation".
   
   
2. We can't measure internal cylinder pressure (except on a test stand), but we can measure cylinder-head temperature (CHT), which has nearly a 1:1 relationship with pressure (the higher the CHT, the higher the pressure). The concern over high CHTs isn't primarily about damage from heat, but about the internal pressure that the heat is hinting at.
   
   
3. Leaning your engine increases the CHT (and internal pressure) up to a certain point, then starts lowering it again. So there's a point somewhere in the middle that you're running your CHTs at the hottest possible, and either enriching or leaning the mixture from there will cool them (and lower the pressure).
   
   
4. If you don't have a CHT gauge, you can use exhaust-gas temperature (EGT) as a proxy. As you lean, the EGT initially climbs and then falls again. The highest EGT is called "peak EGT".
   
   
5. Unlike with CHT, the actual EGT temperature doesn't matter (it varies hugely based on small differences in probe placement). What we care about is how far below the peak it is. When it's below peak EGT on the lean side, we call it "lean of peak" (LOP), and when it's below peak EGT on the rich side, we call it "rich of peak" (ROP).
   
   
6. At any given power setting, the highest CHTs occur around 50°F ROP. Either enriching or leaning from that point will lower the CHTs. Peak power (highest RPM with a fixed-pitch prop like the C172's) will occur around 150° ROP. Best fuel efficiency will occur around 50°F LOP (but not all engines will operate smoothly there).
   
   
7. The higher your engine's power output, the higher the peak CHT (and pressure), and the larger the area you need to avoid around 50° ROP. We call the area you need to avoid the red zone. The red zone is fairly wide at 75% power, minuscule at 65% power, and completely gone by 60% power. So if you cruise at lower power settings (65% power or below), you can put the mixture pretty-much anywhere you want without hurting your engine. At 75% power, you'll want to keep it at or above 150°F ROP (peak power) or down around 50°F LOP (best economy) to avoid the wider red zone. Taxiing is very low power, so lean almost to cutoff to save your plugs. Above 75% power, you shouldn't normally lean at all (with some exceptions for mid-elevation takeoffs, e.g. 3,000 ft density altitude or above, where your engine can still put out slightly above 75% power).
   
   
8. A nice trick that works with a fixed-pitch prop and some engines is called LOP-WOT (lean-of-peak/wide-open throttle). With that approach, you climb normally to cruise altitude, leave the throttle fully open, and gently pull back the mixture until you get to your target RPM for your power setting. Because the throttle is wide open, you know the engine's running at the leanest possible setting, so you don't need EGT or CHT to confirm. You do need to make sure you have the RPM right from the POH power tables. If the fuel-air distribution among your cylinders isn't good, the engine will start shaking very noticeably, so this won't work for you, and you'll be better off flying on the ROP around peak power.
   
   
9. The old advice to "lean to roughness and then enrich slightly" isn't ideal, because every engine starts running roughly somewhere different (if at all; at 2500+ RPM, my engine will lean right to idle cutoff without roughness). In some cases that technique might land you right on 50°F ROP, which is the worst place to operate your engine. Leaning to peak RPM/power with a fixed-pitch prop is safer if you want to stay on the ROP side.

This is a lot to digest, but it can also make you much more confident managing your engine. If you're able to fly LOP, you might also add up to an hour to your endurance, which makes a big difference for IFR flight planning and alternate selection. But even flying ROP, it's good to understand what you're doing and why.


Cheers, David

 

[William Pete Hodges]

Here is a typical flight that keeps you out of the RED FIN. At takeoff FULL RICH, WOT, Max Prop. At 500 Feet Reduce RPM 100 RPM to 2600 and note EGT on the hottest cylinder. (lets say 1300F) Maintain climb at WOT, 2600 RPM, FULL RICH and watch as the EGT starts going down. When the EGT drops by about 30F, Lean the mixture to bring it back to the same temp at 500 feet (1300F). Keep climbing at WOT and 2600 RPM while periodically leaning to maintain 1300F. Keep an eye on CHTs keeping them under 375F, if too high then enrichen the mixture 50F or so as required to keep them cool. At cruise altitude level off and reset engine RPM to desired cruise at WOT, but leave the mixture alone for a couple of minutes for the air speed to stabilize. Now you are ready for the BIG MIXTURE PULL.

Before your flight you should have established a cruise altitude, true air speed, and engine setting that you are looking for. For me, at 9500 or 10500 feet I am looking for 2500 RPM and 8.0 GPH. I can simply PULL the mixture to 8 GPH. But if I was uncertain or wanted a higher power setting, say 9 GPH instead of 8, I would PULL the mixture smoothly and relatively quickly until I feel the airplane slow down a bit due to a reduction of power. That reduction of power means I have entered the LOP region, so stop right there and let the engine stabilize for a minute or two. Now determine Peak EGT from the lean side by enrichening the mixture slowly until the first cylinder to reach Peak EGT occurs, this is the richest cylinder, then lean back the intended number of degrees F on the lean side for the power you are pulling on the RED FIN chart. Determining Peak EGT from the lean side is a quicker process and allows you to spend less time inside the RED FIN then you would have checking Peak EGT from the rich side. It is also really easy to determine engine power produced if you are near Peak EGT on the lean side.

If you know the GPH fuel flow, and you are near Peak EGT on the lean side, then you know the power your engine is producing is 14.9 HP per GPH of fuel flow. 8 x 14.9 = 119.2 / 200 (Rated HP) = 59.6% Load. You can ball park this using 15 HP per GPH, so 8 x 15 = 120 / 200 (Rated) = 60%. This works on any engine runnning LOP near Peak EGT, from Peak to about 75F LOP. Deeper into the LOP region power drops off more sharply and specific fuel consumption increases the leaner you go. Depending on how much power you are pulling that may be the only option to stay out of the RED FIN on the LOP side (see RED FIN chart). Once you have determined Peak EGT it is easy to set LOP or ROP with precision. Use LOP for better fuel economy with some reduction in airspeed, or ROP for more power and better speed. The engine can be run continuously at any power setting either LOP or ROP provided you keep the engine operating outside of the RED FIN.

So now you are getting ready to reduce altitude so how do you do that? One way is to leave the engine RPM alone and reduce throttle setting as you decend to maintain the same MP you had at altitude. You will also need to adjust the mixture to maintain the same GPH fuel flow. The same RPM, MP, and GPH means the same power setting and the same Fuel/Air Ratio during descent. Another way is to reduce RPM first, and check the mixture is where you want, then do the same thing as before to maintain the new lower power setting as you descend. A third way is to do nothing at all. Descend normally at WOT with the mixture left alone. As the engine gets more and more air it will be running leaner and leaner as you descend. When you hear the engine running rough or the sound changes a bit then make mixture adjustments as necessary to keep the engine smooth on the LOP side. As you get closer to your destination reduce RPM as much as possible to keep the engine from shock cooling as the load produced is going down. When you get in the pattern then configure the aircraft for landing and go-around as normal and land.

I hope this gives you a better understanding of how to apply the RED FIN chart to LOP and ROP operations.

Have FUN! Fly SAFE! Petehdgs

 

[William Pete Hodges]

Dominic, The chart can be adjusted for different engines by using a HP Ratio to the GPH fuel flows. For example, the chart is set up for 200HP. For a 300 HP engine multiply the GPH values on the chart by 1.5. 300/200 = 1.5 so 8 GPH on the chart becomes 12 GPH and 10 GPH on the chart becomes 15 GPH. If the HP is less, like 150, then 150/200 = 0.75 so 8 GPH on the chart becomes 6 GPH and 10 GPH on the chart becomes 7.5 GPH. When I have some time I'll consider calculating different charts for different HP engines but for now it is what it is. Hope this helps.

 

[David Megginson]

Here is a typical flight that keeps you out of the RED FIN. At takeoff FULL RICH, WOT, Max Prop. At 500 Feet Reduce RPM 100 RPM to 2600 and note EGT on the hottest cylinder. (lets say 1300F) Maintain climb at WOT, 2600 RPM, FULL RICH and watch as the EGT starts going down. When the EGT drops by about 30F, Lean the mixture to bring it back to the same temp at 500 feet (1300F). Keep climbing at WOT and 2600 RPM while periodically leaning to maintain 1300F. Keep an eye on CHTs keeping them under 375F, if too high then enrichen the mixture 50F or so as required to keep them cool. At cruise altitude level off and reset engine RPM to desired cruise at WOT, but leave the mixture alone for a couple of minutes for the air speed to stabilize. Now you are ready for the BIG MIXTURE PULL.

Before your flight you should have established a cruise altitude, true air speed, and engine setting that you are looking for. For me, at 9500 or 10500 feet I am looking for 2500 RPM and 8.0 GPH. I can simply PULL the mixture to 8 GPH. But if I was uncertain or wanted a higher power setting, say 9 GPH instead of 8, I would PULL the mixture smoothly and relatively quickly until I feel the airplane slow down a bit due to a reduction of power. That reduction of power means I have entered the LOP region, so stop right there and let the engine stabilize for a minute or two. Now determine Peak EGT from the lean side by enrichening the mixture slowly until the first cylinder to reach Peak EGT occurs, this is the richest cylinder, then lean back the intended number of degrees F on the lean side for the power you are pulling on the RED FIN chart. Determining Peak EGT from the lean side is a quicker process and allows you to spend less time inside the RED FIN then you would have checking Peak EGT from the rich side. It is also really easy to determine engine power produced if you are near Peak EGT on the lean side.

If you know the GPH fuel flow, and you are near Peak EGT on the lean side, then you know the power your engine is producing is 14.9 HP per GPH of fuel flow. 8 x 14.9 = 119.2 / 200 (Rated HP) = 59.6% Load. You can ball park this using 15 HP per GPH, so 8 x 15 = 120 / 200 (Rated) = 60%. This works on any engine runnning LOP near Peak EGT, from Peak to about 75F LOP. Deeper into the LOP region power drops off more sharply and specific fuel consumption increases the leaner you go. Depending on how much power you are pulling that may be the only option to stay out of the RED FIN on the LOP side (see RED FIN chart). Once you have determined Peak EGT it is easy to set LOP or ROP with precision. Use LOP for better fuel economy with some reduction in airspeed, or ROP for more power and better speed. The engine can be run continuously at any power setting either LOP or ROP provided you keep the engine operating outside of the RED FIN.

So now you are getting ready to reduce altitude so how do you do that? One way is to leave the engine RPM alone and reduce throttle setting as you decend to maintain the same MP you had at altitude. You will also need to adjust the mixture to maintain the same GPH fuel flow. The same RPM, MP, and GPH means the same power setting and the same Fuel/Air Ratio during descent. Another way is to reduce RPM first, and check the mixture is where you want, then do the same thing as before to maintain the new lower power setting as you descend. A third way is to do nothing at all. Descend normally at WOT with the mixture left alone. As the engine gets more and more air it will be running leaner and leaner as you descend. When you hear the engine running rough or the sound changes a bit then make mixture adjustments as necessary to keep the engine smooth on the LOP side. As you get closer to your destination reduce RPM as much as possible to keep the engine from shock cooling as the load produced is going down. When you get in the pattern then configure the aircraft for landing and go-around as normal and land.

I hope this gives you a better understanding of how to apply the RED FIN chart to LOP and ROP operations.

Have FUN! Fly SAFE! Petehdgs

All good advice. If you have a simpler plane, like a C172 or PA-28-161, some of this won't apply, and you can use a simpler technique:

• Keep the throttle wide open at least until you're at your cruising altitude.
• Like @William Pete Hodges suggested, check your EGT right after takeoff, and above (say) 3,000 ft AGL, lean just enough to keep the EGT there.
• At cruise altitude, for LOP, leave the throttle wide open and lean to your target RPM; for ROP, set just below your target RPM with the throttle, then lean to peak RPM.
• During descent, you can leave the engine leaned as long as it doesn't stumble (just throttle back to your descent RPM).

 

[Pilawt]

check your EGT right after takeoff, and above (say) 3,000 ft AGL, lean just enough to keep the EGT there

My only suggested change would be to note, if you have the opportunity, what the EGT is on your engine full rich at climb power on a standard day at or near sea level, then lean to that target during the climb, even if departing from a higher elevation or warmer conditions. Many is the time I take off from my 900'-MSL field on a hot day with density altitudes pushing 4,000' or more, and I'll start leaning to that "happy place" right away (180 hp O-360-A4M, fixed pitch).

Proper leaning in the climb produces noticeably more power, which allows climb at a higher airspeed, which lowers CHT.

 

[Tom-D]

Something I wonder about, 135 operators spend a lot money on gadget to extend their engine life. when they are bound by TBO and must replace on time.

 

[David Megginson]

Something I wonder about, 135 operators spend a lot money on gadget to extend their engine life. when they are bound by TBO and must replace on time.

Perhaps they want to reduce the risk of paying $15K for a full top overhaul halfway to TBO.

Also, I don't think even Part 135 operators are required to overhaul at TBO, unless it's in their own ops specs; they can continue to operate their engines "on condition". More here:

https://www.avweb.com/ownership/the-savvy-aviator-4-debunking-tbo/

 

[Larry Vrooman]

Sitting on a hot taxiway with no wind at idle, you're way below 65% and can certainly harm your engine by improper leaning.

Man where do you come up with this stuff

There are potentially two ways to harm the engine with improper leaning on the group.

The first is the theoretical issue that I think Salty is suggesting where on a hot day on a hot taxiway at idle with no wind (or a tail wind), there might not be enough air passing over the cylinders to keep them cool. I've never actually seen that, but it could happen. In that case using excess fuel to cool the engine is an option.

The second is running an overly rich mixture at low power on the ground with excessively low combustion temps that then result in lead compounds condensing out on the exhaust valve seats and stems as the exhaust gasses exit the cylinder. IMHO that is a far bigger risk in the long term as it leads to top overhauls due to warped valves and sticking valves.

 

[RoscoeT]

There are potentially two ways to harm the engine with improper leaning on the group.

The first is the theoretical issue that I think Salty is suggesting where on a hot day on a hot taxiway at idle with no wind (or a tail wind), there might not be enough air passing over the cylinders to keep them cool. I've never actually seen that, but it could happen. In that case using excess fuel to cool the engine is an option.

I challenge you to note visible differences in CHT with aggressive ground leaning vs. none while at taxi/warm up RPM.

 

[David Megginson]

The first is the theoretical issue that I think Salty is suggesting where on a hot day on a hot taxiway at idle with no wind (or a tail wind), there might not be enough air passing over the cylinders to keep them cool. I've never actually seen that, but it could happen. In that case using excess fuel to cool the engine is an option.

As I understand it, the main way excess fuel "cools" an engine is by reducing CHT through a richer mixture (once you're on the rich side of 50°F ROP). When you're idling on the ground, CHT is already so low that it wouldn't (I think) make a meaningful difference where you landed on mixture curve. The concept of the extra fuel carrying away the heat out the exhaust was a bit of an old pilots' tale.

For those of you with modern engine monitors, what are the highest CHTs you see while idling/taxiing on the ground? How much does the mixture affect those?

 

[Ed Haywood]

I have been flying for 28 years. In that time I have flown with at least 25 CFI's for training, BFR's, rental checkrides, etc. That includes 3 FAA DPEs, several retired military pilots, and several who were also A&P/IAs.

I have never heard of leaning for taxi. Not once.

 

[denverpilot]

I have been flying for 28 years. In that time I have flown with at least 25 CFI's for training, BFR's, rental checkrides, etc. That includes 3 FAA DPEs, several retired military pilots, and several who were also A&P/IAs.

I have never heard of leaning for taxi. Not once.

Must not live at high DA. Wanna foul plugs, don’t do it up here... it’ll be an enjoyable wait after you fail the mag check. LOL.

 

[Ed Haywood]

Must not live at high DA. Wanna foul plugs, don’t do it up here... it’ll be an enjoyable wait after you fail the mag check. LOL.

I do not now, but in 1994 I flew with the Peterson AFB flying club out of KCOS. By far the most anal retentive checkrides I have ever experienced. Looking at some of my old checklists from there, looks like they did specify to lean 1" after start above 3000 MSL.

 

[chemgeek]

I have been flying for 28 years. In that time I have flown with at least 25 CFI's for training, BFR's, rental checkrides, etc. That includes 3 FAA DPEs, several retired military pilots, and several who were also A&P/IAs.

I have never heard of leaning for taxi. Not once.

Many owners have known about aggressive leaning for ground ops for at least 20 years. One of the first places I remember reading about this practice was in Light Plane Maintenance. Ever since I learned to use this technique, I've never had to fish lead balls out of my plugs, and have never been grounded due to lead fouling. It's especially important for operations at larger airports where taxiing a little plane all over creation to reach the active can result in extended ground ops.

 

[denverpilot]

I do not now, but in 1994 I flew with the Peterson AFB flying club out of KCOS. By far the most anal retentive checkrides I have ever experienced. Looking at some of my old checklists from there, looks like they did specify to lean 1" after start above 3000 MSL.

Guess they didn’t know how to read the POH.

It says something in it... for most aircraft... and it’s not that.

 

[David Megginson]

I have been flying for 28 years. In that time I have flown with at least 25 CFI's for training, BFR's, rental checkrides, etc. That includes 3 FAA DPEs, several retired military pilots, and several who were also A&P/IAs.

I have never heard of leaning for taxi. Not once.

I'm not sure I see your point. I'm sure they were excellent pilots, but they couldn't have known more about engine operations than was available to them at the time.

On that note, Alan Turing was one of the most-brilliant mathematicians of the 20th century, and the star of Bletchley Park's code breaking, but his friends learned quickly that he was hopeless at calculating how to split a bill at a cafe and stopped letting him try. We all have our strengths and weaknesses, and being good at one thing doesn't make you an expert in everything else.

 

[Larry Vrooman]

As I understand it, the main way excess fuel "cools" an engine is by reducing CHT through a richer mixture (once you're on the rich side of 50°F ROP). When you're idling on the ground, CHT is already so low that it wouldn't (I think) make a meaningful difference where you landed on mixture curve. The concept of the extra fuel carrying away the heat out the exhaust was a bit of an old pilots' tale.

For those of you with modern engine monitors, what are the highest CHTs you see while idling/taxiing on the ground? How much does the mixture affect those?

Just to be clear...we are in total agreement on this. I was just pointing to where Salty was coming from. I was also being nice by saying it was mostly theoretical.

Here in NC on a 95-100 degree day flying around at low altitude, with my IO-320 warm enough to start stumbling at low rpm due to heat and vapor pressure issue in the fuel lines, my CHTs are maybe 285 tops.

I am also leaning aggressively on the ground with EGTs in the 1200 degree range in an effort (successful) to prevent lead fouling. If anything that supports the idea that leaner mixtures are effective for keeping CHTs down, by staying out of the "best power" range for any given RPM.

Now...to be fair that might be partially do to my 1967 7KCAB still having a generator and needing to keep the rpm up around 1000-1200 rpm to produce electricity, but that same option is also open to anyone with an alternator who is taxiing on a hot taxi way.

 

[Larry Vrooman]

I have been flying for 28 years. In that time I have flown with at least 25 CFI's for training, BFR's, rental checkrides, etc. That includes 3 FAA DPEs, several retired military pilots, and several who were also A&P/IAs.

I have never heard of leaning for taxi. Not once.

I hate to break it to you but that is, at best, an ad populum logical fallacy. At worst, it's pretty clear evidence that pilot training is badly flawed.

There are a few reasons that CFI's don't teach or advocate leaning engines below 5000'.

First, it's one more thing to teach and airplanes fly pretty well for the most part below 5000' without leaning.

On the other hand, that situation changes when you own the aircraft and both a) find yourself footing the bill for maintenance, and b) don't have salaried mechanics on staff to address the issues that come up when pilots don't lean on the ground.

Second, if a CFI teaches the student to lean on the ground there is an implied risk that the student once he or she is doing unsupervised solo flight might forget to enrich the mixture on take off. That could be harmful to the engine.

That said, CFIs should be doing two things to mitigate that risk. CFIs should be teaching student pilots to not just pick up, but actually use and follow checklists. That includes a pre-take off checklist that requires the mixture to be either full rich, or at high density altitudes of 5000', above lean for best power. In addition, CFIs should be teaching students to lean *aggressively* on the ground, to a point just short of causing the engine to stumble at the rpm used during taxi. In the vast majority of cases, that will be lean to the point that if the student ignores the checklist, or fails to actually follow through with the mixture as a check list item, the engine will stumble when the student advances the throttle for take off. The CFI then needs to let both those things happen with the student to hammer home the importance of properly setting the mixture just prior to takeoff.

Another complicating factor are some aircraft where the checklist requires the engine to be leaned to best power during the run up. In my 7KCAB leaning to best power at 1800 rpm will leave the mixture rich enough to allow take off at full throttle - but with an excessively lean mixture. That means the CFI also need to teach the pilot to re-lean to just short of a stumble at taxi RPM, if there is some distance or time between the run up area and the start of the taek off, or put it to full rich if the takeoff is imminent. In either case, the pre-take off checklist will again cover it.

Third, CFIs are largely self replicating. Many come from 141 programs that are really geared toward churning out pilots who learn procedures that generalize well to later flights in larger aircraft used by scheduled air carriers. However, the reality is that most spend the next 1000-1500 hours instructing. Since they never learned how to properly learn, they can't teach what they don't know.

The example you gave - an experienced 28 year pilot with 3 checkrides and a lot of BFRs under his belt - is a pretty good indication that the level of knowledge around leaning on the ground just isn't adequate. But saying "most CFI's don't teach it" doesn't make it the right thing to do or the right way to operate an engine on the ground.

 

[Larry Vrooman]

Must not live at high DA. Wanna foul plugs, don’t do it up here... it’ll be an enjoyable wait after you fail the mag check. LOL.

Exactly.

The CFIs who do teach students to lean for best power when taking off at 5000 feet or above are probably also the CFIs who teach students how to clear a fouled plug - by running up to 1500-1800 rpm and then aggressively leaning the mixture until the plugs clear.

 

[Larry Vrooman]

I challenge you to note visible differences in CHT with aggressive ground leaning vs. none while at taxi/warm up RPM.

You'll get no argument from me. I was playing devil's advocate by adding the usual explanation I hear from people who think full rich is needed on the ground to keep the engine cool. Note use of the the "theoretical", as opposed to practical.

----

Running full rich at low power settings on the ground will foul your plugs and cause other more serious issues, especially with 100LL in low compression engines designed for 80/87 octane. Those lower compression ratios result in lower combustion temps and in turn the lead compounds in the combustion gasses condensing on valve seats and stems.

 

[smv]

That means the CFI also need to teach the pilot to re-lean to just short of a stumble at taxi RPM, if there is some distance or time between the run up area and the start of the taek off, or put it to full rich if the takeoff is imminent. In either case, the pre-take off checklist will again cover it.

I accommodate this scenario by structuring the checklist in such a manner that the runup checklist is separated from the takeoff checklist by the "Pre-Takeoff Brief". Since the briefing is not something we rush through (making those critical decisions now, on the ground) I make a point of re-leaning for "ground operations" prior to conducting the briefing. So far it seems to have stuck because after a couple times of doing that, the students remember to re-lean on their own.

I also strongly advocate aggressive leaning on the ground to the point where we sometimes have to enrich a little just to back-taxi up our unusually steep sloped runway without the engine stumbling.

 

[ateamer]

I was taught to lean for taxi, on the first taxi, of my first lesson. Been doing that ever since, and the only time I’ve had fouled plugs was on rentals that someone else had beforehand.

 

[David Megginson]

I accommodate this scenario by structuring the checklist in such a manner that the runup checklist is separated from the takeoff checklist by the "Pre-Takeoff Brief". Since the briefing is not something we rush through (making those critical decisions now, on the ground) I make a point of re-leaning for "ground operations" prior to conducting the briefing. So far it seems to have stuck because after a couple times of doing that, the students remember to re-lean on their own.

I also strongly advocate aggressive leaning on the ground to the point where we sometimes have to enrich a little just to back-taxi up our unusually steep sloped runway without the engine stumbling.

Absolutely right — if you're going to lean on the ground, lean big, so that the engine will just sputter and stop if you try to take without enriching again.

In my PA-28-161, I also push with a flat palm for take-off or go-around (at lower elevations) so that there's no lever left behind.

 

[sarangan]

I hate to break it to you but that is, at best, an ad populum logical fallacy. At worst, it's pretty clear evidence that pilot training is badly flawed.

There are a few reasons that CFI's don't teach or advocate leaning engines below 5000'.

First, it's one more thing to teach and airplanes fly pretty well for the most part below 5000' without leaning.

On the other hand, that situation changes when you own the aircraft and both a) find yourself footing the bill for maintenance, and b) don't have salaried mechanics on staff to address the issues that come up when pilots don't lean on the ground.

Second, if a CFI teaches the student to lean on the ground there is an implied risk that the student once he or she is doing unsupervised solo flight might forget to enrich the mixture on take off. That could be harmful to the engine.

That said, CFIs should be doing two things to mitigate that risk. CFIs should be teaching student pilots to not just pick up, but actually use and follow checklists. That includes a pre-take off checklist that requires the mixture to be either full rich, or at high density altitudes of 5000', above lean for best power. In addition, CFIs should be teaching students to lean *aggressively* on the ground, to a point just short of causing the engine to stumble at the rpm used during taxi. In the vast majority of cases, that will be lean to the point that if the student ignores the checklist, or fails to actually follow through with the mixture as a check list item, the engine will stumble when the student advances the throttle for take off. The CFI then needs to let both those things happen with the student to hammer home the importance of properly setting the mixture just prior to takeoff.

Another complicating factor are some aircraft where the checklist requires the engine to be leaned to best power during the run up. In my 7KCAB leaning to best power at 1800 rpm will leave the mixture rich enough to allow take off at full throttle - but with an excessively lean mixture. That means the CFI also need to teach the pilot to re-lean to just short of a stumble at taxi RPM, if there is some distance or time between the run up area and the start of the taek off, or put it to full rich if the takeoff is imminent. In either case, the pre-take off checklist will again cover it.

Third, CFIs are largely self replicating. Many come from 141 programs that are really geared toward churning out pilots who learn procedures that generalize well to later flights in larger aircraft used by scheduled air carriers. However, the reality is that most spend the next 1000-1500 hours instructing. Since they never learned how to properly learn, they can't teach what they don't know.

The example you gave - an experienced 28 year pilot with 3 checkrides and a lot of BFRs under his belt - is a pretty good indication that the level of knowledge around leaning on the ground just isn't adequate. But saying "most CFI's don't teach it" doesn't make it the right thing to do or the right way to operate an engine on the ground.

I agree, during taxi one should lean until the engine is about to stumble. Part of the problem is that most POH do not say much about leaning except in the context of short field takeoff from high density altitude airports. They imply that the mixture should stay at full rich except in special cases. The important thing to realize is that most normally aspirated engine cannot be damaged by any amount of leaning below 75% power. At high density altitudes, it is best to lean at full throttle prior to takeoff. It is more commonplace to lean at the runup RPM and then richen it by a couple of turns.

 

[David Megginson]

I agree, during taxi one should lean until the engine is about to stumble. Part of the problem is that most POH do not say much about leaning except in the context of short field takeoff from high density altitude airports. They imply that the mixture should stay at full rich except in special cases. The important thing to realize is that most normally aspirated engine cannot be damaged by any amount of leaning below 75% power. At high density altitudes, it is best to lean at full throttle prior to takeoff. It is more commonplace to lean at the runup RPM and then richen it by a couple of turns.

Below 65% power it's not an issue, for sure. Between 65% and 75% there's a bit of a ongoing discussion.

In any case, just try to avoid the area near 50°F ROP (highest CHT) by a decent margin if you're between 65% and 75% power, and you should be fine. It doesn't have to be complicated: with a fixed-pitch prop and no engine monitor (e.g. a PA-28-161 or C172), that means aim for peak RPM (which will come in around 150°F ROP) or richer if you're flying ROP, or LOP-WOT (at or a bit below 50°F LOP) if your engine's fuel/air distribution permits smooth operation there. That's giving you a 100°F EGT margin on either side of the hot spot.

 

[bobmrg]

I have been flying for 28 years. In that time I have flown with at least 25 CFI's for training, BFR's, rental checkrides, etc. That includes 3 FAA DPEs, several retired military pilots, and several who were also A&P/IAs.

I have never heard of leaning for taxi. Not once.

Make this the first time, then, and ALWAYS lean when operatimg at less than 65%/

Bob Gardner

 

[Jeff767]

The old adage - “It depends” - Turn off the master in an aircraft with electronic ignition the engine will quit because you’ve robbed the ignition of power. To keep the engine running you must switch to the alternate power source, usually a backup battery, then can shut off the master and the engine will continue to run. If you have an primary electrically failure in flight, need to switch to the back up battery ASAP to continue to fly.

Every aircraft I have flown with a Electronic ignition required at least one of the two ignition sources to be wired to a always hot source of power. This insures that the engine will continue to run with the master off. I am getting ready to install a Surefly system and it has that same requirement even when backed up by a mag on the other side.

 

[Daleandee]

In my PA-28-161, I also push with a flat palm for take-off or go-around (at lower elevations) so that there's no lever left behind.

Yep ... going around means everything (throttle, mixture, carb heat) goes to the firewall.