Mixture Control
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At the risk of stating something obvious, and probably why a lot of organisations teach, and attempt to push people towards rich operations...
There was a historical "myth" that running LoP (lean of peak) would damage engines. This is probably why many POH of old planes often prohibit this. I have put myth in quotes, because, although LoP if done right and accurately, with accurate engine instrumentation has been proved to actually be better for the engine, than running ROP (or over-rich for that matter). Doing it wrong however, could cause both short and long term problems with the engine.
Most spam-cans (pa28/c172/152) that I flew during my training were often 40+ years old, with engine instrumentation somewhat basic and old. Temps being drawn from only one CHT/EGT probe. (whose accuracy I could only guess was roughly working as the needle was moving...) If anyone has ever flown with a digital engine management system with each cylinders displayed, they will know that there is a range of temperatures, especially in old(er) engines. So flying without each cylinders, would be leaving you with 3 unknown temps which may be causing damage without you having the faintest idea.
If you cannot accurately manage the engine temps, you do run the risk of making things worse for yourself. Running on the rich side, with the exception of fouling the plugs (rare above 1000rpm), you don't really have that much risk, and therefore a more cautious option for most of us flying crappy, old and tired planes.
If however, you do have a fancy digital engine management with proper calibration, and installation - you are better (so data tells me) running LoP, but this is often a continual adjustment to ensure proper operations. You can't get complacent about it - saying "oh I did that about 30 minutes ago in the cruise" because any slight difference (OAT, ALT, Pressure) will cause fluctuation and as you are running on the edge, you do need to manage it actively to ensure proper operation. (which is why I tend to prefer ROP operations - and enjoy the scenery a little more)
On a side note, a number of Mooney acclaim and ovations have been reported to have had shortened engine lifespan as a direct result of running over-lean, (so people tell each other) but I can only attribute that to poor engine management (despite their fancy kit).
Back to the main question however, leaning on the ground.... Do you ever remember your instructor telling you to maintain at least 1000rpm whilst taxying, and that you had to close the throttle, and use the brakes to slow down before returning to at least 1000rpm? Well that is so as to not foul the plugs at low rpm. An alternative to this, is to reduce the amount of excess fuel being fed to the engine, hence the leaning on the ground. The main reason why instructors that I know are not so keen on teaching to lean on the ground is because if you are a newly qualified PPL, with 3 friends in the plane, all excited and distracting you, the likelihood of you forgetting something is increased, and they believe (quite rightly in my mind) that forgetting the mixture would not be a great idea on takeoff!
With regards to being on final, I was always taught to go full rich as part of my downwind checks. I don't think it is a good idea to leave it till the go-around, or that moment when you suddenly need full power during final due to severe downdrafts / windshear, because chances are you'll need it full rich, and not have the power you need, when you need it, as a result.
There was a historical "myth" that running LoP (lean of peak) would damage engines. This is probably why many POH of old planes often prohibit this. I have put myth in quotes, because, although LoP if done right and accurately, with accurate engine instrumentation has been proved to actually be better for the engine, than running ROP (or over-rich for that matter). Doing it wrong however, could cause both short and long term problems with the engine.
Most spam-cans (pa28/c172/152) that I flew during my training were often 40+ years old, with engine instrumentation somewhat basic and old. Temps being drawn from only one CHT/EGT probe. (whose accuracy I could only guess was roughly working as the needle was moving...) If anyone has ever flown with a digital engine management system with each cylinders displayed, they will know that there is a range of temperatures, especially in old(er) engines. So flying without each cylinders, would be leaving you with 3 unknown temps which may be causing damage without you having the faintest idea.
If you cannot accurately manage the engine temps, you do run the risk of making things worse for yourself. Running on the rich side, with the exception of fouling the plugs (rare above 1000rpm), you don't really have that much risk, and therefore a more cautious option for most of us flying crappy, old and tired planes.
If however, you do have a fancy digital engine management with proper calibration, and installation - you are better (so data tells me) running LoP, but this is often a continual adjustment to ensure proper operations. You can't get complacent about it - saying "oh I did that about 30 minutes ago in the cruise" because any slight difference (OAT, ALT, Pressure) will cause fluctuation and as you are running on the edge, you do need to manage it actively to ensure proper operation. (which is why I tend to prefer ROP operations - and enjoy the scenery a little more)
On a side note, a number of Mooney acclaim and ovations have been reported to have had shortened engine lifespan as a direct result of running over-lean, (so people tell each other) but I can only attribute that to poor engine management (despite their fancy kit).
Back to the main question however, leaning on the ground.... Do you ever remember your instructor telling you to maintain at least 1000rpm whilst taxying, and that you had to close the throttle, and use the brakes to slow down before returning to at least 1000rpm? Well that is so as to not foul the plugs at low rpm. An alternative to this, is to reduce the amount of excess fuel being fed to the engine, hence the leaning on the ground. The main reason why instructors that I know are not so keen on teaching to lean on the ground is because if you are a newly qualified PPL, with 3 friends in the plane, all excited and distracting you, the likelihood of you forgetting something is increased, and they believe (quite rightly in my mind) that forgetting the mixture would not be a great idea on takeoff!
With regards to being on final, I was always taught to go full rich as part of my downwind checks. I don't think it is a good idea to leave it till the go-around, or that moment when you suddenly need full power during final due to severe downdrafts / windshear, because chances are you'll need it full rich, and not have the power you need, when you need it, as a result.
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Slightly ROP is the worst place to be (25 - 50F ROP) because the fuel burns the fastest then and with a fixed advance ignition the cylinder head reaches the maximum temperature and pressures within the combustion chamber (BMEP) is at the highest. That is why training aircraft suffer many cylinder head failures/cracking.
I will try to post the Lycoming graph which you will find in almost all POH's for the 'general training aircraft.' (EGT / CHT / Percentage Power / Specific Fuel Consumption.) When leaning the mixture, the CHT peaks first (slightly ROP) if you continue leaning to PEAK EGT; the CHT DECREASES. It is safer to run at peak EGT than 'slightly ROP.' (Providing less than 75% power)
Follow the graph and check CHT at 50F LOP and look to the right to find the ROP temperature that gives the same CHT and you will find it to be off scale on the rich side. IE full rich will result in a hotter CHT than 50F LOP.
Looking at the fuel consumption graph, it is flat from peak EGT to 50F LOP then becomes worse. (GAMI et al have found that there is a very slight improvement.) 50F LOP is not for better economy as HP drops off rapidly, it is for cooler CHT as that also drops off rapidly.
Last edited by Chuck Notyeager; 29th Jun 2017 at 07:03.
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For those that haven't found it yet, here is the John Deakin article about leaning. It was written nearly twenty years ago and remains valid today. Expect to spend several hours to absorb what is in there.
https://tinyurl.com/ybjoov4e
https://tinyurl.com/ybjoov4e
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At the risk of stating something obvious, and probably why a lot of organisations teach, and attempt to push people towards rich operations...
There was a historical "myth" that running LoP (lean of peak) would damage engines. This is probably why many POH of old planes often prohibit this. I have put myth in quotes, because, although LoP if done right and accurately, with accurate engine instrumentation has been proved to actually be better for the engine, than running ROP (or over-rich for that matter). Doing it wrong however, could cause both short and long term problems with the engine.
Most spam-cans (pa28/c172/152) that I flew during my training were often 40+ years old, with engine instrumentation somewhat basic and old. Temps being drawn from only one CHT/EGT probe. (whose accuracy I could only guess was roughly working as the needle was moving...) If anyone has ever flown with a digital engine management system with each cylinders displayed, they will know that there is a range of temperatures, especially in old(er) engines. So flying without each cylinders, would be leaving you with 3 unknown temps which may be causing damage without you having the faintest idea.
If you cannot accurately manage the engine temps, you do run the risk of making things worse for yourself. Running on the rich side, with the exception of fouling the plugs (rare above 1000rpm), you don't really have that much risk, and therefore a more cautious option for most of us flying crappy, old and tired planes.
If however, you do have a fancy digital engine management with proper calibration, and installation - you are better (so data tells me) running LoP, but this is often a continual adjustment to ensure proper operations. You can't get complacent about it - saying "oh I did that about 30 minutes ago in the cruise" because any slight difference (OAT, ALT, Pressure) will cause fluctuation and as you are running on the edge, you do need to manage it actively to ensure proper operation. (which is why I tend to prefer ROP operations - and enjoy the scenery a little more)
On a side note, a number of Mooney acclaim and ovations have been reported to have had shortened engine lifespan as a direct result of running over-lean, (so people tell each other) but I can only attribute that to poor engine management (despite their fancy kit).
Back to the main question however, leaning on the ground.... Do you ever remember your instructor telling you to maintain at least 1000rpm whilst taxying, and that you had to close the throttle, and use the brakes to slow down before returning to at least 1000rpm? Well that is so as to not foul the plugs at low rpm. An alternative to this, is to reduce the amount of excess fuel being fed to the engine, hence the leaning on the ground. The main reason why instructors that I know are not so keen on teaching to lean on the ground is because if you are a newly qualified PPL, with 3 friends in the plane, all excited and distracting you, the likelihood of you forgetting something is increased, and they believe (quite rightly in my mind) that forgetting the mixture would not be a great idea on takeoff!
With regards to being on final, I was always taught to go full rich as part of my downwind checks. I don't think it is a good idea to leave it till the go-around, or that moment when you suddenly need full power during final due to severe downdrafts / windshear, because chances are you'll need it full rich, and not have the power you need, when you need it, as a result.
There was a historical "myth" that running LoP (lean of peak) would damage engines. This is probably why many POH of old planes often prohibit this. I have put myth in quotes, because, although LoP if done right and accurately, with accurate engine instrumentation has been proved to actually be better for the engine, than running ROP (or over-rich for that matter). Doing it wrong however, could cause both short and long term problems with the engine.
Most spam-cans (pa28/c172/152) that I flew during my training were often 40+ years old, with engine instrumentation somewhat basic and old. Temps being drawn from only one CHT/EGT probe. (whose accuracy I could only guess was roughly working as the needle was moving...) If anyone has ever flown with a digital engine management system with each cylinders displayed, they will know that there is a range of temperatures, especially in old(er) engines. So flying without each cylinders, would be leaving you with 3 unknown temps which may be causing damage without you having the faintest idea.
If you cannot accurately manage the engine temps, you do run the risk of making things worse for yourself. Running on the rich side, with the exception of fouling the plugs (rare above 1000rpm), you don't really have that much risk, and therefore a more cautious option for most of us flying crappy, old and tired planes.
If however, you do have a fancy digital engine management with proper calibration, and installation - you are better (so data tells me) running LoP, but this is often a continual adjustment to ensure proper operations. You can't get complacent about it - saying "oh I did that about 30 minutes ago in the cruise" because any slight difference (OAT, ALT, Pressure) will cause fluctuation and as you are running on the edge, you do need to manage it actively to ensure proper operation. (which is why I tend to prefer ROP operations - and enjoy the scenery a little more)
On a side note, a number of Mooney acclaim and ovations have been reported to have had shortened engine lifespan as a direct result of running over-lean, (so people tell each other) but I can only attribute that to poor engine management (despite their fancy kit).
Back to the main question however, leaning on the ground.... Do you ever remember your instructor telling you to maintain at least 1000rpm whilst taxying, and that you had to close the throttle, and use the brakes to slow down before returning to at least 1000rpm? Well that is so as to not foul the plugs at low rpm. An alternative to this, is to reduce the amount of excess fuel being fed to the engine, hence the leaning on the ground. The main reason why instructors that I know are not so keen on teaching to lean on the ground is because if you are a newly qualified PPL, with 3 friends in the plane, all excited and distracting you, the likelihood of you forgetting something is increased, and they believe (quite rightly in my mind) that forgetting the mixture would not be a great idea on takeoff!
With regards to being on final, I was always taught to go full rich as part of my downwind checks. I don't think it is a good idea to leave it till the go-around, or that moment when you suddenly need full power during final due to severe downdrafts / windshear, because chances are you'll need it full rich, and not have the power you need, when you need it, as a result.
Restrictions on running LoP were not so much a "myth" as a certain limitation to specific types, I might be wrong but I think this was specifically related to higher-powered engines in Ye Olde Worlde Days when lower-octane fuel was in common usage. Higher-powered engines are more sensitive to detonation and for correct operation higher-octane fuels are a necessity. Just why we've ended up with having to run 180hp 4-cylinder engines on 100-octane fuel that requires toxic additive is a mystery to me (like many things in aviation, "Cos that's the way we've always done it") however we have what we have. Running LoP on certain engines or general cack-handed operation of the mixture control was a sure-fire way of inducing detonation which causes catastrophic engine damage in the medium to long term no mattery what type. Hence we ended up with people who were terrified of attempting to run LoP or even trying to lean the mixture at all.
Most engines in common GA aircraft today will quite happily run LoP (if done correctly in accordance with the POH and manufacturer's data, as alluded to in the previous posts with) without any detriment to the engine operation or lifespan if done correctly.
Often in horizontally-opposed engines different cylinders will run slightly leaner or richer than the others (and at a different temp), so unless you have a fancy engine analyser leaning can often be a bit of a compromise as one or two cylinders may start rough running as they're running a bit leaner than the other cylinders.
P.S. I was always taught that closing the throttle completely on taxiing before applying brakes was to save wear on the brakes, as even at 1000rpm there's enough thrust there to work against the brakes - saves Mx costs!
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Interesting Chuck! I am quite surprised to hear your thoughts on this! So you're suggesting that they may have been running slightly ROP and that caused damage? That is surprising indeed! I may be reading what you linked wrong, but I don't see that significant a temp change between LOP and ROP. I was always under the impression that ROP operations would always enable a slight element of cooling from excess fuel going in. Interesting!
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Interesting Chuck! I am quite surprised to hear your thoughts on this! So you're suggesting that they may have been running slightly ROP and that caused damage? That is surprising indeed! I may be reading what you linked wrong, but I don't see that significant a temp change between LOP and ROP. I was always under the impression that ROP operations would always enable a slight element of cooling from excess fuel going in. Interesting!
The reason why the CHT peaks first is the fuel burns the fastest at that fuel/air (Stoicometric ratio sp?) and because of the fixed ignition most of the heat goes into the cylinder head and not into pushing the piston down. (Ineffective crank angle plays a part here, transferring linear motion to rotating motion.) Peak EGT slows the combustion and most of the heat/energy is used to force the piston down the cylinder. The cylinder head has to absorb less heat and thus runs cooler.
Below 75% power, if you are scared of LOP operation, then do your engine a favour and stay at peak EGT.
Last edited by Chuck Notyeager; 29th Jun 2017 at 15:18.
Never quite understood what ROP and LOP meant. Which Peak are you talking about?
If it is the peak of propeller rpm, then that is not really 'peaky' at all. It increases by about 50rpm then drops considerably as you go from Rich to ICO.
.
If it is the peak of propeller rpm, then that is not really 'peaky' at all. It increases by about 50rpm then drops considerably as you go from Rich to ICO.
.
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The 'Peak' referred to is peak EGT.
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Rich/Lean of Peak refers to EGT.
Peak EGT is to the rich side of the best economy range IIRC - RPM wise I think that is when you set cruise power (e.g. 2200rpm), lean to peak RPM then keep leaning until RPM falls back to the original set value... however I could be wrong as I've never used that method in practice. ISTR that Best Power is lean to peak RPM then nudge it slightly rich, however my memory is somewhat hazy after several nights without sleep (teething toddler & pregnant wife about to pop No2 - not good
) - happy to be corrected where wrong.(As said previously, all for NA fixed-pitch props)
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Rich/Lean of Peak refers to EGT.
Peak EGT is to the rich side of the best economy range IIRC - RPM wise I think that is when you set cruise power (e.g. 2200rpm), lean to peak RPM then keep leaning until RPM falls back to the original set value... however I could be wrong as I've never used that method in practice. ISTR that Best Power is lean to peak RPM then nudge it slightly rich, however my memory is somewhat hazy after several nights without sleep (teething toddler & pregnant wife about to pop No2 - not good) - happy to be corrected where wrong.
(As said previously, all for NA fixed-pitch props)
Peak EGT is to the rich side of the best economy range IIRC - RPM wise I think that is when you set cruise power (e.g. 2200rpm), lean to peak RPM then keep leaning until RPM falls back to the original set value... however I could be wrong as I've never used that method in practice. ISTR that Best Power is lean to peak RPM then nudge it slightly rich, however my memory is somewhat hazy after several nights without sleep (teething toddler & pregnant wife about to pop No2 - not good
) - happy to be corrected where wrong.(As said previously, all for NA fixed-pitch props)
Peak EGT, is peak EGT. Period. The idea of setting '2200rpm' then leaning past max power (read more RPM) to (less power) the same RPM means you are at the same point in the power delivered arc in the Lycoming graph above. Nothing else.You might or might not have gone past peak EGT, you might or might not be ROP. The method proves nothing and is just another OWT. By your own admission, you have heard about it but not tried it. Now you need not try it nor use it.
In a carbureted GA 'trainer' in cruise pull the red knob back till the engine runs rough, then richen only enough for smooth operation. If you have an EGT gauge then use it to set max EGT and leave it there provided it runs smooth. (Mr Lycoming's advice, not mine.) The reason for insisting on smooth running should be obvious, torsional vibration from uneven power pulses is bad.
Mr Lycoming says max power happens at 100 to 150F ROP, GAMI/Atkinson et al have found this figure is actually about 75 to 90F ROP so unless you have a fancy engine monitor, rather run 100 to 150 ROP if in a race.
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I appreciate this doesn't answer any questions, but during my PPL not once was I ever shown or told how to lean, I was told that seeing as the price we pay per hour is "wet" then to leave it fully rich as the school are cautious of damage to the engine.
I have to be honest since finishing my ppl I've been mainly focusing on ATPL exams but in the little bit of hour building I've done I've tried leaning it a few times whilst in the cruise. I know that I still leave it on the rich side but without having been taught what to do I feel it's probably better to be on the safe side as I'd be worried of over doing it
I have to be honest since finishing my ppl I've been mainly focusing on ATPL exams but in the little bit of hour building I've done I've tried leaning it a few times whilst in the cruise. I know that I still leave it on the rich side but without having been taught what to do I feel it's probably better to be on the safe side as I'd be worried of over doing it
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Stefan, did you ever read the POH for the aircraft you're flying? It's a good habit to get into for any type or sort of aircraft you're flying now, or are going to fly in the future.
When you follow the advice in there, you really can't go wrong. Plus, the manufacturer has taken into account the type of engine (fixed or variable pitch, injection or not) and the instruments fitted (RPM, EGT, CHT or not) and tailored its advice particularly for that situation. So you don't have to read through two pages of how to lean for Peak EGT or ROP or LOP when you don't even have an EGT fitted.
And if anybody questions your leaning practices, you can simply point to the POH and tell them that you're flying the aircraft according to the manufacturers advice. That'll shut the majority of critics up.
A word of caution here. You need to either do as you're taught, or you need to follow the manufacturers advice - assuming those two are not the same. But if you're just going to be making up your own compromises you may actually be putting yourself in danger. This doesn't apply so much to leaning, although there may be a few corner cases where leaning "just a bit" may be more damaging to the engine than not leaning at all or leaning all the way. But it applies more to flying in general. If you make up your own compromises which are not taught or described anywhere, you are in fact a test pilot. (A far more extreme example would be an outside loop down. You either don't do it at all, or you need to do it with full gusto, headache be damned. But if you do it halfheartedly you'll find yourself exceeding Vne really quickly, with no easy way out. And the cows getting bigger awfully fast.)
Fully agree. Best performance for the lowest fuel burn requires careful management of MAP, RPM and Mixture. However, it also requires a fair portion of your attention, so you should also know when to compromise because your attention is required elsewhere. That's why, somewhere during the approach, we shove both RPM and mixture all the way forward. Two less things to worry about in case of a possible go-around.
When you follow the advice in there, you really can't go wrong. Plus, the manufacturer has taken into account the type of engine (fixed or variable pitch, injection or not) and the instruments fitted (RPM, EGT, CHT or not) and tailored its advice particularly for that situation. So you don't have to read through two pages of how to lean for Peak EGT or ROP or LOP when you don't even have an EGT fitted.
And if anybody questions your leaning practices, you can simply point to the POH and tell them that you're flying the aircraft according to the manufacturers advice. That'll shut the majority of critics up.
I know that I still leave it on the rich side but without having been taught what to do I feel it's probably better to be on the safe side as I'd be worried of over doing it
Long story short, the bottom line is that the Mixture control is as much and engine control as the Throttle is.
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@BNI: If you want to stay happy as a Pistonpilot, learn and understand what the mixture knob does! I am very surprised you have training without intense teaching on how to manage fuel. Am I right, you have not gone through that part of the training were you scrub 15 traffic circles in 45 minutes?
Yes, it is indeed correct to point you to the POH, but it may be misleading as well. If you do training in an aircraft which was designed i.e. for 87 octane use and now fly on 100LL, the POH is not written for that and at latest, your FI should teach you what to do with the excess lead. If you drive an old 172 or 28 with TCM or Lycosaurus, you will get into trouble over time when not leaning on the ground -> thy collect lead on the sparks pretty quickly.
Yes, it is indeed correct to point you to the POH, but it may be misleading as well. If you do training in an aircraft which was designed i.e. for 87 octane use and now fly on 100LL, the POH is not written for that and at latest, your FI should teach you what to do with the excess lead. If you drive an old 172 or 28 with TCM or Lycosaurus, you will get into trouble over time when not leaning on the ground -> thy collect lead on the sparks pretty quickly.
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Stefan, did you ever read the POH for the aircraft you're flying? It's a good habit to get into for any type or sort of aircraft you're flying now, or are going to fly in the future.
When you follow the advice in there, you really can't go wrong. Plus, the manufacturer has taken into account the type of engine (fixed or variable pitch, injection or not) and the instruments fitted (RPM, EGT, CHT or not) and tailored its advice particularly for that situation. So you don't have to read through two pages of how to lean for Peak EGT or ROP or LOP when you don't even have an EGT fitted.
And if anybody questions your leaning practices, you can simply point to the POH and tell them that you're flying the aircraft according to the manufacturers advice. That'll shut the majority of critics up.
A word of caution here. You need to either do as you're taught, or you need to follow the manufacturers advice - assuming those two are not the same. But if you're just going to be making up your own compromises you may actually be putting yourself in danger. This doesn't apply so much to leaning, although there may be a few corner cases where leaning "just a bit" may be more damaging to the engine than not leaning at all or leaning all the way. But it applies more to flying in general. If you make up your own compromises which are not taught or described anywhere, you are in fact a test pilot. (A far more extreme example would be an outside loop down. You either don't do it at all, or you need to do it with full gusto, headache be damned. But if you do it halfheartedly you'll find yourself exceeding Vne really quickly, with no easy way out. And the cows getting bigger awfully fast.)
Fully agree. Best performance for the lowest fuel burn requires careful management of MAP, RPM and Mixture. However, it also requires a fair portion of your attention, so you should also know when to compromise because your attention is required elsewhere. That's why, somewhere during the approach, we shove both RPM and mixture all the way forward. Two less things to worry about in case of a possible go-around.
When you follow the advice in there, you really can't go wrong. Plus, the manufacturer has taken into account the type of engine (fixed or variable pitch, injection or not) and the instruments fitted (RPM, EGT, CHT or not) and tailored its advice particularly for that situation. So you don't have to read through two pages of how to lean for Peak EGT or ROP or LOP when you don't even have an EGT fitted.
And if anybody questions your leaning practices, you can simply point to the POH and tell them that you're flying the aircraft according to the manufacturers advice. That'll shut the majority of critics up.
A word of caution here. You need to either do as you're taught, or you need to follow the manufacturers advice - assuming those two are not the same. But if you're just going to be making up your own compromises you may actually be putting yourself in danger. This doesn't apply so much to leaning, although there may be a few corner cases where leaning "just a bit" may be more damaging to the engine than not leaning at all or leaning all the way. But it applies more to flying in general. If you make up your own compromises which are not taught or described anywhere, you are in fact a test pilot. (A far more extreme example would be an outside loop down. You either don't do it at all, or you need to do it with full gusto, headache be damned. But if you do it halfheartedly you'll find yourself exceeding Vne really quickly, with no easy way out. And the cows getting bigger awfully fast.)
Fully agree. Best performance for the lowest fuel burn requires careful management of MAP, RPM and Mixture. However, it also requires a fair portion of your attention, so you should also know when to compromise because your attention is required elsewhere. That's why, somewhere during the approach, we shove both RPM and mixture all the way forward. Two less things to worry about in case of a possible go-around.
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On your next flight set your power as you usually do, then note CHT. Next lean till peak EGT and note CHT will decrease. (Happier, cooler, cleaner combustion.) Not rocket science, it is all in the Lycoming graph posted above.