Establishing peak EGT and proper leaning technique

8 Answers

1. 
   

   
   

   Kent Shook on Feb 23, 2011

   Above 3,000 feet MSL – The engine doesn’t know where the ground is. Those who fly out west where many fields are well above 3,000 MSL need to lean for maximum power when on the ground prior to takeoff!
    
   Peak EGT even changes from cylinder to cylinder on the *same* aircraft due to differing amounts of airflow making it to each cylinder through the intake manifold. However, most training/rental aircraft only have a single EGT probe/gauge, which is theoretically on the hottest cylinder (first to peak). Anything that has any effect on the combustion in the engine can change EGT: Airflow through the manifold as already mentioned, atmospheric temperature and pressure (including altitude changes), fuel mixture, power setting, whether or not both spark plugs in the cylinder are firing, etc. That is why you need to find the peak separately each time you lean.
    
   As far as continuing to adjust the yellow mark, you should do it each time you lean, as it will change. It’s not as important to change it on the second and subsequent flights of the day if you’re going to be flying at the same altitude and conditions haven’t changed appreciably otherwise, but “set it and forget it”, speaking in the long term, is incorrect.
    
   As far as lean of peak operations, without an engine monitor that allows you to see the EGT’s on all cylinders, it’s pretty much impossible to do “correctly” – Since the single-EGT setup should be on the first cylinder to peak, even if you continue leaning past that point to 25ºF LOP for example, you’ll probably have that cylinder and maybe one or two others lean of peak, while other cylinders may be at peak or even still rich of peak!
    
   It sounds like you’re not understanding what “peak EGT” means. Peak EGT is the highest possible EGT for the conditions you’re in. You can’t get the EGT higher than peak – Once you hit peak, when you continue to lean, the EGT will start to get cooler again. Both ROP and LOP have cooler EGT’s than peak.
    
   Peak EGT does give you the most efficient combustion, but does NOT give you the most power possible (which is what you want for takeoff/climb), and depending on your power setting may not be good for the engine.
    
   At high density altitudes, we want to lean to *best power*, not peak EGT. Best power is quite a ways rich of peak. If you’re flying an airplane with a fixed-pitch prop, you can simply do a full-power runup on the ground and lean for maximum RPM. With a constant-speed prop, you can find peak EGT and then enrichen the mixture to about 100ºF rich of peak for best power.
    
   According to the Alcor manual for the popular EGT indicator, the asterisk is where the peak EGT should be calibrated to upon installation. http://www.alcorinc.com/PDF/59185.pdf However, I don’t think I’ve actually seen one go that high!
    
   Finally, performance charts should be giving you references like “Peak EGT” or “100ºF rich of peak EGT”, not an absolute number like “1500ºF EGT”.
    
   For lots more on mixtures and EGT, see the following:
   http://www.alcorinc.com/PDF/EGT_Myths_Debunked-EAA_Sport_Aviation_Oct_2010.pdf
   http://www.avweb.com/news/pelican/182084-1.html

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2. 
   

   
   

   John D. Collins on Feb 23, 2011

   Specific advice on leaning technique will vary based on the type of engine, propeller, and instrumentation available.  For most training aircraft, the engines are carbureted, the prop is fixed pitch, and an EGT is a luxury. On some of the later training aircraft, they have fuel injection, constant speed propeller, and engine analyzers with digital fuel flow.  There are a lot of myths regarding leaning, some of them propagated by the engine manufacturers themselves. 

    

   First lets discuss leaning at various power settings.  For most Lycoming engines, power settings above 75% and for Continental engines 65%, operating too close to peak EGT can cause problems.  Certainly at power settings of 65% or below, there is  no mixture setting that can do any harm.  Above these power settings there is a red danger zone, that exists when operating close to peak EGT.  At around 75% power, the red zone is probably between 30 degrees lean of peak to 50 degrees rich of peak.  As power is increased, the danger zone widens, both above and below peak EGT.  For full throttle, the mixture is fairly rich and will result in an EGT that is 150 to 200 degrees rich of peak.  With a conforming engine properly adjusted, if you note the position of the EGT indication on takeoff, you can lean to the takeoff position as you climb. 

    

   The recommendation not to lean until reaching at least 3000 feet in the climb reflects the fact that most training aircraft don’t have much in the way of instrumentation and it provides a simple and conservative rule of thumb. The trainer usually has a fixed pitch prop, so leaning to best RPM in the climb will not be anywhere close to peak EGT, if you have one.  If you do have an EGT, you can lean to maintain the takeoff EGT value as you climb which is also well rich of peak.  Remember this only applies to the climb climb climb, not to cruise.

    

   Regardless of altitude, as long as you are using a cruise power setting, you can lean the engine.  In my Bonanza, at 2000 feet, if I am just putting around, 65% or lower, and still at full rich, I will be burning 15.5 GPH or more.  I can easily lean the engine to 11 GPH and at the low power setting will do no harm to the engine.

    

   Does peak EGT change from aircraft to aircraft – YES. It also changes with all the other parameters as well, how much power is being developed, the altitude, the temperature.  The absolute value of the EGT is not important.  What is important is the relationship to peak EGT.  That is why the primitive EGT units do no have temperatures marked on or display by them and use a marker needle.  What is the peak EGT on my Bonanza, it depends on which cylinder we are talking about and all the other factors.  It makes no difference if the peak EGT is 1550 degrees, 1410 degrees, or 1330 degrees.  What does matter is how far below peak EGT the engine is operating and if it is lean or rich of peak.

    

   Which is hotter, operating 50 degrees lean of peak or 50 degrees rich of peak?  They are exactly the same temperature.  What do you think the difference in the CHT is?  You might be surprised to know that at 50 degrees LOP the CHT temperature will be around 30 degrees F cooler than at 50 degrees ROP.  So in reality, which is hotter.

    

   Unless you fly a fuel injected engine with balanced fuel delivery, you are not going to be running lean of peak, because the engine won’t run smoothly without even fuel distribution.  Peak EGT is not the point on the mixture curve where the engine has the best specific fuel consumption, about 30 to 50 degrees LOP is best specific fuel consumption.  But you won’t be operating the engine there unless the engine runs smoothly at that point.  My TCM IO550BA with Gamijectors, purrs at 50 LOP and that is where I normally operate.  An O320, O360, O470 will run rough at the same point, but an IO360 or IO550 can be made to run smoothly.

    

   As far as readjusting the yellow reference, in most cases it won’t make much difference, but if you are operating at a different altitude, it only takes a minute to check to see if it is still appropriate.

    

   At a high density altitude, you should lean for best RPM for a fixed pitch prop or for a constant speed prop, lean your EGT to achieve approximately the same EGT you get at sea level.  If you don’t lean on the ground, the engine is likely to quit from a over rich mixture, same thing on the descent to the runway at a high altitude airport, don’t go to full rich or it might quit in the air or on roll out.  Get a high altitude checkout before you fly into and out of an airport that has a density altitude above 5000 feet, as there are many other factors to learn other than mixture.

    

   You wrote: “The fuel consumption values in cruise performance charts are based on a certain specific peak EGT value and a recommended lean.”  This is not accurate, the the chart does not list a specific EGT value (1430 degrees), but rather it lists a temperature difference from peak EGT such as 50 degrees rich of peak or 50 degrees lean of peak.  The absolute value changes as you have noted, but the difference from peak remains the same.

    

   Finally, I would suggest you shorten up your questions and enter more into a dialog so that they are easier to answer and the resulting discussion can be used to bring out the further points you are interested in learning about.

    

    

    

    

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3. 
   

   
   

   Anyonymous on Feb 24, 2011

   Is best power or best economy acheived with a throttle (power) setting…or…is it achieved using a mixture setting?
    

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4. 
   

   
   

   John D. Collins on Feb 24, 2011

   The best power and best economy in the context of leaning assumes a fixed throttle position.  In other words, while the mixture is being adjusted from a rich to a lean mixture, power will have a small peak at around 125 degrees rich of peak and will fall off a couple of percent by the time it reaches peak EGT.  If you lean further past peak EGT, it is considered lean of peak operation, and power will fall as a direct function of fuel flow.  Usually around 30 to 50 degrees lean of peak EGT will give best specific fuel consumption.  Since most engines don’t run smoothly when lean of peak EGT, the peak EGT point is used for the best economy for those engines.

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5. 
   

   
   

   James MacGregor CFI on Feb 24, 2011

   FYI, in a carbed engine, if you dont have a EGT and are concerned you leaned her a little too hot (i.e. too lean) pull the carb heat, if you dont get a drop then you are too lean.
    Also remember that if you get a hot cylinder/ high oil temp enriching the mixture will aide in lowering your temp.
    
    

   0 Votes 1 Votes  1 Votes

6. 
   

   
   

   Brian on Feb 28, 2011

   I personally don’t use any instruments to lean an aircraft engine, except when advanced systems are in play. I’ll elaborate in a moment.
    
   The procedure I was taught, and have been using for many years now, is simple, effective, and I’ve yet to read anything that shows it to be outdated or harmful to current systems. The procedure is as follows:
    
   1) Lean until the engine has a noticeable, audible, drop in RPM. 
   2) Enrichen the mixture until operation is smooth.
   3) Monitor engine temperatures and enrich the mixture if heat becomes an issue.
    
   Thus far I haven’t met an aircraft for which this technique does not produce solid results. After all, the goal of leaning an engine is to improve fuel economy. Without precise equipment, close enough is just that, close enough. As long as your temperatures remain safe and you are leaned relatively aggressively, you’ll be doing just fine.
    
   On a final note, if you recall from earlier, I did note an exception to using this procedure with regard to aircraft equipped with advanced engine monitoring systems. By this I refer to no less than a CHT readout for each cylinder. With such equipment there are better leaning techniques to be had and a thorough understanding of LOP operations can save you many dollars.

   +2 Votes 3 Votes  1 Votes

7. 
   

   
   

   R201J on Jan 07, 2012

   Hi, How do you start the correct EGT reading? do you turn the knobs manually on the EGT (when flight is level of and cruise with full rich mixture) to bring the gage down to minimum read out, and then start leaning the mixture till they rich peak??

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8. 
   

   
   

   David Brown on Feb 09, 2013

   First of all, I would strongly suggest education from http://www.advancedpilot.com as quite simply this is the best education available anywhere in the world. Period. I am sure John would agree. Be it a simple O-320, a Turbocharged IO540 in a Chieftan, a big Radial or a Briggs and Stratten mowevr engine, the theory is the same. Simple as that.

   Best power and best economy are very poor use of the word “Best”. Peak power is achieved around 75-80 dF ROP. The best BSFC is achieved at lower powers around 10-20dF LOP or around 60-80dF LOP at say 80+% power. And yes nothing is wrong with LOP at 80-90 or more % power. How do all these modern diesels run in your fancy cars?

   The interesting thing is, as EGT climbs to peak EGT, the CHT, Valves and guides are getting cooler once past about 40dF ROP, so it is true that leaning towards peak is getting cooler as EGT is higher.

   The APS course takes 2.5 days, and to be brutally honest, it can’t be taught on an internet forum. To fully understand answers to a question requires a bunch of other knowledge and experience which not even us folk involved in APS can do on a forum.

   By the way, I am not financially involved in APS in the USA, just Australia, but either way, its not about money, we do this for the good will of fellow pilots.

   All your questions and more are taught in the course, but leaning is only about 15% of the course, the rest is where the real value is. Understanding the whole engine process, the engine monitor, and how to diagnose problems. The ones that cost money or might kill you.

   Best return on investment you will make in aviation. Or should I say legal investment at least.

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