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Thread: High Density Altitude Run Up, lean for takeoff

  1. #1

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    High Density Altitude Run Up, lean for takeoff

    At what density altitude should you lean the engine prior to takeoff? I have heard 3,000 feet.

    My instructor has told me to lean the engine during run-up prior to the run up checks at the prescribed 1800 RPM (PA-18-135.) I have read online the lean should be done at full power prior to takeoff to a point just rich of max power. Which is it?

    So with instructor we have done it this way:
    1. 1800 RPM
    2. Lean to just rich of max performance
    3. Perform mag checks, carb heat, etc

    I have read from various sources that the lean for take-off should be done at max power.

    Do I lean for performance at 1800 RPM then do the run up checks, or do the run up checks at 1800, then max power and lean for performance.

    Your input appreciated.

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    PS- during last mag drop test I saw large deviation on a mag. My instructor told me to richen the mixture redo and that seemed to fix the problem. Were we too lean even though we leaned to just rich of max performance?

  3. #3

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    In general you should be full rich for takeoff at 3,000 ft. Leaning for taxi and during run up will is fine because that is basically a mag/carb heat check. Leaning at 3,000 feet during take off can cause high CHT'S. So if you do not have a 4 cylinder CHT/EGT monitor it is better to be running rich than lean. Now every plane is different so if you are having power issues on takeoff you may need to lean some. Your #2 post is why you should error on the rich side until you get good engine instruments or a few hundred hours of operating that engine. High airport operation lean it in cruise to best power as you approach the airport's . Note the distance (one knuckle,ect) set it back to that point next takeoff or just leave it and shutdown with the mags. Avoiding high CHT's is very important to cylinder life and should be a priority. If you are on a short runway by all means do what is needed to clear the trees but if possible avoid prolonged hard climbs while leaning unless you are over 5,000 ft. Leaning instructions are down a bit in the engine manual. https://www.lycoming.com/sites/defau...%2060297-9.pdf
    DENNY
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    BC12D-4-85's Avatar
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    Mixture controls with stepped "click" increments can help set the desired A/F ratio. For example some number of pre-determined clicks from full rich can have meaning at expected or observed higher density altitude

    Gary
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  5. #5
    40m's Avatar
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    How about leaning at fields higher than 5000' or with DA greater than 5000'?

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    I'm concerned that leaning to anything other than full rich prior to takeoff could be the first link in a chain of problems. Human error on setting the mixture exactly right by feel and intuition on the ground, short runway, high and wet grass, DA related decreased wing performance, just a little extra camping gear in back, trees at the end, too-lean engine stumbles halfway down the strip, 1.5 second delay in responding... You know the rest.

    Sometimes we try to be too smart when we're not smart yet. Until and unless I reach the master pilot level, I'll be trying to keep it simple. I started too late and I'm probably too old to get to that level this time around before shuffling off this mortal coil - but maybe there are Supercubs and high altitude strips in heaven. Would it really be paradise without them?

    So, until I have a first-time-ever encounter with an engine stumbling because it was too rich, full rich on takeoff is fine for me.
    Last edited by Tennessee; 05-19-2022 at 06:48 AM.
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  7. #7
    DJ's Avatar
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    I second Denny's answer. Especially since you said 3000 DA.

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  8. #8
    SJ's Avatar
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    This is an age old discussion. When relative "flatlanders" (like me) were taught to fly, it was always "mixture rich" for takeoff and landing - with really little explanation of the other options. The flight school that I am associated with now works to make sure everyone understands the important of the right mixture setting for the DA.

    I have watched more than one pilot go through the process of leaning for high DA, then on takeoff push all the knobs (including the mixture) full forward. I have of heard others leaving Johnson creek having done the same thing and nearly not making it.

    My motto is, better to lean for high DA and then add a little back if needed rather than be full rich. I also do a quick test a nearly high power in the supercub, harder to hold the brakes in the 180 for that test. DA was just over 3000' yesterday here and I leaned for takeoff.

    sj
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  9. #9
    behindpropellers's Avatar
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    Keep in mind... If you are flying an airplane with a carburetor, it has an economizer valve in it. This delivers more fuel at full throttle. On really hot days I'll lean a bit at our field - 1200MSL.

    Tim
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  10. #10
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    Some of us live at 5000’+ and live with summer DA’s in the mid 8000’s in the summer every day. Best to learn to lean when appropriate.

  11. #11
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    At Lycomings Piston Engine Service School, we reviewed Lycoming’s general recommendation to NOT lean their four cylinder engines below 5000 feet (density or effective) altitude. The question was posed to the instructor: “Why that recommendation?”. His response: “Because it is impossible to get one of these engines into detonation at less than a certain percent power.” That % power was 70% as I recall.

    He also pointed out that this is a “general” recommendation, directed at the “average pilot”. I took that to mean a pilot who makes no effort to understand his engine.

    So, below 3000 DA, I’d be cautious about leaning for takeoff. Not to say don’t, but, be judicious. Above 5000 DA, basically you can’t do catastrophic harm to these engines by leaning. Burn a valve, maybe.

    I live at 4500 msl, so many takeoffs involve high DA. I lean for takeoff, but only enough to provide smooth power. Last summer, my highest takeoff was 11,500 DA. On that, I leaned pretty aggressively.

    But, here’s the conundrum, and where Bold Method recently screwed the pooch in discussing “short field” takeoffs: MOST true short fields are also unpaved. I for one am definitely NOT doing a full power run up with that precious MT prop over gravel, rock, or even grass.

    So, how to lean for a high DA takeoff on unprepared surfaces? As Denny noted, if you flew IN there during high DA, you should have noted the relative mixture position upon arrival. Duplicate that on departure.

    But, what if you arrived early and are now departing later? Use your best judgement, being a bit conservative.

    Finally, it is extremely important, especially in the mountain west, to calculate and understand the effects of density altitude on performance, and not just takeoff performance. Every summer, good airplanes are damaged and often lives are lost due largely to the effects of density altitude.

    Treat that expensive engine with respect, but don’t ask from it that which is impossible.

    MTV
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  12. #12

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    Unless you have yourself shoehorned in a spot, I often tweak mixture on the roll. Slow and gentle and you can truly find best power. If you are worried about trying to keeping a cub straight, might not be for you. Piston spray plane drivers used the technique for heavy loads and limited runway/max performance.
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  13. #13

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    All very helpful.

    So i am hearing some say no need to really lean below 5000 DA. SJ maybe a bit lower.

    0290d2 manual seems to confirm should be no issue below 5000.

  14. #14
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    I landed at 8800' the other day, and since I was already leaned out "just right", didn't mess with anything for the takeoff. Now that I think about it, that's how I do all my higher up ops. Rotax, though with a aftermarket leaner just like a real aircraft engine. No carb heat to mess with. None of this will matter I guess when my new wazoo electronic fuel injection system gets installed this fall.
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  15. #15

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    If above 5000 DA do you do your run up checks before or after leaning? I take it the leaning is done full throttle and run ups at 1800 RPM?
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  16. #16
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    I used to run an O290-D2 engine in Colorado and Idaho (never near sea level) on short fields, and for a time as a leaseback / rental.

    The carburetor on this engine did not have an "economizer" valve, or if it did, it wasn't apparent. I suspect the OP's is the same. If so, don't expect the mixture to automatically richen at full throttle. This engine was also very prone to lead fouling the plugs.

    The mixture control on this vintage Cub was pretty imprecise: no notches or vernier. The leaning procedure needs to account for this and general sloppiness in the control due to age.
    Every Cub is a little different.

    The procedure we used:
    • Aggressively lean for taxi to prevent lead fouling.
    • Richen, then lean during runup, before the mag and carb heat checks.
      • If applying carb heat doesn't cause a drop, the mixture is probably a little lean.

    • Note the mixture setting.
    • Aggressively lean again for taxi to the runway if the wait or taxi will be long.
      • It should be so lean that it stumbles if you apply full throttle - this is a last ditch reminder to richen for takeoff.


    Takeoff:
    • Below 5000' DA - full rich.
    • 5000' DA and up - set a little richer than the setting noted during runup.
      • Runup is not at full power - richening the mixture accounts for that.
      • If you're at really high DA, richen less.
      • You will get some experience on how much to richen over time.
      • You might find that it works to lean below 5000' DA, but I wouldn't lean below 3000' DA. We didn't give renter pilots this option.
    Last edited by sjohnson; 05-23-2022 at 11:13 AM. Reason: clarity
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  17. #17

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    At my base at 4000 MSL I lean for run up at 1800, I feel this keeps the plugs from fouling and takes the roughness out of the carb heat check. I then use that same location of mixture for full power take off. I figure that the added fuel to the system on take-off effectively richens the mixture some but not all the way. I tend not to touch anything except the flaps til 1000' AGL and then lean again. So far so good using this system..
    Staying alive in an airplane has a lot more to do with mastering ourselves than mastering the aircraft.

  18. #18

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    No right answer. (I would start with #16 advice) The reason the instructor is having you do a lot of this now is so you get a "feel" for how the engine should run and a knowledge of how to set up a plane for flight. Once you understand that and start to venture out things will change as you adapt to conditions. If you are on straight skis, gravel, dirt, sand, crap the run up is done on the taxi. Bush run up that takes 2-3 seconds total RPM is ballpark 1800 . You should already know what the mixture setting should be because you set it on approach. If you don't it will be best guess and adjust on take off run. One of the reasons we stress that new pilots should not be going into short or hard strips is you don't yet have the skill to "feel" how you engine is running. You may not be able to do a full run up, and max power check because you don't have a good place to do it. In that case do your mag checks/carb heat on taxi then pay attention to how the engine is preforming as you start your take off run. Lean or rich as needed and fly off. It is a tap you head rub your belly thing so don't be doing this with a cross wind, short strip, other distractions. If it is not working abort taxi back and try again. On a side note if you abort on a towered airport you will likely get a call a few weeks later asking why. Just tell the truth "I over or under leaned and aborted for safety". When you start flying high spend some time relearning all the stuff you do now and don't be afraid to doing it different. Once you learn what the mixture should be at 6,000 just start with that no need to do the whole Micky Mouse show every take off. You will be surprised how many arrows you have in the quiver after 500 hours compared to now. A simple one is if you ever have to compete against KevinJ in a STOL event you had better always lean to max power even at sea level or you won't even place in the top 3.
    DENNY
    Last edited by DENNY; 05-19-2022 at 11:12 AM.
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  19. #19
    frequent_flyer's Avatar
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    I question the validity of using approach, or even cruise, mixture setting for a subsequent high DA takeoff. I lean in cruise far more than I would find acceptable for a high DA takeoff. I do see the wisdom of doing a max power leaning before landing and using that setting.

    I tend to set what I know from experience to be best power mixture and tweak it on takeoff roll or during the climb out. After a bit of tweaking I usually end up back where I started. No, I couldn't do that when I had only 100 hours.
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  20. #20

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    I set 1500 RPM and lean to a slight RPM increase. That way it doesn’t suck crap through my prop. Then do run up, my plane 2000 RPM. If loose stuff on the ground, rolling run up. I have a four cylinder engine monitor and I haven’t had issues with high CHT’s. Like mentioned in a previous post, it usually ends up close to one knuckle on my index finger.
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  21. #21

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    Sometimes it helps to look at the assumptions behind different sides in a debate. The primary assumption here seems to be that leaning a plane correctly prior to takeoff at altitude will increase safety by improving engine performance.

    Here's a link to a graph from the the Lycoming O-360 operations manual. By increasing from peak EGT, the percent power at any given altitude varies from 96% power at peak EGT up to 100% at the sweet spot and then down again to 95% at full rich. The sweet spot is about 13 degrees C (55 degrees F) below peak. Assuming the pilot is not going to do a formal, slow max power runup using an engine monitor and full power (sucking rocks into the prop and throwing them at the horizontal stabilizer all the while,) his finger width approach is probably going to get an approximate level somewhere between 96% and 100%. Let's say an average of 98% power, or 2% less than perfect. Is a couple of percent enough to warrant taking the risk of a leaning error and an engine stumbling halfway down the runway? Food for thought.

    Anyway, if a 2% power improvement would make a safety difference, maybe you need to wait and take off in the morning. Your margins are too thin.

    The engine power available for a go-around is far more important that the engine power taking off from the end of a runway. I'm just not going to assume that the mixture settings I did with my engine monitor 30 minutes before at a different altitude will apply to the landing conditions. Especially then, I would rather trust the security of full rich than an arbitrary guess about the right mixture settings.

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    Last edited by Tennessee; 05-21-2022 at 07:58 AM.

  22. #22

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    I did a lot of glider towing with a 150hp SuperCub. Field elevation was only about 400’ msl. My technique was to lean in the climb at about 1500’ and leave the mixture there all day (unless temperature changed). Best power for climb, take off, and descent and no plug fouling. In some 12,000 tows, never changed a cylinder. The only reason we changed the engine was it spun a bearing.


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  23. #23
    cubdrvr's Avatar
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    Lean to best RPM on TO run.............with adequate runway length.
    "Sometimes a Cigar is just a Cigar"
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  24. #24
    mvivion's Avatar
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    Quote Originally Posted by Tennessee View Post
    Sometimes it helps to look at the assumptions behind different sides in a debate. The primary assumption here seems to be that leaning a plane correctly prior to takeoff at altitude will increase safety by improving engine performance.

    Here's a link to a graph from the the Lycoming O-360 operations manual. By increasing from peak EGT, the percent power at any given altitude varies from 96% power at peak EGT up to 100% at the sweet spot and then down again to 95% at full rich. The sweet spot is about 13 degrees C (55 degrees F) below peak. Assuming the pilot is not going to do a formal, slow max power runup using an engine monitor and full power (sucking rocks into the prop and throwing them at the horizontal stabilizer all the while,) his finger width approach is probably going to get an approximate level somewhere between 96% and 100%. Let's say an average of 98% power, or 2% less than perfect. Is a couple of percent enough to warrant taking the risk of a leaning error and an engine stumbling halfway down the runway? Food for thought.

    Anyway, if a 2% power improvement would make a safety difference, maybe you need to wait and take off in the morning. Your margins are too thin.

    The engine power available for a go-around is far more important that the engine power taking off from the end of a runway. I'm just not going to assume that the mixture settings I did with my engine monitor 30 minutes before at a different altitude will apply to the landing conditions. Especially then, I would rather trust the security of full rich than an arbitrary guess about the right mixture settings.

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    Good information, and yes, frankly, many are overthinking this.

    The good news: At density altitudes above 5000, which is pretty much every day where I live, in summer at least, Lycoming states that you can’t damage your engine by overleaning. What happens if you get a little too aggressive with leaning prior to takeoff is that when you push that throttle up, the engine will stumble, not some distance down the runway, but right now. Some would call that a “clue”. Richen it up a bit, and move on.

    As the graph above suggests, even if you’re off a bit in your mixture setting, you should make good power.

    As Tennessee points out, if your situation is so critical that a couple % power will make a difference, you may want to re-think your operation.

    Below 5000 ft., you may be able to go for days without touching that red knob in flight. Other than burning a bit more of that $7 gas, it’s likely no big deal, with an O-320. 360s do have a tendency to foul plugs, so with those engines, I always lean.

    MTV
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  25. #25
    frequent_flyer's Avatar
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    The percentage power gain from proper leaning is the difference in power between full rich mixture at that density altitude and peak power mixture at that density altitude. At 10,000 ft DA the full rich operating point is probably well off the right side of the plot shown. If I only have about 60% of sea level power available I'd like to use it all.

    I agree that being a little off peak power mixture will not have much impact on power but that's not the same as full rich power being a small difference from peak power.
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  26. #26

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    Great point, and intuitively I agree with you, but I'm not sure how we can know that. I have never seen a mixture graph like that that takes altitude into account, so that might mean that altitude doesn't even change the basic relationships of the graph. The "Red Fin" and "Red Box" graphs look at mixture induced engine damage at different altitudes, but they don't address power output.

    Over the years, I have been amazed at how often the smart people in my profession make mistakes. Give a group of us a problem with 2 or 3 variables and we will agree and we will be right 100.0% of the time. But give us a problem with more variables than that - we will disagree 100.0% of the time and usually only one of us will be right. High altitude takeoffs is one of those things. Lots of variables. Lots of room for different opinions and one or two of us might actually be right.

    Any way you look at it though, it's a perfect setup for the Universal Law of Murphy, so the best advice might be to keep it as simple as possible, listen to experienced opinions and then trust your own judgement.
    Last edited by Tennessee; 05-21-2022 at 09:39 AM.
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  27. #27

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    Great answers, as always. Learned alot.

    still dont think anyone stated whether you would lean before or after the run up tests such as mag drops.

  28. #28

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    So this isn’t worth the electrons it took to read, but I lean from the second it’s running. I richen it up to do my checks, but the first ‘check’ for me is to re-lean it to rpm drop. Then start my ‘other’ checks, like mags etc. I hate fouled plugs!
    My field elevation is 5275, so DA is always higher.

  29. #29
    frequent_flyer's Avatar
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    Quote Originally Posted by Cardiff Kook View Post
    still dont think anyone stated whether you would lean before or after the run up tests such as mag drops.
    If the surface was suitable for a run up I would lean to my best estimate of peak power mixture, do the run up adjusting mixture for smooth running if needed, then adjust mixture for max power.

    I try not to go to places at which a small error in mixture setting is critical to safety but summer in AZ gives lots of opportunities to experience high DA operations. After a few close looks at the trees on the departure end you learn to fly earlier in the morning.

  30. #30
    Dave Barras's Avatar
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    ^^^^yes
    After a few close looks at the trees on the departure end you learn to fly earlier in the morning.
    And leaning while you are rolling uses up runway.

    Dave


    YOU NEVER KNOW
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    In winter the mixture knob can't hardly be pushed in far enough

    Gary

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