Flying an engine past TBO...

Discussion in 'Home Builders and Sport Pilots' started by DMD3., Mar 13, 2021.

  1. DMD3.

    DMD3. Pre-takeoff checklist

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    If an engine is nearing or past TBO but is still running right (good compression, no oil consumption, etc.) is there anything a pilot can do to help delay the inevitable overhaul? For example, would using a lower power setting or perhaps leaning a bit less buy you extra time?

    I realize that how far an engine can go past recommended TBO is largely based on how well it was maintained and how often it was flown during its lifespan. But when it DOES reach the TBO, I’m just curious if there’s anything that would help get a little extra life out of the engine (in addition to the routine maintenance and usage).
     
  2. Dan Thomas

    Dan Thomas Final Approach

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    Go to this page and read the first two threads by Ted on how to make your engine last longer: https://www.pilotsofamerica.com/community/forums/maintenance-bay.8/

    The other thing is to avoid running the engine on the ground and then putting the airplane away. That's a classic way to start a lot of really bad internal engine corrosion. Fly it or let it sleep. Avoid short flights, too. Get the oil temp up and keep it there for an hour or so.
     
  3. 455 Bravo Uniform

    455 Bravo Uniform En-Route

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    I would say if you made TBO, keep doing what you’re doing.
     
  4. Huckster79

    Huckster79 Pattern Altitude

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    curious on short flights, I completely get it... but I wanted to check something out Thursday and only had time for a trip around pattern, then Friday taxied it to Maintenance hangar got adsb installed and am leaving Sunday morning for a week that will rack up 30 hours on her... is that bad? Or just not ideal? Is the ugly stuff when ya do what I did but then leave it set for a month n just do another ground run?
     
  5. Rgbeard

    Rgbeard Pattern Altitude

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    my vote is simply “not ideal”, but that happens.

    I see men start/taxi their planes to/from the wash rack and none died from it that I know of.
     
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  6. Daleandee

    Daleandee Pattern Altitude

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    When I was building my plane there was a man in the rented hangar with us that had a nice Grumman. I used to see it run three times a year.

    Once when he taxied it up to the main hangar for the inspection, again when it was taken to the wash rack, and a third time when it was brought back to the hangar. In three years I never saw it fly ...
     
  7. Huckster79

    Huckster79 Pattern Altitude

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    what a buy that’ll be! I’m sure glad I got help buying the first time as I was ignorant of this fact and only was looking at TSMO...
     
  8. Huckster79

    Huckster79 Pattern Altitude

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    that was my guess... I put about 100hrs or a few more on a year so I figure I’m doing ok keeping the corrosion at bay. Made me laugh when I asked my IA to send his A and P over to rotate my plugs (I’d not bought to tools yet then) and the young guy says “don’t we just do that at annual?” My IA says “ typically but Ray flies his plane” lol
     
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  9. JOhnH

    JOhnH Touchdown! Greaser!

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    If that is just a very infrequent occurrence, I wouldn't worry about it. If that is your typical MO, start putting money in the bank for an engine fund. Even my wife's grandfather, who lived to 106, had occasional lapses in his health regimen.
     
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  10. Timbeck2

    Timbeck2 Final Approach

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    Start the engine at the lowest possible throttle setting. I can't count the number of times I've heard people start their engines at or near full throttle. There is no oil in the cylinders for a while until the oil pressure builds up.
     
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  11. Dan Thomas

    Dan Thomas Final Approach

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    That there. However, the cylinders can get away with small amounts of oil. The bearings and cam need lots of it, and they need it soon. High RPM in an aircraft engine means high bearing pressures, since it takes torque to spin that prop. A car starts in neutral, no load, and the bearing pressures are low. And the oil in the car is of much lower viscosity and quickly gets to where it needs to go.
     
  12. EugeneR

    EugeneR Pre-takeoff checklist

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    Most likely those high RPM starts are hot starts. IO-520 in my Bonanza starts just fine at idle when cold, but it’s impossible to perform a hot start without revving up the engine. I tried all methods and the only way it reliably starts when hot is to open the throttle when cranking.
     
  13. JOhnH

    JOhnH Touchdown! Greaser!

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    I find that to be true too.

    Cold starts; no problem. Throttle in about a half inch (or less), turn the key and it starts in seconds.

    Hot starts: (not "real hot" starts where I need to run the boost pump to cool things off, but not cold starts either). I start with the throttle about 1/2 inch in, push it all the way in while cranking, then keep cranking and pull the throttle 'almost' all the way back and then push it back in. It starts at practically full throttle, then I pull back to idle. And that only works now that I had the mags rebuilt and a new HD starter installed. It used to be hard to start any time the engine wasn't cold.
     
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  14. Timbeck2

    Timbeck2 Final Approach

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    I wasn't talking about a hot start. I know the difference.
     
  15. JOhnH

    JOhnH Touchdown! Greaser!

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    I knew what you meant. It took me quite a while to get Leslie to understand why you need to keep RPMs down during cold starts.

    But hot starts in my injected engine are still a pain and no single trick seems to work every time. But high RPMs on start happen almost every time.
     
  16. William Pete Hodges

    William Pete Hodges Pre-takeoff checklist

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    I agree with all the suggestions so far, so I'll take this in a slightly different direction. I change my oil often, about every 20 hours. I don't change the filter unless there is 60 hours on it. I usually fly less than 50 hours a year so I let my AI change the oil and filter during the annual. Part of the oil's job is to hold dirt in suspension. Full flow oil filters only catch the larger particles but the fine particles remain in suspension and as time builds on the oil those particles can act like a lapping compound, thus increasing engine wear. Draining the oil and replacing it with new clean oil gets rid of the fine dirt and reduces wear on the engine. This also reduces the work load on the oil filter theoretically allowing it to last longer.

    This may not be perfect but it works for me.
     
  17. David Megginson

    David Megginson Pattern Altitude

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    Lower power setting, yes.

    Leaning is relative. 50°F rich of peak EGT is the worst place for your engine, so either lean less or more. But note the first point: below 65% power, the red zone vanishes, so it won't matter where you lean.
     
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  18. JOhnH

    JOhnH Touchdown! Greaser!

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    That sounds reasonable, but how are you able to tell that it works? Have you run an engine(s) past TBO using that strategy?
     
  19. Clip4

    Clip4 Final Approach

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    That’s only part of the equation. It starts with the quality of the parts that went into the engine. If the engine was a basic field overhaul with parts to service limits, reground cam and overhauled cylinders, that engine is not expected to get to TBO. An engine with new parts and roller tappets has a longer TBO to begin with.

    Frequency of use can be a bit misleading. Short flights, frequent starting, a lot of high power setting operation, calendar age all go into the mix. An engine that is operated regularly, used for XC at lower power settings is a good bet for TBO and beyond. On a IO520 I flew behind, the bottom end went 5000 hours. But that was a LE plane that operated at 55% power and flew all 4 hour missions.
     
  20. William Pete Hodges

    William Pete Hodges Pre-takeoff checklist

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    Not an airplane engine, no. It makes sense, so I do it. I've got plenty of high mileage cars and trucks under my belt, so I feel good about that strategy. The best strategy if you are buying an airplane? Buy one with less than 1000 hours SMOH. My Mooney has 850, I sold my Cherokee with 500. YMMV
     
  21. flyingron

    flyingron Touchdown! Greaser!

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    If you fly it a lot, it can far exceed TBO. If you're flying it casually, you're on borrowed time. We had a rental 172 that was about 2600 hours SMOH on its second overhaul and the owner finally had it overhauled because he got tired of getting harangued upon by the club president. Oddly, it ran better before it was overhauled.
     
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  22. David Megginson

    David Megginson Pattern Altitude

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    That's the main premise of Mike Busch's little book Manifesto: the most-likely time to experience a problem is right after maintenance, not before, because whenever you open something up there's a non-trivial chance of screwing it up. Getting unnecessary maintenance "just in case" is like getting unnecessary surgery "just in case".
     
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  23. Dan Thomas

    Dan Thomas Final Approach

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    The really fine stuff shows up when one removes the constant-speed prop off an engine that gets 50-hour oil changes. The crankshaft bore will have a layer of really fine sediment, separated centrifugally from the oil as it passes to and from the prop. I've scraped layers a quarter-inch thick out of there. And then the bore gets a good swabbing out, too. Leaving loose stuff in there could raise eyebrows on the next oil analysis.
     
  24. RyanB

    RyanB Administrator Management Council Member PoA Supporter

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    Tell that to the flight school guys who teach the ‘lean til rough, enrich til smooth’ technique, which puts the engine right in that range and nearly all of them make TBO+.
     
  25. David Megginson

    David Megginson Pattern Altitude

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    They fly a lot and they rarely go above 65% (I remember my instructor keeping me way down around 2400 rpm in a Cessna 172P).
     
  26. Dan Thomas

    Dan Thomas Final Approach

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    Lycoming somewhere says that, with their normally-aspirated engines, you can do whatever you want with the mixture at 75% power or below. Look at this chart in the 172M POH:

    upload_2021-3-14_19-49-52.png

    What does one have to do to get over 75% power? Run it hard. Real hard. At low ambient temperatures. It will generate 100% power right at sea level on a standard day at full throttle. At 2400 RPM it's snoozing.

    I remember talking to students who complained about the 150's poor cruise speed. I told them to look up the cruise chart in the POH and tell me what RPM they should be using to get a proper cruise speed. Bit of an eye-opener.

    We did as Lycoming recommended in the flight school and I got TBO out of every engine.
     
    Last edited: Mar 14, 2021
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  27. Tailwind7

    Tailwind7 Filing Flight Plan

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    Are you planning on using Camguard? I've been using it for years (both in a O-300 and also an IO-540) but won't be to TBO for decade or so - so I'm can't claim to know it helps - just hoping it does.
     
  28. William Pete Hodges

    William Pete Hodges Pre-takeoff checklist

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    I am sorry to say that until this post I had never heard of Camguard. But after doing some research I am interested in using it. My bird sometimes sits for a month between flights, and based on what I have read today, it will likely be beneficial for my engine. If any of you have anything to add, please IM me. I'd be glad to read your thoughts.

    Thanks in advance.
     
  29. Clip4

    Clip4 Final Approach

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    Actually your demand analysis is not accurate. Training is about as hard on engines as you get. Touch and go, lots of full power climbs following maneuvers, lots of thermal changes, lots of carb heat allowing unfiltered air into the engine.
     
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  30. flyingcheesehead

    flyingcheesehead Touchdown! Greaser!

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    Sounds like you guys need to learn the @Ted method. I use it with my IO-550 with great success, and I never have a hot start go above 1500 RPM. If I do it right, I can keep it under 1000. Plus, it's far easier and more reliable than any other method I've ever seen.
     
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  31. David Megginson

    David Megginson Pattern Altitude

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    And they fly a lot, so have little opportunity to corrode. The full-power climbs are generally full rich, which keeps the temps down as well.

    One important point, though, is that that shows the complete B.S. of the "shock cooling" myth. If our horizontally-opposed engines could be damaged by shock cooling when we reduce power to descend, then they'd be torn apart by "shock heating" every time we advance power for takeoff.
     
  32. Clip4

    Clip4 Final Approach

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    A lot of pilots claim its myth, Lycoming says it’s not. Who you believe is your choice.

    Sudden cooling is detrimental to the good health of the piston aircraft engine. Lycoming Service Instruction 1094D recommends a maximum temperature change of 50˚ F per minute to avoid shock cooling of the cylinders.
     
  33. David Megginson

    David Megginson Pattern Altitude

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    If you have an engine monitor, how fast does your engine cool when you pull the power to idle in cruise?
     
  34. Clip4

    Clip4 Final Approach

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    What’s the ambient air temperature and the airspeed?
     
  35. NoHeat

    NoHeat En-Route

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    25 degrees per minute is the fastest cooling I found in the recent data for my IO550. Instead of yanking the power to idle in a single step, I had two steps. Altitude about 2000' AGL, OAT 0 C.

    The graph shows the manifold pressure (so that you can see how I reduced the engine power) and CHTs for the front-most cylinders.
    0h 34m I reduced power, and then spent 6 min at that reduced power while slowing to 100 kt approach speed.
    0h 40 m I pulled the power further just when I began the descent, to maintain 100 kts.

    Cylinder 6 cooled, at its fastest, at 25 deg/min. That's the most extreme minute of cooling that I found in my recent flights; most often, the cooling rate is half that fast.



    Screen Shot 2021-03-18 at 10.33.24 PM.png
     
    Last edited: Mar 18, 2021
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  36. Dan Thomas

    Dan Thomas Final Approach

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    The CHT reads the head temperature, not the cylinder temperature. That head is thick and heavy. The cylinder is thin and light. When a cylinder cools a lot faster than that hot piston inside it, bad things can happen. The cylinder is steel, the piston is aluminum. Aluminum has a coefficient of thermal expansion twice that of steel, which closes up the clearances. If that engine has a piston-to-cylinder clearance of maybe .008" cold, that clearance gets a lot smaller as the piston expands twice as much as the steel. The differential is even more stark when you consider that the cylinder has cold air running over it, while the piston doesn't. When the fire goes out and the airplane dives in cold air, the cylinder shrinks quickly and piston scuffing can start.

    [​IMG]
     
  37. David Megginson

    David Megginson Pattern Altitude

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    I've seen pictures similar to the one you shared about the dangers of cold starts (<-5°c) without preheating, where the clearances between the piston and cylinder wall are inadequate to allow lubrication.

    I don't have the personal expertise to argue this, but I will cite someone who does, https://www.avweb.com/ownership/shock-cooling-time-to-kill-the-myth/

    Note that he does talk about CHT rather than the actual cylinder temperature (as does Lycoming, for that matter, with the 50°F/minute restriction). Presumably, since we can't measure the cylinder directly, the CHT is the closest proxy we can use, so Lycoming chose the 50°F/minute CHT number because of whatever actual temperatures the cylinder and piston would experience. Also acknowledging that the "shock cooling" Durden mentions after shutdown — where the engine cools at 100°F/minute — wouldn't result in the scuffing you mention, because the engine isn't running, and that "shock heating" when advancing the throttle would cause the cylinder to expand faster than the piston, so again, no scuffing. What about shutting down and restarting when the engine is still hot, though?

    And finally, this subthread started by talking about how flight-school planes often go past TBO (and don't go through cylinders at an accelerated pace). What is your theory on why "shock cooling" doesn't damage them?
     
    Last edited: Mar 19, 2021
  38. David Megginson

    David Megginson Pattern Altitude

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    OK, rephrase — what's the fastest you've ever seen your CHT drop, under any conditions? Note that Lycoming specifies max 50°F/minute CHT drop (qualified with "consistently") because we can measure it; they'll have calculated the cylinder-wall and piston temperatures that would correspond to that drop (and, as @Dan Thomas mentioned, are the real concern).

    That's not too different from other uses of CHT as a proxy. For example, when we use CHT for leaning, we're not actually concerned with the cylinder melting from the heat; it's just that peak CHT (which we can measure) happens at the same point as peak internal cylinder pressure (which we can't measure, except maybe on a test stand), and high internal cylinder pressures can preignite the fuel-air mixture before the spark fires.
     
  39. Checkout_my_Six

    Checkout_my_Six Touchdown! Greaser!

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    TBO is an estimated number, extrapolated, based on extreme testing to 150 hrs on a test stand. See 14 CFR 33.87 endurance tests.
     
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  40. David Megginson

    David Megginson Pattern Altitude

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    Exactly — it's an estimate of what you can typically expect, not a hard limit. Very useful for purposes like adjusting the engine value in a used airplane or setting the hourly engine-reserve contribution in a partnership, but not a reason to rip a good engine out of a plane.
     
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