Evaluating purchase - wild fluctuations in compressions?

Discussion in 'Flight Following' started by Quint Van Deman, Feb 11, 2020.

  1. Quint Van Deman

    Quint Van Deman Filing Flight Plan

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    Hello all--

    I'm reviewing an airplane for purchase that has a known long in the tooth engine - a Lycoming IO-540 with 1600hrs/18 years. I'm valuating it in a way that if I had to overhaul it the day after purchase I wouldn't be mad. That said, I'm really perplexed by what I see in the compression readings, comparing year over year:
    2016 - 78,75,72,74,76,73
    2017 - 78,78,79,76,76,80
    2018 - 74,78,80,80,80,77
    2019 - 74,66,64,68,74,75

    If I look at 2019 alone, I'm thinking "she's going downhill fast" - but if I look at the trends they are all over the map. Conceptually I don't see any logic in how a cylinder (#3) could read 72 in '16, and then go UP in '17 and '18 to 80, just to then fall precipitously to 64 in '19.

    The owner provided an engine oil analysis that gave reason to believe there was a bit of life left in the engine, but ultimately, two questions:
    1) Is there a good guide for interpreting compression checks over time?
    2) If I want a best gauge of the engine condition is it compression, oil analysis, other?

    Much appreciated.
     
  2. Getonit

    Getonit Pre-takeoff checklist

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    I wouldn’t worry about it this annual, coming from airplane owner who is reasonably involved with his annuals. The test isn’t done on a computerized testing mechanism and there Is a lot of room for errors to be introduced.
     
  3. Grum.Man

    Grum.Man En-Route

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    Things such as engine temperature, amount of hours flown since last annual, proper testing technique, can all affect the readings. I wouldn't so much focus on the fluctuations as I would a general trend which is tough to determine given the readings. Did the same shop do all the compression test? I find 80/80 hard to believe on an engine with any amount of run time much less 1600+ hours. So if you remove 18' numbers you get a better trend. If they have a documented oil analysis history from an independent lab that is probably the best condition monitor in addition to a bore scope and compression testing. Cutting the filter and manually checking for metal is also a good idea.
     
  4. AA5Bman

    AA5Bman Pre-takeoff checklist

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    Is 80/80 a “thing”? I always thought there was no such thing as a cylinder that held perfect compression. But what do I know (not much).
     
  5. Grum.Man

    Grum.Man En-Route

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    Yea but usually not for long.
     
  6. Domenick

    Domenick Pre-takeoff checklist

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    The differential compression test is very susceptible to test technique. To do a proper test a calibrated orifice is needed and the prop should be rocked to find the highest value. If you just set the cylinder to what you think it TDC, it's easy to be off.

    You could have an A&P who you trust do an independent test.
     
  7. Salty

    Salty Final Approach PoA Supporter

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    My personal opinion is that you can't use compression tests as a trend per se. It's kind of a pass / fail type thing. If you're temperature goes up from 98.5 to 98.8 do you drive to the ER? But if you have a 100 degree temp for a few days, maybe you should do something.
     
  8. cowman

    cowman En-Route PoA Supporter

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    There can be huge variations in compressions depending on temperature at the time. If they drag it in cold on a very cold day the compressions can be a lot lower than if you fly it in and they check it hot.
     
  9. Bell206

    Bell206 En-Route

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    No. I've done 100s of compressions checks and never got the same number on the same engine. Too many variables. But remember the differential pressure check is only a leak check not a cylinder condition check. That requires further work if the leak check is excessive. As to your numbers, there is basically no set minimum. I believe Lycoming recommends any reading below 60psi be checked further and Continental recommends calculating a minimum acceptable pressure each time you perform the check as the trigger to look further. Now if one cylinder tanks down to 20 psi and/or you hear air leaking then that would be a clue to investigate further.
    All the above to include owner operation, boroscope checks, usage, etc. However, the best "gauge" from my view point is the opinion you get from your mechanic who will be maintaining your new aircraft as he's the one that will give you any engine bad news at your next annual.
     
  10. Kristin

    Kristin Cleared for Takeoff

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    There is also the issue of the orifice size in the testing unit. They can have .040 and .060. Above 5" bore diameter, .060 is specified, but mechanics usually only have one tester and often don't pay too much attention to the size of the orifice in their compression tester. A compression done warm with a .060 tester is followed by one done cold with a .040 tester, the difference could be quite noticeable.
     
  11. NordicDave

    NordicDave Pattern Altitude PoA Supporter

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    The compressions look very normal. They will float due to:

    * Any given position of the piston rings as they rotate. Occasionally all the ring gaps will line up, producing a much lower than anticipated value.
    * How warm the engine might be during the test sealing the rings.
    * Different IA performing the test, variation in test technique
    * Proper calibration of the pressure gauge and master orifice
    * A lead deposit stuck to the exhaust valve/seat

    Wavering 8-10 PSI is not much of a concern. Only when you hear compression test air hissing through the exhaust or back through the induction system or oil breather or oil filler. Venting into the crankcase could indicate some or all of the ring gaps have aligned. In this case, and if the exhaust valve shows no signs of stress, it's recommended to go fly for a few hours and retest.

    Also bore scope the cylinders, especially #3 to see if the exhaust valve is getting burnt or a deposit is stuck on valve face or seat.
     
    Last edited: Feb 12, 2020
  12. Quint Van Deman

    Quint Van Deman Filing Flight Plan

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    Cool, thank you all for the opinions...I think what I gathered is 'data point' but not 'definitive proof'...which means further inspection/verification required. Much appreciated!
     
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  13. chemgeek

    chemgeek Pattern Altitude

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    I would monitor #2, 3 and 4 at the next annual, or investigate further if you also see fouled plugs or other issues in those cylinders. All the stuff in the 70s is just insignificant variation. Without other symptoms, these compression readings alone do not suggest any urgent action.
     
  14. Tom-D

    Tom-D Taxi to Parking

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    Have you determined the cause of leakage ?