tachometer issues

Discussion in 'Avionics and Upgrades' started by mr_happyland, Nov 5, 2021.

  1. mr_happyland

    mr_happyland Pre-takeoff checklist

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    I've been trying to lockdown issues with my tach and curious for input.

    Long story short, a few months ago, the analog tach on my Grumman Tiger took a dump and I needed to replace. I didn't (and still don't) wanna go the digital tach route. After my mechanic installed a new Mitchell analog tach, it seemed to be reading lower than in the past. I verified with a handheld digital tach reader that the new tach was indeed reading between 100-150 RPM too low.

    We have since tried a few things, the latest being replacing the old tach cable. It still reads 100 RPM low (compared to the handheld digital tach). The next step would be to send the "new" tach into Mitchell for them to have a look.

    I know an analog tach isn't perfectly accurate, but do you guys think it's worth sending it in to Mitchell? Should I just live with it in its current state knowing it reads about 100 too low? For the time being, I still don't want to switch to a digital tach. I might do so down the road with a JPI 900 or Garmin 275, but for now I wanna stay analog.

    What say you?
     
  2. SixPapaCharlie

    SixPapaCharlie May the force be with you

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    Why not digital?
     
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  3. William Pete Hodges

    William Pete Hodges Pre-takeoff checklist PoA Supporter

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    IMG_20201201_132255729.jpg Have you called Mitchell and asked them? 1% of 2500 is 25. 100-150 is 4-6% error. That is too much for a precision instrument. It sounds like your tach is defective or was not calibrated properly. If the meter is new and reads that far off, I imagine they want to help you fix it.

    PS I like analog instruments. They respond quickly and are easy to scan across without actually reading them. I have a CGR30P combo in my bird, and I retained the OEM tach and have a mechanical MP gauge as backup. The CGR30P has two large analog/digital meters on the top row. MP and RPM. I like it and after using it for a year I am now comfortable referencing it first, but I still log tach time off the old meter.
     

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    Last edited: Nov 6, 2021
  4. Bell206

    Bell206 Final Approach

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    Just to ask but have you checked the digital reader against another aircraft for accuracy?
    Was the 100-150 low at all RPMs or at a specific RPM?
    Is this a new tach from Mitchell or just new to the aircraft?
    Would call them first.
     
  5. Joe_B1

    Joe_B1 Line Up and Wait

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    Call them and explain it. You paid for a precision istrument.
     
  6. Domenick

    Domenick Line Up and Wait

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    Most old analog tachs read a bit low. Did you check you last tach against the optical tach? Was the Mitchell replacement new, overhauled, or rebuilt?

    I'll repeat what Bell206 said, "...have you checked the digital reader against another aircraft for accuracy?"
     
  7. Ed Haywood

    Ed Haywood Line Up and Wait

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    Mitchell is a great company, and they know their stuff where analog tachs are concerned. Call 'em and discuss. Most likely they will offer to send you another tach.

    In the meantime, like Bell said, is it off at all RPM or just one setting? Analog tachs are designed to be accurate at cruise RPM. They are going to be off at other parts of the range. If you are checking a tach at runup RPM, it is going to be off. That's just the nature of analog tachs.

    I also prefer analog tachs. In addition to the old school looks, I like not being dependent on electrical power to monitor MP and RPM.
     
    Last edited: Nov 6, 2021
  8. Dan Thomas

    Dan Thomas Final Approach

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    Calibrate the optical tach checker under a fluorescent light. Set it on the two-blade setting. It should read 3600 RPM. The 60-Hz grid frequency is stable to, IIRC, .0033%. Good enough.

    A tach that reads 100 RPM low is junk. Period. In Canada we have to check magnetic-drag-type tachs yearly for accuracy, and they must be within 4% when the engine is running at cruise range midpoint. That's max 100 RPM low at 2500. The magnets in them get weak with age, and if this Mitchell thing is low it's of poor quality, and no amount of cable-changing will fix that. A cable must keep up with the engine or it snaps. All it can do is get jerky. I don't know why people persist in buying mechanical tachs for any airplane with an electrical system. If you want the classic look in an electronic tach, this is available:
    https://www.aircraftspruce.com/catalog/inpages/tso_tachometer.php

    [​IMG]

    It's TSO'd and can be installed under a field approval.
     
  9. mr_happyland

    mr_happyland Pre-takeoff checklist

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    Thx for all the input. Just to answer a few of the questions...

    The tach is new. My mechanic has spoken with someone at Mitchell and we've gone thru all the steps they've suggested. The next step would be to send it in to them. As far as the accuracy of the handheld digital tach reader, it belongs to my mechanic and I assume he's verified it's accuracy over the years. Beyond that, my new tach reads consistently low at full power cruise settings, unlike my previous tach which matched up with POH numbers perfectly. The new tach seems to be closer to matching the digital reader at idle, yet once I'm cruising it seems off by at least 100 RPM.

    The obvious next thing seems to just send it in to Mitchell. That means a couple weeks of downtime. I guess what I was mainly wanting to query was whether or not an analog tach that is 100 RPM off is worth sending it in? Since I've already gone as far as I have trying to resolve the issue, might as well.

    Side note, as far as digital tachs go, I like the aesthetics of the analog instruments, plus I'm not a fan of standalone EI digital tach. I've found the "exact" accuracy of it distracting, plus it's lighting is annoying on night flights. YMMV. One day maybe I'll spring for a comprehensive digital engine monitor system. Until then...analog.
     
  10. Bell206

    Bell206 Final Approach

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    Sounds like the calibration setting has slipped. Good luck.
     
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  11. mr_happyland

    mr_happyland Pre-takeoff checklist

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    I've never seen this digital tach before. Only the EI one. Interesting.