Questions about HOBS vs Engine TT vs Airframe TT

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cmeleg
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Questions about HOBS vs Engine TT vs Airframe TT

Post by cmeleg »

We are going through our first annual with Serial 0034 ... have a question:

At some point in the past in my logbooks, Airframe TT started using the value of HOBS (5 or so years ago)... My A&P is telling me that is likely wrong. My Engine TT is 600 ish hours but my HOBS is over 1000. I am asking because @ 1000 hours there are some exhaust parts that are required to be replaced for airworthiness.

Any advice on what values should be used? (and where they come from)

Any help would be greatly appreciated!

GimpyPilot
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Re: Questions about HOBS vs Engine TT vs Airframe TT

Post by GimpyPilot »

My books are based on tach time from the VM1000. Some A&Ps don't realize that the VM1000 maintains an extrapolated tach time based on RPM, so they just record HOBBS time, which of course screws up your books and maintenance times. My mechanics have always used tach times. I would recommend the same for all maintenance and repair intervals.

At last annual, my times were 621.4 tach and 839.5 hobbs. It seems that perhaps someone have adjusted the tach math in your VM1000 because a delta of 400 hours between tach and hobbs seems unusual unless you typically operate at low RPM, or routinely experience very long ground holds.
Last edited by GimpyPilot on Wed Jul 24, 2019 9:20 pm, edited 2 times in total.

cmeleg
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Re: Questions about HOBS vs Engine TT vs Airframe TT

Post by cmeleg »

Thank you! On the VM 1000... which value is tach? I believe at startup there are 2 values that display. Yes, low RPMs and long ground holds are common as this airplane has taught a few people how to fly in its prior life and now my kids use it for that same purpose.

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Re: Questions about HOBS vs Engine TT vs Airframe TT

Post by GimpyPilot »

To my recollection, Tach time is displayed numerically inside the tachometer arc, when the unit is powered up. I'm going to be flying tomorrow afternoon, and I will confirm, however the manual says this:
When the engine is not running, the tachometer digital display reads the total accumulated engine hours to a maximum of 5999.9 hours. Engine hours are accumulated any time RPM is greater than 1500.

cmeleg
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Re: Questions about HOBS vs Engine TT vs Airframe TT

Post by cmeleg »

I found out that the first set of numbers are the software revision (1011 for me)... Thanks GimpyPilot for all your awesome help!

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XL2driver
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Re: Questions about HOBS vs Engine TT vs Airframe TT

Post by XL2driver »

Yes indeed, the TACH time from the VM1000 is the only time to use for all maintenance scheduling.
The TACH is usually running at 20-30% less than the Hobbs time, depending on what kind of operation you have and how long your taxi times are. Essentially the tach barely moves at anything under 1500 RPM.
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Re: Questions about HOBS vs Engine TT vs Airframe TT

Post by BRADLOUTH »

Use HOBBS for FLIGHT log time ( PIlot)
Use Tach time for all mechanical logs...

Difference is ground running time.

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