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Discrepancy between tach and Hobbs on an Acclaim

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i am looking at an Acclaim that has a large gap in time between the tach time displayed on the G1000 and Hobbs meter at the back. The Hobbs shows around 930 hours while the tach time is at 1400. How can this happen? Is this a serious concern? Also, the airbag seatbelt expired. Would this affect the airworthiness?

thank you,


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A few hours difference can be normal as the hobbs is clock time and a tach is mechanical time.  What I mean by that is the tach is usually designed to cound hours at an average of 2400 RPM.  Sometimes we are below and above that, hence the difference.

The only other thing would be time with the master on, but the engine not running.   But over 400hours?

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Tach on g1000 is the only thing that matters, I’m not sure why a Hobbs is even installed in the aircraft anymore, although I’m sure someone will explain here shortly..

But the tach on the g1000 is all that matters. 
seatbelts can be disconnected and labeled inop, and won’t affect airworthiness if this is done. 

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So time while the master switch is on is also recorded by the tach? Even if the airplane is connected to external power? Maybe  someone was practicing on the G1000 while on the ground.

thank you both for the responses.

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Check the POH...

It will describe what is used to turn on the clock...

And what the clock is actually measuring...

In some long bodies... an oil pressure switch was used to power an actual clock on the back wall of the baggage compartment... so it is not exactly measuring flight time...

Since the Hobbs meter was traditionally a revolution counter... expressed in units of time...  the standard of measuring engine hours is a bit fuzzy... and allows for a lot of flexibility... as it was only accurate at one rpm...

So... the POH and maintenance manuals will be the place to get the right hours associated with the proper maintenance intervals...

The magic of the G1000 system... it can know the difference between taxiing on the ground... and flight hours which typically start at 65kias in a long body...

It would be disappointing if taxi time at a big airport got counted the same as actual flight time... 

It would be even more disappointing if OHs had to occur on exact time schedules, like at TBO...

So... as a GA pilot/owner... not such a big deal...

But, as a commercial operator, stuck in a long line, waiting on releases at a big airport.... that could get more annoying...


Most engine monitors know the difference between being turned on... and being used in flight.... turning on an engine monitor doesn’t necessarily have it store a data file...


To be sure what was collected on the various run clocks a plane may have... read the POH...

The engine log is probably pretty clear about where the data was taken from...

PP thoughts only, my engine hour meter is in the baggage compartment...  out of sight, out of mind... :)

The Acclaim is the ultimate Mooney...

Go Acclaim!

Best regards,



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18 minutes ago, Yariv said:

Thank you Carusoam:-). At this point I do not have the logbooks for inspection yet, and no access to the POH.

this is from a late 90s M20R POH, when G1000s were still really new... (update 10/2003)

Find line 25... here...  then find it in the actual POH of the plane you are looking at...

Best regards,




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I thought the Hobbs time in the back was connected to an oil pressure switch?  It could have failed 500 hrs ago or someone disconnected it. I’ve seen some really low hr ovations in the past that look like they they’ve been worked longer than indicated.   It wouldn’t take more than a min for someone to dis or reconnect the wire going to it. 

The real question is how’s  the plane being advertised...1400 hrs or 900. If 900 the PO has some explaining to do. 

I agree with LLC above, not sure why Mooney was still installing them a decade after the g1000 was shoehorned in. 

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2 hours ago, Yariv said:


i am looking at an Acclaim that has a large gap in time between the tach time displayed on the G1000 and Hobbs meter at the back. The Hobbs shows around 930 hours while the tach time is at 1400. How can this happen? [...]

I posted this in another thread but I think it is worth repeating here.  This was an Ovation, not an Acclaim but I don't know why they would be different:

The Hobbs meter in my G1000 Ovation 2 is activated by the extra set of contacts in the landing gear airspeed safety switch, which in theory should closely track the time airborne.  This would be great, but because of an apparent design shortcut it shares its power with the cabin lighting, which occasionally blows the 5A fuse in my plane.  [...]  Over time the Hobbs and tach time readings have diverged as a result.  I think the Hobbs is useless as-is [...]

I think the G1000 derives tach time by counting engine (magneto) revolutions as it would for the tach itself.  It is always less than elapsed time from engine start to shutdown by a few tenths, but it is very close to flight time shown on FlightAware.

I also saw various logbook entries that showed several unsuccessful attempts to troubleshoot this problem.  When I bought the plane I replaced a blown cabin light fuse and a broken cabin light fixture.  I don't know if that was the fix but he cabin lighting and the Hobbs meter have worked without a problem since then.  I check the cabin lights after every flight to make sure that (1) they still work, and (2) they are turned off.

Earlier electrical schematics show that for pre-G1000 Ovations the Hobbs was connected to the tachometer.

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