Hobbs meter running fast?

[Mark89114]

Has anybody experienced this?  Either my record keeping is getting sloppy or something else is going sideways.  I have never experienced huge differences between Hobbs and tach.  But recent flights aren't adding up.  7.7 on Hobbs and flight timer is showing 5:35, which is right.  There is no way I ground run to run the number up that much.  Not sure how Hobbs is wired in, except I am guessing original 2000 ovation install in luggage compartment.

[carusoam]

From the O POH...

25. HOUR METER
Hour meter - located on baggage compartment bulkhead and indicates elapsed time while engine is running. Location may vary depending on installed systems. 

 

1) Where is your Hobb's meter, G1000? What is your flight timer?

2) the hour meter and its description indicates the engine being on.  Could be an oil pressure switch or as simple as a wire from the ignition switch or another engine on sensor...  oddly includes warm-up, taxi, run-up, TO roll, flight time, landing roll-out, taxi time, and final shut down...

3) a flight timer clearly indicates something different.  Ground operations are not part of a flight timer. 

4) We have discussed this before. I just don't recall the detail.  The O1 has only the meter on the back wall.  Conveniently out of view and out of mind of the inevitable OH that is implied by using another hour....

5) What has changed that would make your flight timer be much shorter than your engine timer?  Longer taxis than usual lately?

6) Have you gotten new technology like ADSB that is actually measuring the time the flight occurs.  Gear up time only?

Interesting question...

Best regards,

-a-

Edited April 23, 2017 by carusoam

[jaylw314]

Tach time is just a rev counter for the engine.  The time counter converts revs into hours using an arbitrary "cruise" rpm, somewhere between 2400-2600 rpm.  As such, if you are doing pattern work or use rpm's that are much lower, your tach time will always be significantly lower than your Hobbs meter.  If you ever rent a plane that charges by tach time, you can save money this way  I used to rent a fixed pitch plane by tach time, and the tach time was often 20-30% lower than Hobbs when doing pattern work.

Edited April 23, 2017 by jaylw314

[Scottknoll]

A little late, but hopefully this helps someone in the future. Found this while looking for something else. I had this issue and it turned out to be a loose cannon plug for the RPM indication. My analog RPM gauge was a little bouncy, and at times up to 3000. JPI was reading normal, and it was obviously an indication issue. But the with the Hobbs being tied to RPM, it showed a lot of extra flight time. Tightening up the cannon plug fixed the issue.

Edited November 3, 2022 by Scottknoll

[A64Pilot]

There are three timers being discussed and I believe confused here.

1. Hobbs meter, a simple timer that should be hooked to an oil pressure switch, whenever the engine has oil pressure it runs at a set rate, RPM has nothing to do with it, it’s a simple electric timer, it’s on or off.

2. Tach, this is a mechanical device similar to an old fashioned speedometer, the higher the RPM, the quicker it counts time, it operates off of RPM. “normal” cruise RPM should correlate to real time pretty much.

3. Fight timer, a device meant to count flight time, not engine time, can be wired in different ways, mine on my J model counts time whenever the gear is up, so all the time the engine is running but the gear is down, it doesn’t count.

So the three won’t ever agree, Hobbs may accumulate time the fastest followed by Tach, then flt timer.

[PT20J]

Part of the confusion is probably that later Mooneys use a B&D electric tach that did not include an integral hours counter but instead drove an external counter that looked exactly like a standard Hobbs meter but had three wires instead of two, the third wire connecting to the tach. 

[carusoam]

On 11/3/2022 at 11:15 AM, Scottknoll said:

A little late, but hopefully this helps someone in the future. Found this while looking for something else. I had this issue and it turned out to be a loose cannon plug for the RPM indication. My analog RPM gauge was a little bouncy, and at times up to 3000. JPI was reading normal, and it was obviously an indication issue. But the with the Hobbs being tied to RPM, it showed a lot of extra flight time. Tightening up the cannon plug fixed the issue.

Great follow up Scott!

Thanks for sharing it…

Best regards,

-a-