Cm Hobbs meter running for six weeks while airplane off and hangared!

[tom paul]

I just flew my plane for the first time since December. I was at the hanger in late January for an oil and filter change, and I ran the engine up to temperature both before and after the oil change. My hobbs meter was showing about 1240 hours, and I have a photo of that indication. I take a picture with my phone before and after every flight. Today I went out to do some pattern work and dust off the old skill set, not to mention the aircraft. I was shocked to find the hobbs meter indicating to 2272 hours! This number actually adds up to the number of hours since I did the oil change. The plane was sitting with a battery charger connected all of the time.
I did notice something new when I got in the plane this morning. I heard an occasional click coming from the panel. It happened every few seconds with apparent regularity. I traced it to the hobbs meter and saw a little flag indicator changing on the left of the face. I didn’t build the airplane and thus have a limited understanding of the systems. I am considering that perhaps there is a oil pressure sensor that turns on the hobbs which has malfunctioned.

This seems like a pickle to be in. I need to fix the issue and somehow wind the hobbs back to where it was indicating TTSN engine and airframe.
I look forward to hearing anyones thoughts and experience with something like this.

[Tankerpilot75]

I also purchased my RV from the original builder and noticed immediately a significant difference between Hobbs time and Tach time. Much to my surprise I discovered the Hobbs meter ran when ever the panel was powered up - there wasn’t an oil pressure switch to provide power to the Hobbs. As a result, when ever the panel was powered (say to practice and gain familiarity with the EFIS or gps systems) the Hobbs meter ran.

This wasn’t acceptable to me so I ordered an oil pressure switch and new Hobbs meter. Before I installed them (after confirmation of Tach hours with log book entries) I powered the new Hobbs meter long enough to have the hours match tach time. Installed switch and new Hobbs meter, annotated log book and pressed forward. This was about 400 hours ago and as expected there is now a slight difference between Tach and Hobbs hours (~3.5 hours) but at least the two measurements are tracking actual run time - not panel power time.

In your situation I’d first check to see if other items have power to them when the charger is connected. If so you need to correct that first. If not then someone probably hot wired the Hobbs to the battery. Remember, these are amateur built aircraft and anything is possible.

Good luck.

[Ironflight]

I don’t know of a way to run the Hobbs backwards - my guess is that you can’t because, well….Hobbs are used on all sorts of machinery to record time, and if it was easy to run them backwards, no one would ever know how much time was on a tractor, or a backhoe, or an airplane, or….. (I coudl be wrong of course - someone will come along and say that they connected the battery backwards and ran it in reverse….but I have never heard that such a Ethan’s works!).

What I would do is make a logbook entry that shows what Hobbs time correlates to at this point in time. Then you’ll give yourself (and future owners) an accurate benchmark. Hobbs time is arbitrary anyways - some folks put old Hobbs meters in new machines and just keep track of the delta. Our RV-6 has had Hobbs, and hasn’t. It’s had five different EFIS/EIS installations over the thirty year life. It’s had two engines, with one of them overhauled. I spent an afternoon going through all the logbooks, correlating and sleuthing to figure out my best estimate on what the actual airframe time was, then built a spreadsheet to show my work, printed it out, and put it in the logbook. So from now on, we (and any possible future owners) know how many hours are on the airframe based on the current indicated time on the EFIS.

Paul

[BobTurner]

The standard Hobbs on a rental 172 is wired thru a fuse directly to the battery, and controlled by an oil pressure switch. Otherwise a few pilots would turn off the master in flight, to cut their rental charges.

[gasman]

Originally Posted by tom paul

I just flew my plane for the first time since December. I was at the hanger in late January for an oil and filter change, and I ran the engine up to temperature both before and after the oil change. My hobbs meter was showing about 1240 hours, and I have a photo of that indication. I take a picture with my phone before and after every flight. Today I went out to do some pattern work and dust off the old skill set, not to mention the aircraft. I was shocked to find the hobbs meter indicating to 2272 hours! This number actually adds up to the number of hours since I did the oil change. The plane was sitting with a battery charger connected all of the time.
I did notice something new when I got in the plane this morning. I heard an occasional click coming from the panel. It happened every few seconds with apparent regularity. I traced it to the hobbs meter and saw a little flag indicator changing on the left of the face. I didn’t build the airplane and thus have a limited understanding of the systems. I am considering that perhaps there is a oil pressure sensor that turns on the hobbs which has malfunctioned.

This seems like a pickle to be in. I need to fix the issue and somehow wind the hobbs back to where it was indicating TTSN engine and airframe.
I look forward to hearing anyones thoughts and experience with something like this.

If all power to the aircraft is cut off with the Master Solenoid, and the master switch WAS off, then how did the hobbs run during this time.

When you connected the charger, did you reach in the oil door and connect the + clip to the wrong side of the master solenoid..... or to the battery side of the starter solenoid??

[PilotjohnS]

I would buy a new hobbs, run it at home till it reads the correct hours. change it out on the plane, and note in log book what occurred.

[gasman]

Originally Posted by BobTurner

The standard Hobbs on a rental 172 is wired thru a fuse directly to the battery, and controlled by an oil pressure switch. Otherwise a few pilots would turn off the master in flight, to cut their rental charges.

In the 70's, I was along for the ride with a friend in a rented turbo Arrow. In front of me was the hobbs and a pop-in panel that Piper used to cover a future radio location. Curious, I popped the cover off and THERE, was an inline fuse to the hobbs.....

We agreed to remove the fuse. 10 minutes later, guilt was the conversation and the fuse was replaced......

[AlexPeterson]

Originally Posted by gasman

If all power to the aircraft is cut off with the Master Solenoid, and the master switch WAS off, then how did the hobbs run during this time.

When you connected the charger, did you reach in the oil door and connect the + clip to the wrong side of the master solenoid..... or to the battery side of the starter solenoid??

Most hobbs are set to record engine run time, not master on time. Standard practice is as discussed earlier in this thread - direct from an always-hot bus.

So, to run it backwards, first find a black hole - the next part gets a bit tricky...

[Mike S]

Originally Posted by PilotjohnS

I would buy a new hobbs, run it at home till it reads the correct hours. change it out on the plane, and note in log book what occurred.

Easier is to install a new unit, and make an entry in the log stating Hobbs was replaced at XXX hours.

Then current Hobbs + XXX will be the total time.

And, wire it up through an oil pressure switch.

[jneves]

Originally Posted by Mike S

Easier is to install a new unit, and make an entry in the log stating Hobbs was replaced at XXX hours.

Then current Hobbs + XXX will be the total time.

And, wire it up through an oil pressure switch.

This method works, but you can also just make a log entry that actual time = Hobbs minus XXXX. Same outcome in both scenarios a quick calc is needed. I did this on the last plane after a few hours of going through the logs to confirm the actual TT. Also made reference in the notes section of the log book for future use by who ever needed it