Hobbs Meter off

[Sue Bon]

So I flew to London to see my sister and when I landed, my Hobbs Meter read 3807:16.

A couple days later, I flew back home - a 2'50" flight - and it read 3812:10. Total flight time 4'54" according to the meter. I didn't check it before I left, so I don't know when the discrepancy happened. It was windy during my ground time, but the pitot cover was on.

Any ideas why that happened? I've done a couple of short flights since then and everything was normal.

[Schllc]

All of my planes but one had a g1000 and hobbs.
The Hobbs have been questionable to nonsensical in all of them.  
My current plane had an issue with the meter and it was actually rolling over and flipping, never look at it so I didn’t notice until one day I heard this click click click in the back.
I looked at the meter and it was literally clocking .1 hours every few seconds. 
I don’t trust any of them,  but my experience is pretty limited…

[kortopates]

We’d have to start with what kind of “Hobbs” meter you have. I assume it’s not the Mooney Hour meter which is actually tach time.


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[Sue Bon]

[A64Pilot]

I’ve never seen a hobbs like that. But then I’m not in Europe.

A Hobbs meter is nothing more or less than a simple electric clock, most often an oil pressure switch provides a ground for the clock, so that whenever the engine has oil pressure, and the Master switch is on the clock runs. That’s all there is to one.

If it’s malfunctioning, replace it. There isn’t anything else you could do. If there are times when it stops, check wiring. It’s possible the oil pressure switch could be bad, not likely but possible. Always check wiring on anything electric, often the fault is a loose ground.

A Hobbs differs from tach time as tach time is a mechanical device driven by the tach, when the engine is idling, it runs slow and at T/O RPM it runs a little fast.

Think of it like an Odometer in an older car.

I personally have never seen a Hobbs do anything but stop working, All Hobbs I’m familiar with were made by Hobbs and I’ve only seen one quit working, due I believe to just age. If a clock stops working after getting 50 years old, I can’t find fault in it.

Do make a logbook entry that states Hobbs hours when you changed it, and this is one of the few things I would keep to one day show the potential buyer, same if a tach goes bad, sometimes they do.

Personally I don’t put much into times on electronic displays, because if you have the password, you can change the hours. Not everyone is honest.

On edit, check to see that your isn’t just connected to the Master switch by turning the Master on without starting the engine and seeing if it’s running, many have some kind of indicator like a little wheel that moves a bit every few seconds or something.

Stupid question, but is your airplane right hand drive? I ask as I believe I’m looking at the right side of the panel, but there seems to be a Mag switch there. I had never thought of it before, but why not?

[Pinecone]

Hobbs is the company that makes the time counters.  They can be connected and controlled in several ways.

For most aircraft, the meter counts time based on how it is switched on and off.  Some are wired directly to the Master Switch, so any time the Master is on, it is counting time.  More common is an oil pressure switch, so the meter counts time when the engine is running.   Some twins have them wired to the gear, so they count Gear Up time.

In a Mooney, they look like the standard timer, but they are wired through the tach and the rate at which they count varies based on the RPM.  According to a table that Frank Crawford supplied to me, at 2500 RPM, the meter is counting time in a 1:1 ratio with clock time.  Lower RPM the counter runs slower and higher RPM it runs faster.  So it is counting Tach Time.

Tach Time is normally counted on the Tach itself, and it can have a number of different specific RPM where it is 1:1

The Garming G1000s I fly (C-182s) count Tach Time on the Engine Page.  If you want to have "Hobbs" time, you need a separate meter.

My JPI 830 counts both.  Hobbs time based on engine running and Tach Time based on a 1:1 RPM that can be set.  With mine set to 2500 RPM, there is very small difference in the JPI and the ships times.

[Sue Bon]

It counts flight time. It's definitely not counting when I'm on the ground, taxiing or doing the runup, etc. I don't know if it counts by RPM or oil pressure or RAM air. Someone told me once that his plane was parked into high wind (maybe 40 kt?) without the pitot cover on and he found a couple of extra hours in the morning. That's why I mentioned the pitot cover in my original post. Someone else mentioned a squat switch that maybe triggered it to run when my plane was moved by airport ground personnel, but I don't have a squat switch.

I'll ask my maintainer. Maybe I need to replace it. I also don't want to use digital time because I don't want the hassle when it comes time to sell.

The photo is from the lower left corner of the panel  Here is the whole pic. I took it to show the avionics guy that my backup horizon wasn't fixed as promised...

 

[Fly Boomer]

19 hours ago, Sue Bon said:

So I flew to London to see my sister and when I landed, my Hobbs Meter read 3807:16.

They make others, but this is the form factor I have seen most frequently:

 

[Sue Bon]

14 minutes ago, Fly Boomer said:

They make others, but this is the form factor I have seen most frequently:

 

I misspoke (miswrote?). It's not a Hobbs. It's just a counter, but it's the counter I use for 50- and 100-hour maintenance appointments. I have to figure out what makes it spin/count.

[Fly Boomer]

35 minutes ago, Sue Bon said:

I misspoke (miswrote?). It's not a Hobbs. It's just a counter, but it's the counter I use for 50- and 100-hour maintenance appointments. I have to figure out what makes it spin/count.

Sorry. when I looked at your instrument, I got hung up on the shape, style, and brand, and lost track of your question.  As others have posted, it could be driven in any number of ways.  I was intrigued by the one that was registering miles while the airplane was tied down -- I would expect that the instrument would be unable to respond to anything if the master was off.

[A64Pilot]

1 hour ago, Sue Bon said:

I misspoke (miswrote?). It's not a Hobbs. It's just a counter, but it's the counter I use for 50- and 100-hour maintenance appointments. I have to figure out what makes it spin/count.

Most likely it’s like my clock that counts flight time, it’s connected to the gear, whenever the gear is up and I assume Master on, the flight timer runs, it stops when the gear is put down. 

I know this is how hours of engine operation is usually counted for turbines, hours on a turbine is flight time, not engine run time.

When I put MVP-50T’s in the turbine aircraft we built I set it so the timer didn’t run until 90% Ng from memory, an engine speed your not likely to get taxing around, because they were fixed gear.

It’s likely I assume that the System you have accepts flight time as engine time and therefore your timer is connected to the gear. It could be tied to pitot I guess, but as that’s a flight instrument it would be more difficult to get approval for that, and I know the Brits if that’s what your under are strict, very strict.

Connecting to the gear would be as simple as connecting it to the gear up light, gear up light on, timer runs.

It’s real common to call any timer a Hobbs, we all do it. It’s like calling any tissue a Kleenex or a bandage a band aid or whatever.

[kortopates]

It’s not the original Mooney tach, but an alteration. I’d check aircraft records - probably installed a long time ago.
It sorta look like a tach except for 60 min at top with quartz suggest more of a clock. But if it’s a tach, i’d expect it to be showing RPM and can’t tell from the pict.


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[Fly Boomer]

3 minutes ago, A64Pilot said:

Most likely it’s like my clock that counts flight time, it’s connected to the gear, whenever the gear is up and I assume Master on, the flight timer runs, it stops when the gear is put down. 

I know this is how hours of engine operation is usually counted for turbines, hours on a turbine is flight time, not engine run time.

When I put MVP-50T’s in the turbine aircraft we built I set it so the timer didn’t run until 90% Ng from memory, an engine speed your not likely to get taxing around, because they were fixed gear.

It’s likely I assume that the System you have accepts flight time as engine time and therefore your timer is connected to the gear. It could be tied to pitot I guess, but as that’s a flight instrument it would be more difficult to get approval for that, and I know the Brits if that’s what your under are strict, very strict.

Connecting to the gear would be as simple as connecting it to the gear up light, gear up light on, timer runs.

Swiss.

[A64Pilot]

Just now, Fly Boomer said:

Swiss.

OK, I know the Brit exchange Officers we would get at the test activity were astonished at how we could do as we please. Even the Military in England if they wanted to change brands of tire if the builder specified a tire brand had to get permission from the builder. She said London so I assumed Brit.

I assume she too, maybe I shouldn’t.

[Sue Bon]

2 minutes ago, A64Pilot said:

I assume she too

Swiss. Grew up in the US (California). Moved here (Switzerland) in 1995.

[buddy]

21 hours ago, Schllc said:

All of my planes but one had a g1000 and hobbs.
The Hobbs have been questionable to nonsensical in all of them.  
My current plane had an issue with the meter and it was actually rolling over and flipping, never look at it so I didn’t notice until one day I heard this click click click in the back.
I looked at the meter and it was literally clocking .1 hours every few seconds. 
I don’t trust any of them,  but my experience is pretty limited…

My Hobbs did the same thing and I disconnected it.

[MatthiasArnold]

As mentioned before, most of the G1000 equipped models have two hour meters:
A traditional Hobbs located in the baggage compartment and the "software" meter integrated in the G1000.

The traditional Hobbs is actuated by the "airspeed safety switch" which also tries to block the gear from being retracted while on ground for most newer Mooney models.
The airspeed safety switch has two parallel micro switches actuated by its differential pressure membrane (connected to static and pitot) and the threshold is set to something around 60kts.

I have no idea whether the electric wiring is similar for a J.
However, I observed two different errors (occurring occasionally):

• continue counting while on the ground (like your issue)
• not counting while in the air

Both could be traced back to the similar root cause:
The airspeed safety switch with its micro switches, not the Hobbs meter (which simply counts once voltage is applied).

Long story short: if the working principle for your J is the same, it might not solve the issue by exchanging the Hobbs meter, there might be other root causes...

Best,
Matthias

BTW: will cross the Swiss Alps this Saturday north bound. However not in the Grenchen region but more to the east (Como, Bellinzona, Disentis, Bifertenstock, Obersee )..

[Sue Bon]

2 hours ago, MatthiasArnold said:

BTW: will cross the Swiss Alps this Saturday north bound. However not in the Grenchen region but more to the east (Como, Bellinzona, Disentis, Bifertenstock, Obersee )..

What a beautiful flight!

[A64Pilot]

Next time I go fly I’ll drop the gear early and see if the clock stops counting time, if it does then it’s the gear that operates it, if it doesn’t, then it’s the airspeed safety switch.

You could do the same check of course by simply leaving the gear down and flying a circuit or two, if the clock never starts it’s the gear, if it does it’s the A/S safety switch.

I’m pretty sure my Davtron clock is OEM, there is even a flight timer reset button on the panel so I’m assuming it’s wiring is IAW Mooney drawings

[Pinecone]

Here in the US, normally Tach Time is used for 50 and 100 hour servicing.

Hobbs is used for logging purposes.  FAA allows logging time starting with the first movement of the aircraft for flight to the when it stops moving before engine shutdown.  So, if you log using the hour meter in the early Mooneys, or flight time (take off to touchdown) you are shorting your logged time.

In the USAF we logged take off to touchdown plus 5 minutes.  But, for civilian purposes, could log an additional few tenths every flight.

[A64Pilot]

Normal Hobbs in piston US airplanes run whenever there is oil pressure, normal old school tachs are mechanical and count time as a function of RPM.

US anyway normal tach and hobbs ought to be close, most piston US aircraft don’t have Hobbs meters. I’ve always wondered why have one. I’ve assumed rentals do because crank the engine and the hobbs runs, where the tach is slow until takeoff and they charge based on Hobbs? Guessing here

When we get to turbines in the US Hobbs count flight time, in the old turbine we built we put the Hobbs oil pressure switch in the torque sense line of the PT-6 or Garrett, the Hobbs switch closes from memory about 30 PSI, a torque you don’t hit until takeoff run and drop below on final, so it counted flight time. The ones with MVP’s I used Ng speed to start the timer

Pratt anyway schedules engine maintenance based on flight, not running time and crop dusters spend a lot of time on the ground running refilling the hopper so for them it’s a big difference.

Attached is a photo of the Hobbs oil pressure switch.

Now I ASSUME that in Europe all aircraft count flight time for scheduled maintenance instead of engine run time and maybe their Hobbs are wired differently, or is Mooney an outlier as a single piston with a Hobbs that only counts flight time? Just speculating I don’t know, my J doesn’t have a Hobbs

Most piston single engines don’t have retracts or A/S safety switches, so they would need hardware that I’ve not see to count flight time.

Army we counted flight time from takeoff to landing, or maybe it was takeoff until engine shut down, 20 years ago so I forget

I had an instructor arguing with the FSDO that he should be allowed to add .3 to each of his flights because the FAA counted flight time from engine start to shut down, you could tell they were annoyed. I told him, John put whatever you want in your book, no one cares. But it’s not changing your Army flight time.

 

[Sue Bon]

1 hour ago, Pinecone said:

Here in the US, normally Tach Time is used for 50 and 100 hour servicing.

Hobbs is used for logging purposes.  FAA allows logging time starting with the first movement of the aircraft for flight to the when it stops moving before engine shutdown.  So, if you log using the hour meter in the early Mooneys, or flight time (take off to touchdown) you are shorting your logged time.

In the USAF we logged take off to touchdown plus 5 minutes.  But, for civilian purposes, could log an additional few tenths every flight.

You are talking about block time vs. flight time. I use block time in my personal log book, but flight time for the 50/100 hour service.

[Sue Bon]

Just got word from my maintainer. He says the counter is driven by a sensor on the pitot line. They suspect the sensor remained closed for some reason. As it only happened once, they recommend to watch it and if it continues to happen, they will replace it. 

This has been an interesting discussion! Thanks to all of you

 

[Sue Bon]

1 hour ago, Pinecone said:

In the USAF we logged take off to touchdown plus 5 minutes.  But, for civilian purposes, could log an additional few tenths every flight.

We are Swiss. We record it exactly down to the minute  

I'm only half joking. I usually add 10 minutes before takeoff and 3 minutes after landing. I'm the exception. Most people here really do write down the exact minute they go off block and the exact minute that they are back on block.

[Pinecone]

Most planes I have flown (piston) have a Hobbs activated by engine oil pressure.

In the USAF and Gliders, take off and touchdown times were logged.

In my plane, I use the automatic timer in my GTN-650 which counts time from power on (right after engine start) to the when I shut the avionics switch off.