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How is the Hobbs meter hooked up?


urbanti

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I'm not aware of Hobbs meters being part of the equipment in classic Mooneys.  Hobbs meters can be hooked up to measure time the master switch is on (rentals!), how long the engine has produced oil pressure, or how long the aircraft has been airborne.

 

Mine was added to record flight time since that's what TBO is based on (not tach time), but it wasn't original; it was added later.  Now, this same function can be recorded by the Garmin 430/530 by recording time above a certain ground speed (usually 40 Kts.).

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  • 1 month later...

A question, related to the subject. I installed after market Hobbs, activated by oil pressure sensor some 20 years ago. Recently, I've noticed that it's recording slower than engine running time, sometimes as much as 50%! Is it the meter or the sensor? Or....

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How are you making the comparison?

The oil pressure sensor is an on/off switch.

If you compare the engine hours to your watch, how well does it do?

I'm expecting that the switch is either working or not. Or, the hour meter itself is slowing down

What type of Hobbs meter did you get. An electric clock that runs when the oil P switch is on? A mechanical clock that may have dirty or wear issues?

Expect a loose wire in the system...

The M20C has an hour meter that is really measuring engine revolutions. It is a mechanical part of the rpm gauge.

Best regards,

-a-

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It's the Honeywell Quartz available at Spruce and others. I recognized something was amiss when I took a local flight. Tach measured 1 hour. Hobbs showed 0.6! Normally I'd expect 1.2 Hobbs in the example above. Had worked predictably for 2500 hours. Where's the oil pressure switch located on IO360A3B6D? You think that's where I'd find the loose wire?

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I'd guess that the person adding it may not have bothered with the elaborate oil pressure routine.

Connecting it to the avionics bus would essentially work. Unless you rent it out.

Quartz clock isn't going to run slow.

Chase the wires out the back of the meter to see where they go.

If things are documented properly, the POH would be updated with weights and updated wiring diagrams.

If you replaced an existing Hobbs then there probably is a pressure switch on the oil line. Unfortunately, I don't have much knowledge of where to look on your engine for that. For original equipment, your POH would give a good description of it.

Best regards,

-a-

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  • 4 years later...
52 minutes ago, NotarPilot said:

Does anyone know how easy it is to rig up a Hobbs meter to an oil pressure switch?  Seems like it’s pretty straight forward but I have no idea where the oil pressure switch goes.  Does anyone have a picture by chance?

Cessna uses a special fitting at the firewall in the oil pressure line to the gauge.  You could use any number of oil gallery plugs in the engine to mount the pressure switch, then run wires to the hour meter.

Clarence

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Strictly for logging flight time.  I currently use the stopwatch on my MD-93 clock.  As soon as the engine starts I start the time and as soon as I pull the mixture I stop it and write down the elapsed time.  The MD- 93 has an engine run time too that I could probably use as a Hobbs meter though.  When it was installed, the fine people at Pacific Coast Avionics wired it to the master and never asked me in advance if I wanted to wire it to a oil pressure switch.

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10 minutes ago, NotarPilot said:

Strictly for logging flight time.  I currently use the stopwatch on my MD-93 clock.  As soon as the engine starts I start the time and as soon as I pull the mixture I stop it and write down the elapsed time.  The MD- 93 has an engine run time too that I could probably use as a Hobbs meter though.  When it was installed, the fine people at Pacific Coast Avionics wired it to the master and never asked me in advance if I wanted to wire it to a oil pressure switch.

The reason to wire it to the oil pressure switch is to prevent pilots from flying around for free by shutting off the master. For us it doesn't make much difference.

-Robert

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I liked using my portable garmin for that...

The portable held all the details automatically... date, place, hours, great for filling in log book details...

Having a JPI do the logging can work pretty well also. Some are really easy to download, some require bringing a laptop still...

For flight time, I used the BK ADF... switching tanks hourly...  too easy to forget, until the moment the electricity is turned off...

The older you get, the more challenging it can be to collect all the Minor details... let the automation do it...

Best regards,

-a-

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The problem with the GPS (Garmin) type of logging is that they don't work until you get to a certain speed. You're cheating yourself. On Tuesday I clicked off about 30 minutes at SJC San Jose just getting moved from runway to runway while ATC tried to figure out how to work a single engine IFR departure in with their 121 traffic.

 

-Robert

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+1 on auto logging start speed... it is selectable... choose the lower of the speeds... good enough accuracy when all I need is to log time to report annual hours to the insurance company...

 

Definitely need to have a reason to log things...

1) flight time... Engine started, brakes released the first time...

2) engine time via tach... always counting every revolution...

3) engine time via oil pressure activated electric switch and a real clock timer..... (typical LB system...)

4) engine time using the master switch... adds to the inaccuracy...

 

5) tracking a half hour of taxi time would kind of add insult to the injury... long ground holds waiting for a clearance?  I was trying to make those bad memories go away, quickly!  :)

Best regards,

-a-

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The problem with the GPS (Garmin) type of logging is that they don't work until you get to a certain speed. You're cheating yourself. On Tuesday I clicked off about 30 minutes at SJC San Jose just getting moved from runway to runway while ATC tried to figure out how to work a single engine IFR departure in with their 121 traffic.
 
-Robert

You can configure taxi time into the auto logging, I add 0.2 hours for takeoff,landing ground movements.
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