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05-13-2009, 09:04 AM
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Join Date: Apr 2008
Location: Tennessee
Posts: 440
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Hobbs meter...
Hobbs meter is reading when the key is turned on until when it is turned off. this is not dependent on the engine running. That is why you want to be especially vigilant about turning off the master electrical switch when flying rental planes as they all use the Hobbs time for charging you for the rental. This is really easy to forget when the engine is turned off via fuel lever (which it should be). The Hobbs will keep running even tho the engine is not turning . Ask me how I know!
Quote:
Originally Posted by Greg Arehart
I may be wrong, but my understanding was that Hobbs time is from engine on to engine off - basically any time there is oil pressure in the system. Tach time on the Dynon actually counts RPMs and then calculates time based on the user preset RPM. So if I set the "tach RPM" to 2700, I should get a much lower tach time than Hobbs time, since I rarely run at those RPMs! However, if I set the "tach RPM" to, say, 1800, then I might actually get a tach time that is LARGER than the Hobbs time, because I would be (normally) cruising at 22-2400 RPM which would compensate for the time I spend on the ground at 7-900 RPM.
I currently have my tach timer set for 2200 RPM, which is probably a bit lower than I normally cruise.
greg
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05-13-2009, 09:05 AM
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Join Date: Jun 2005
Location: Woodinville, WA
Posts: 1,499
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It as hinted at above, but just a note for those of you comparing tach times, the Dynon and AFS/GRT systems do it totally differently. The Dynon EMS systems actually normalize to an RPM, just like mechanical tach timers do. In a sense, they count actual revolutions of the engine, which is the whole reason that tach time is important. They read higher than 1 hour per hour if you are above your nominal RPM, and lower if you are lower. The system works at 600 RPM or 2700 RPM.
The AFS/GRT system is just a glorified hobbs timer. They just run 1 hour per hour when above a certain RPM. This is not tach time as defined by Lycoming or Continental.
If you cruise at 2300 RPM, but Lycoming says the nominal is 2400, then for every 24 hours you fly, you accumulate one more tach hour on a GRT or AFS than you do on a Dynon.
Of course, it's much harder to count actual revolutions on an engine than it is to just run a timer whenever RPM is greater than X, but we felt that if we were going to call it tach time, it should work like tach time.
Also, as a note, in many planes, hobbs is hooked to oil pressure, not the master. This is so you can't turn off the master and fly around while accumulating no hobbs time. In the Dynon EMS, hobbs runs whenever oil pressure is above 15 PSI. It does not accumulate just because the EMS is on.
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05-13-2009, 10:41 AM
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Join Date: Mar 2008
Location: San Diego
Posts: 457
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With E.I. instrumentation...
Another data point... We have Electronics International instruments for tach and Hobbs time in our RV-6. According to the manuals for these:
The E.I. R-1 tachometer "keeps a running total of time the engine is above 1300 RPM." Above that threshold it is evidently recording real clock time, not RPM-proportional time.
The E.I. SC-5 clock records the "total time the engine has been running." It decides the engine is running if the aircraft power bus is above 13 volts, which will normally be the case only if the alternator is spinning.
So, these are not doing exactly the same thing as a good old fashioned mechanical tach and oil-pressure Hobbs.
And we have 622.13 SC-5 time, 476.9 R-1 time. So tach is 77% of Hobbs, with this setup and our mix of flying.
--Paul
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05-13-2009, 12:08 PM
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Join Date: Jan 2005
Location: 57AZ - NW Tucson area
Posts: 10,011
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I'm not sure that is the standard.
Quote:
Originally Posted by JDanno
Hobbs meter is reading when the key is turned on until when it is turned off. this is not dependent on the engine running. That is why you want to be especially vigilant about turning off the master electrical switch when flying rental planes as they all use the Hobbs time for charging you for the rental. This is really easy to forget when the engine is turned off via fuel lever (which it should be). The Hobbs will keep running even tho the engine is not turning . Ask me how I know! 
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The Hobbs meter is usually connected directly to the battery (bypassing the master switch) and is grounded (ie, switched on) by a pressure switch connected to the engine oil pressure.
This shows a certified 1977 era plane - the hour meter (aka Hobbs) is 2/3 down on the right side -
http://www.aucountry.com/ACA_Folder/...Scan_76-77.jpg
The power to the Hobbs is a fuse under the cowl - not a breaker or fuse that is pilot accessible - so that a student can't reduce his/her bill by pulling a breaker...
I think the plane you are referring to might have been modified by an FBO/owner to get even more money... 
__________________
Gil Alexander
EAA Technical Counselor, Airframe Mechanic
Half completed RV-10 QB purchased
RV-6A N61GX - finally flying
Grumman Tiger N12GA - flying
La Cholla Airpark (57AZ) Tucson AZ
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05-13-2009, 01:11 PM
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Join Date: Apr 2008
Location: Tennessee
Posts: 440
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Hobbs
Well, I guess I'll have to recheck that. the owner is a "good friend". I forgot the master switch and it ran for about 1 1/2 hours extra.  Maybe he modified it?!
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05-13-2009, 01:34 PM
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Join Date: Dec 2006
Location: Atlanta, GA
Posts: 1,231
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Quote:
Originally Posted by JDanno
Well, I guess I'll have to recheck that. the owner is a "good friend". I forgot the master switch and it ran for about 1 1/2 hours extra.  Maybe he modified it?!
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Or the pressure switch could be broken. However, in the distant past I have heard rumors of flight schools that have bypassed the oil pressure switch...
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05-13-2009, 05:17 PM
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Join Date: Jan 2005
Location: 57AZ - NW Tucson area
Posts: 10,011
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Probably not...
Quote:
Originally Posted by breister
Or the pressure switch could be broken. However, in the distant past I have heard rumors of flight schools that have bypassed the oil pressure switch...
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...since the "standard circuit" does not go through the master switch, it is driven directly off the battery. See schematic link above.
I'm not sure how standardized the circuit I mentioned is - does anyone have any different circuits from certified planes?
__________________
Gil Alexander
EAA Technical Counselor, Airframe Mechanic
Half completed RV-10 QB purchased
RV-6A N61GX - finally flying
Grumman Tiger N12GA - flying
La Cholla Airpark (57AZ) Tucson AZ
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05-13-2009, 05:24 PM
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Join Date: Jan 2009
Location: Gilbert, AZ (and missing TX)
Posts: 54
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I have no idea if it's 'standard' but on the rentals I was using the hobbs was ticking anytime the master was on. This was a 1968 Cherokee 180
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05-13-2009, 07:14 PM
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Join Date: Jan 2005
Location: Marion IA
Posts: 1,095
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Quote:
Originally Posted by az_gila
I'm not sure how standardized the circuit I mentioned is - does anyone have any different circuits from certified planes?
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I checked my 1980 Beech Skipper manual - the circuit is as shown above. There is a 5A cartridge fuse in the engine compartment (easily reachable through the oil door) that powers both the Hobbs and the clock.
__________________
Dave Gribble VAF #232
Building RV-9A N149DG (slider, IO-320, IFR)
Restored and Flying Beech Super III N3698Q
Marion IA
Struggling with fiberglass
There is no sport equal to that which aviators enjoy while being carried through the air on great white wings." Wilbur Wright, 1905
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05-13-2009, 08:14 PM
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Join Date: Feb 2005
Location: Bay Pines, FL (based @ KCLW)
Posts: 1,955
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My hobbs is significantly larger than my tach time.
My TT is set for 1500rpm.
It is indicative of the way I fly. I fly alot of very short flights(10-15 mins) but my hobbs would record .4 to include the taxi.
(.2 TT : .4 hobbs)
Current numbers.
Hobbs: 332
TT: 228
__________________
Danny "RoadRunner" Landry
Morphed RV7(formally 7A), N20DL, PnP Pilot
1190+ hours
2019 Donation Paid
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