joegoersch Posted October 3, 2012 Report Share Posted October 3, 2012 I thought the Hobbs meter in my M20J was running slow. If I were to start my engine and then shut it down an hour and a half later, I thought the Hobbs should read 1.5 hours more at engine shut down than it did at start up. But this doesn't seem to be the case, it seems to indicate more like 1.2 hours or so. All the planes in which I trained (152's, 172's, Archers) had Hobbs meters which seemed to indicate clock time from the time the master was turned on (or maybe oil pressure not at zero) until shut down. My mechanic said in Mooneys, it's a little more complicated. Does anyone have a good understanding about how the Hobbs meter is wired in a 201 ? What turns it on/off ? Should it match clock time ? Thanks !!! Quote Link to comment Share on other sites More sharing options...
jetdriven Posted October 3, 2012 Report Share Posted October 3, 2012 It should ony run when there is electrical power applied to it (usually the batt bus) and the oil pressure switch closes (when it has oil pressure). Rental outfits that run the hobbs straight off the master switch IMO are stealing from their customers. That can usually be alleviated by turning off the master switch in flight a few times. The get the idea, and wire it the way it is supposed to be. Quote Link to comment Share on other sites More sharing options...
Parker_Woodruff Posted October 3, 2012 Report Share Posted October 3, 2012 What model year M20J? Is it factory original? Do you also have tach time available? Some are designed to tick on only above a certain RPM so it's more indicative of flight time, and Mooney may have installed it that way, depending on your model year (especially likely if you don't have a tach time). Quote Link to comment Share on other sites More sharing options...
joegoersch Posted October 3, 2012 Author Report Share Posted October 3, 2012 What model year M20J? Is it factory original? 1989, Original Quote Link to comment Share on other sites More sharing options...
jetdriven Posted October 3, 2012 Report Share Posted October 3, 2012 Parker, Bill Eldred from Mooney mentioned that it might not record hours below a certain RPM on the M20Js without a recording tach meter. Perhaps that is what is happening here. Joe, do you have a recording tachometer? Quote Link to comment Share on other sites More sharing options...
joegoersch Posted October 3, 2012 Author Report Share Posted October 3, 2012 Parker, Bill Eldred from Mooney mentioned that it might not record hours below a certain RPM on the M20Js without a recording tach meter. Perhaps that is what is happening here. Joe, do you have a recording tachometer? I have a tach, but it does not keep track of total tach hours (is that what a "recording tach" is ?). All the maintenance (oil change, ADs etc.) are therefore based on Hobbs time. I believe this is standard for M20J's , but I'm not sure. "it might not record hours below a certain RPM on the M20Js without a recording tach"--so presumably if Im taxiing and/or waiting for clearance, the Hobbs isn't recording ? Anyone know at what RPM they are set to record ? Quote Link to comment Share on other sites More sharing options...
Hank Posted October 3, 2012 Report Share Posted October 3, 2012 My tach records hours at the bottom; it never matches my watch but comes close on long-ish XC when I fly at 2500 and don't have to shoot an approach. Haven't seen a Hobbs since I last rented. Maintenance goes by the numbers on the tach, and I log flight time by my watch. Therefore I can conclude that the tach is set to record something along the lines of (RPM/2500), which shows up as tenths of an hour as the numbers roll up and around. But my plane predates most J's by a decade or more. Example: Gave rides on Saturday, flew ~4.5 hours total by my watch while the tach advanced by 2.5; not counting takeoff and climb to 1200-1400 agl, I flew at 15-17"/2300 RPM to avoid running over the Cessnas in front of me. Then flew 2.3 hours out and 2.0 hours back at 2500, and the difference between watch and tach was on the order of 0.2 hours [presumably taxi, runup, pattern, landing & taxi] since I descend Power-On. Quote Link to comment Share on other sites More sharing options...
Parker_Woodruff Posted October 3, 2012 Report Share Posted October 3, 2012 Parker, Bill Eldred from Mooney mentioned that it might not record hours below a certain RPM on the M20Js without a recording tach meter. Perhaps that is what is happening here. Joe, do you have a recording tachometer? That's what I'm guessing as well. My M20K doesn't record below a certain number, either. My tach does not have a time attached to it directly; it's tied into the hobbs meter. My M20K is within a couple years, so I figure that could be the issue here. My M20J was a 1990 AT variant, and it had a tach that recorded time...only it had been replaced about 2 or 3 times over its 6900 hours prior to my purchase. Quote Link to comment Share on other sites More sharing options...
kortopates Posted October 3, 2012 Report Share Posted October 3, 2012 The Mooney Hobbs meter (unless it’s been replaced) is measuring total tach time unlike the Hobbs meter installed on rental aircraft that measures elapsed time triggered by the oil pressure sensor/switch - thus total elapsed time the engine is operating regardless of power setting. Our Mooney Hobbs meter though is not a clock but designed to most accurately measure time when RPM is around cruise and is much less accurate at lower RPM settings. Thus when idling, the Hobbs is recording time at rate of approx about 40% of what it would in cruise (1000 rpm/2500 rpm). It may also be true that they have a minimum RPM but I have never seen a reference that states that. Regardless though, my Hobbs time always pretty much exactly come out to be .2 hrs less than actual clock time for just about any normal flight, short or long, because of the time to taxi to and from the runway from the hangar, do the run up etc. If the flight is a training one with many take-off/landings w/ taxi-backs then the difference between Hobbs time and engine running time can be much greater than 0.2. As a result, the Hobbs meter, measuring tach time like our Mooneys, is purely for measuring Time in Service for maintenance purposes. Thats why back in your student pilot/rental days you often recorded both a tach time for maintenance and Hobbs time based on the oil pressure to be billed. You were paying that extra .2 or more time per flight than what the tach time saw. (I've heard mention of Hobbs time based entirely on the master switch but never seen one for real and wonder if there is any real truth to that - if so, they're being robbed!) We have 3 ways of measuring “Hobbs” time on my aircraft as do most of you. We use the Mooney Hobbs time to measure Time in Service – not our pilot log time. We then use one of GNS-430, to measure engine time, essentially identical to the oil pressure triggered Hobbs, which is what we log as pilot time (our #2 GNS-430W timer measures time from startup, which comes on seconds after engine startup). Our other GNS-430W timer measures Air-Hobbs time with a timer set to start at lift off (triggered with GS>30kts), that way we get actual air borne time just as most advanced aircraft record airborne time based on a gear squat switch. Quote Link to comment Share on other sites More sharing options...
joegoersch Posted October 3, 2012 Author Report Share Posted October 3, 2012 The Mooney Hobbs meter (unless it’s been replaced) is measuring total tach time unlike the Hobbs meter installed on rental aircraft that measures elapsed time triggered by the oil pressure sensor/switch - thus total elapsed time the engine is operating regardless of power setting. Our Mooney Hobbs meter though is not a clock but designed to most accurately measure time when RPM is around cruise and is much less accurate at lower RPM settings. Thus when idling, the Hobbs is recording time at rate of approx about 40% of what it would in cruise (1000 rpm/2500 rpm). It may also be true that they have a minimum RPM but I have never seen a reference that states that. Regardless though, my Hobbs time always pretty much exactly come out to be .2 hrs less than actual clock time for just about any normal flight, short or long, because of the time to taxi to and from the runway from the hangar, do the run up etc. If the flight is a training one with many take-off/landings w/ taxi-backs then the difference between Hobbs time and engine running time can be much greater than 0.2. As a result, the Hobbs meter, measuring tach time like our Mooneys, is purely for measuring Time in Service for maintenance purposes. Thats why back in your student pilot/rental days you often recorded both a tach time for maintenance and Hobbs time based on the oil pressure to be billed. You were paying that extra .2 or more time per flight than what the tach time saw. (I've heard mention of Hobbs time based entirely on the master switch but never seen one for real and wonder if there is any real truth to that - if so, they're being robbed!) We have 3 ways of measuring “Hobbs” time on my aircraft as do most of you. We use the Mooney Hobbs time to measure Time in Service – not our pilot log time. We then use one of GNS-430, to measure engine time, essentially identical to the oil pressure triggered Hobbs, which is what we log as pilot time (our #2 GNS-430W timer measures time from startup, which comes on seconds after engine startup). Our other GNS-430W timer measures Air-Hobbs time with a timer set to start at lift off (triggered with GS>30kts), that way we get actual air borne time just as most advanced aircraft record airborne time based on a gear squat switch. That sounds right. That's consistent with my Hobbs vs actual time. Gee, I thought I knew how a Hobbs meter works--was I wrong ! Thank you for educating me Quote Link to comment Share on other sites More sharing options...
Jeff_S Posted October 4, 2012 Report Share Posted October 4, 2012 Here's an interesting one for you. My aircraft rolled off the line as a 201-AT model, for Advanced Trainer. It went to Flight Safety International and was used as a trainer for 1000 hours before being put to use as personal transportation for one of the CFIs. Although it doesn't work anymore, it has an air-driven Hobbs meter, with a small vane on the belly that would only activate the electrical circuit at a certain air speed. I'd never seen one like that before. Clearly that was used purely to measure flying time for instructional purposes, and is not directly related to engine operations or maintenance. Quote Link to comment Share on other sites More sharing options...
danb35 Posted October 4, 2012 Report Share Posted October 4, 2012 Our Mooney Hobbs meter though is not a clock but designed to most accurately measure time when RPM is around cruise and is much less accurate at lower RPM settings. Thus when idling, the Hobbs is recording time at rate of approx about 40% of what it would in cruise (1000 rpm/2500 rpm). Are you entirely sure of this? I ask because it is completely inconsistent with what I know of how a Hobbs meter works. As I understand it, a Hobbs is a simple hour meter. All it does is record how long it's been turned on to the nearest tenth of an hour, and it has no way of knowing how fast the engine is turning. What you describe is how a mechanical recording tach works (electronic tachs generally record time 1:1 above a certain threshold engine speed). Of course, the Hobbs can be activated by anything. The most common is probably an oil pressure switch, but it could run directly from the master bus, or the avionics bus, or a squat switch, or an airspeed switch. I've seen them installed in combustion heaters and electric standby vacuum systems, obviously to measure the operating time of those devices. But in any case, it will measure 1:1--60 minutes of the Hobbs being activated will record 1.0 hours. The only variable is how it is activated. Quote Link to comment Share on other sites More sharing options...
joegoersch Posted October 4, 2012 Author Report Share Posted October 4, 2012 Are you entirely sure of this? I ask because it is completely inconsistent with what I know of how a Hobbs meter works. As I understand it, a Hobbs is a simple hour meter. All it does is record how long it's been turned on to the nearest tenth of an hour, and it has no way of knowing how fast the engine is turning. What you describe is how a mechanical recording tach works (electronic tachs generally record time 1:1 above a certain threshold engine speed). Of course, the Hobbs can be activated by anything. The most common is probably an oil pressure switch, but it could run directly from the master bus, or the avionics bus, or a squat switch, or an airspeed switch. I've seen them installed in combustion heaters and electric standby vacuum systems, obviously to measure the operating time of those devices. But in any case, it will measure 1:1--60 minutes of the Hobbs being activated will record 1.0 hours. The only variable is how it is activated. I did (before I posted my question) talk to my mechanic about "just" putting in a new Hobbs. It seemed to be cheaper than troubleshooting. I see Hobbs selling for $30. Two wires, two screws--shouldn't be more than 1/2 hour labor--or so I thought. My mechanic said, it's not that easy, more complicated wiring. I didn't pin him down, but it didn't seem like a simple two wire, on or off type of job. That's why the complicated 0,40%, 100% answer seemed to make sense. But I'm hoping a Mooney Mechanic will chime in !!! Quote Link to comment Share on other sites More sharing options...
kortopates Posted October 5, 2012 Report Share Posted October 5, 2012 Are you entirely sure of this? I ask because it is completely inconsistent with what I know of how a Hobbs meter works. As I understand it, a Hobbs is a simple hour meter. All it does is record how long it's been turned on to the nearest tenth of an hour, and it has no way of knowing how fast the engine is turning. What you describe is how a mechanical recording tach works (electronic tachs generally record time 1:1 above a certain threshold engine speed). Yes, you can confirm by looking at your schematic. My mooney "hours meter" is wired into the tach. Its an electric tach working off a Hall sensor. The tach is measuing magento pulse through the hall sensor. Also note it doesn't come close to measuring 1:1 until you're at cruise power. Its only accurate when the number of pulses its measuring are at the rate of cruise power RPM, taxing around, touch and go's etc. and whenever the RPM is way back, the meter is not seeing enough impulses to keep 1:1 and doesn't keep up with clock time. That's why flight schools install oil sensor based hobbs time so that they do get 1:1 time when the engine is running at any power level. But their should still be a tach based timer there too, either on the tach itself or as a separate hours meter connected to the tach.. But like you say, hobbs can run off anything. The standard "hours meter" in our Mooney's is running off the tach but I haven't looked at your schematic to know what you actually have but would expect similar.Can you check your schematic? Quote Link to comment Share on other sites More sharing options...
smccray Posted October 8, 2012 Report Share Posted October 8, 2012 It's my understanding that the Mooney hobbs meter came from the factory wired into an airspeed switch. When the airspeed exceeds a certain level, the hobbs meter starts running; when the airspeed is below that number, even if the engine is running, the hobbs meter is idle. The airspeed switch in my plane died years ago and it was replaced with an oil pressure switch. Quote Link to comment Share on other sites More sharing options...
joegoersch Posted October 22, 2012 Author Report Share Posted October 22, 2012 I found this in the M20J service manual-- Hour Meter (Optional). The hour meter operates operates from the electric tachometer. A Hobbs meter directly from the alternator "Aux." terminal through a may be installed as an option. fused wire. S/N 24-1418 and later models... so now I think the Hobbs just turns on at some certain RPM...but I'm not sure. Quote Link to comment Share on other sites More sharing options...
kortopates Posted October 22, 2012 Report Share Posted October 22, 2012 I found this in the M20J service manual-- Hour Meter (Optional). The hour meter operates operates from the electric tachometer. A Hobbs meter directly from the alternator "Aux." terminal through a may be installed as an option. fused wire. S/N 24-1418 and later models... so now I think the Hobbs just turns on at some certain RPM...but I'm not sure. What Mooney refers to as the Hour Meter is what we've been discussing as the Mooney Hobbs meter installed by the factory - as you site from your J service manual - it runs off the tachometer. The "Hobbs" meter they mention would be the option a flight school would want to install to get total elapsed time of engine operation - not likely anyone would have this, nor want it, unless their Mooney was in a flight school at some time. Quote Link to comment Share on other sites More sharing options...
joegoersch Posted November 2, 2012 Author Report Share Posted November 2, 2012 What Mooney refers to as the Hour Meter is what we've been discussing as the Mooney Hobbs meter installed by the factory - as you site from your J service manual - it runs off the tachometer. The "Hobbs" meter they mention would be the option a flight school would want to install to get total elapsed time of engine operation - not likely anyone would have this, nor want it, unless their Mooney was in a flight school at some time. O.K. Any idea how the Mooney Hour Meter works ? Does it run only when tach is above a certain level ? How is Hour Meter wired to tach ? Quote Link to comment Share on other sites More sharing options...
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