3.3 hours passed on hobbs, 0 hours on tach

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Connor Shanks

Filing Flight Plan
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Connor Shanks
A plane that I fly for aerial surveying recently had new camera equipment installed, and to test the equipment the engine was running while on the ground. There was testing going on for about 2 weeks. Afterwards, the tach time still lined up with the ending time from the previous flight, but the hobbs time had gone up 3.3 hours. I understand that tach time is based on the rpm of the engine, but I don't get how it didn't tick at all because the engine was running. Does tach time not tick at all if just idling on the ground?
 
Connor Shanks said:
Afterwards, the tach time still lined up with the ending time from the previous flight, but the hobbs time had gone up 3.3 hours.
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How is the hobbs wired in the system? If it doesn't have some kind of operational pressure switch (or its inop) it will record as long as the master switch is off. As to the tach not changing on the ground with engines running it can be a number of things such as internals worn out/weak or off calibration. Keep in mind a tach is not a "time-piece" but only an RPM counter calibrated to show "one hour" at a specific RPM.
 
Bell206 said:
As to the tach not changing on the ground with engines running it can be a number of things such as internals worn out/weak or off calibration. Keep in mind a tach is not a "time-piece" but only an RPM counter calibrated to show "one hour" at a specific RPM.
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Is it possible for the tach to not work at say 1000 rpm or less but still function correctly at it's calibrated "one hour" rpm setting?
 
Bell206 said:
How is the hobbs wired in the system?
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I'm not sure how the hobbs is wired, but I don't think the hobbs is the issue. I'm mainly concerned about how the tach functions on the ground.
 
Was it accurate on its next flight?
 
Connor Shanks said:
I'm not sure how the hobbs is wired, but I don't think the hobbs is the issue. I'm mainly concerned about how the tach functions on the ground.
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Well the Hobbs does matter because you're assuming the engine ran for 3.3 hours. Maybe it was only 15 minutes and the rest of the time was the master on and the engine off, in which case the tach would probably not register.
 
dmspilot said:
Well the Hobbs does matter because you're assuming the engine ran for 3.3 hours. Maybe it was only 15 minutes and the rest of the time was the master on and the engine off, in which case the tach would probably not register.
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Oh I see. I was not the one running the tests, but the person running them said the engine was running a minimum of 1.5 hobbs (this was about a month ago so memory fades). If the case be that he did leave the master on for some time, 1.5 on the hobbs should still yield some time on the tach correct?
 
Connor Shanks said:
Is it possible for the tach to not work at say 1000 rpm or less but still function correctly at it's calibrated "one hour" rpm setting?
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Sure. Especially if it's seen its better days. But without checking it against a known reference couldn't tell you it was accurate or still calibrated.
Connor Shanks said:
I'm not sure how the hobbs is wired, but I don't think the hobbs is the issue.
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Well if you use the hobbs for mx it is costing you major bucks on costs if direct off battery or the pressure switch is inop.
Connor Shanks said:
1.5 on the hobbs should still yield some time on the tach correct?
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It should, but you may be only be looking .7 on the tach depending on tach type/calibration. Regardless, if it is a true 3.3 hours discrepancy between the hobbs and tach with 1.5 hobbs time of engine running then it should be wrote up and looked at.
 
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No. Most rental airplanes the Hobbs is on the master switch. It could easily have accumulated 3 hours with the engine not running at all.
 
Connor Shanks said:
A plane that I fly for aerial surveying recently had new camera equipment installed, and to test the equipment the engine was running while on the ground. There was testing going on for about 2 weeks. Afterwards, the tach time still lined up with the ending time from the previous flight, but the hobbs time had gone up 3.3 hours. I understand that tach time is based on the rpm of the engine, but I don't get how it didn't tick at all because the engine was running. Does tach time not tick at all if just idling on the ground?
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My thought is it ran on ground power for 3.3 hours and the motor ran only a little bit or not at all.
The ground power powered the hobbs? I bet they didn’t run the motors at all.
 
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Is this a mechanical tach, or an electronic one? The mechanical type should have recorded something even if at low idle the whole time.

If an electronic tach, some/most of them don’t record anything below a certain rpm. 1300 sticks in my mind for some popular model for some reason. If all the testing was at idle, then that type of tach wouldn’t record anything.
 
RussR said:
f an electronic tach, some/most of them don’t record anything below a certain rpm.
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Interesting. I did not know that. Then again I never messed with an electronic tach. Are all electronic tachs like this with a minimum RPM cut-off on the recording side?
 
RussR said:
If an electronic tach, some/most of them don’t record anything below a certain rpm. 1300 sticks in my mind for some popular model for some reason.
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EI CGR-30P, if memory serves. I don't recall what JPI uses.
 
Bell206 said:
Interesting. I did not know that. Then again I never messed with an electronic tach. Are all electronic tachs like this with a minimum RPM cut-off on the recording side?
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asicer said:
EI CGR-30P, if memory serves. I don't recall what JPI uses.
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I looked it up, yes the CGR-30P uses 1300 rpm. A client of mine had one, which is probably why I thought of it. From the manual:

upload_2021-11-4_21-33-54.png
If you think about it, it's probably about the same overall as a mechanical tach, when the plane is used in "normal" operations. A mechanical tach will count up faster the faster the engine is running, up to 1:1 with "real time" at some rpm like 2300 (depends on the model). The ratio is proportionate, meaning at half that rpm it records time at half speed - so 1150 in this case. So it's recording time all the time the engine is running, but very slowly when you're sitting there idling, and not "real time" until takeoff/cruise.

The electronic tach, on the other hand, will record "real time", but only above 1300 rpm. Meaning at 1000, it's not recording anything.

For normal flight, I bet the two work out pretty much the same. But in long-duration engine runs, probably not.
 
A Hobbs is just an electric timer. It runs whenever it has power. I've seen it wired up to all kinds of things from oil pressure switches, to gear squat switches, to cabin heaters, etc...
 
Connor Shanks said:
A plane that I fly for aerial surveying recently had new camera equipment installed, and to test the equipment the engine was running while on the ground. There was testing going on for about 2 weeks. Afterwards, the tach time still lined up with the ending time from the previous flight, but the hobbs time had gone up 3.3 hours. I understand that tach time is based on the rpm of the engine, but I don't get how it didn't tick at all because the engine was running. Does tach time not tick at all if just idling on the ground?
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My friend is a DOM at a aerial surveying company. I have helped do annuals at his shop on my friends Cherokee. I am told this is a million dollar camera.
IMG_8563.JPG

IMG_0504.JPG

I got to ride along during checkout, my buddy with the Cherokee is a check airmen.
IMG_0506.JPG
 
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RussR said:
The electronic tach, on the other hand, will record "real time", but only above 1300 rpm. Meaning at 1000, it's not recording anything.
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The electronic tach will definitely give you a more accurate time-in-service record considering it doesnt record below 1300 and most aircraft can't get off the ground at that RPM either. But I wouldnt use it for billing or pilot time as you'll short yourself in most cases. Do you know if JPIs and other engine monitors cut off time in a similar manner?
 
Post # 15. Probably spent 3 hours on aux power in the shop during install.
 
flyingron said:
A Hobbs is just an electric timer. It runs whenever it has power. I've seen it wired up to all kinds of things from oil pressure switches, to gear squat switches, to cabin heaters, etc...
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Add airspeed switch to your etc............Our Mooney 201 had one, and it was flimsy. I think it broke when hit by a wash brush.
 
Connor Shanks said:
Is it possible for the tach to not work at say 1000 rpm or less but still function correctly at it's calibrated "one hour" rpm setting?
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Possible but highly unlikely. Inside the tach there is a worm gear driven by the cable, and a pinion gear driven by the worm. If these gears wear excessively they will not mesh and the counter will stop. Frequently the worn gears will start jamming and break the cable. There is no reason for this to be speed dependent.
 
kgruber said:
Add airspeed switch to your etc............Our Mooney 201 had one, and it was flimsy. I think it broke when hit by a wash brush.
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This is how mine is wired in the Seneca. I think it's calibrated to 80 knots or so.
 
flyingbrit said:
Possible but highly unlikely. Inside the tach there is a worm gear driven by the cable, and a pinion gear driven by the worm.
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FYI: you'll find in most aircraft tachometers the pointer is not geared direct to the drive cable. Most have a coupler of some sort with the more common being a magnetic coupler or a flyweight set up. Over time the magnets/calibration springs get weak and no longer drive the drag cup connected to the pointer at lower RPMs. Same issue with the older flyweights not pushing on the pointer paddle. There has been several documents issued over the years addressing the loss of accuracy and function of OEM tachs to the point where it could cause certain engine/prop failures.
flyingbrit said:
There is no reason for this to be speed dependent.
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If talking about the recording side of the tach it is directly related to speed and tachs are identified by specific RPMs that will result in the indication of one hour at a specific RPM. Any operation outside of that stated/calibrated RPM will result in more or less time recorded on the tach.
 
I did not explain it well. You are of course correct that the rpm pointer is coupled magnetically and will read lower as the magnet gets weaker with age.
However the "hour" counter is coupled through the gears and not the magnetic coupling. Therefore it is not affected by magnet aging. As long as the gears still mesh properly, the counter will work as it always did regardless of engine rpm. Note that the counter is not really counting hours but actually is counting revolutions, which are converted to hours (at a particular speed) by the mechanism.
Yes, the "hours'" will not read accurately at idling rpm. My point was that the "hours" should not fail to advance at all due to the gear driving. The OP observed that the "hours" did not change at all even though the engine was running. There is no logical explanation for this.
 
flyingbrit said:
There is no logical explanation for this.
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As you stated worn gears drives/mounts on the counter mechanism plus the calibration ratio. But if the engine was ran for 1.5 hobbs hours even at idle there should have been at least a .1 difference unless as noted above its an electronic tach or the mechanical internals are fubar.
 
When I was renting out my Cessna 152, I noticed that Tach readings were about 80 percent lower than the Hobbs. As an owner, I always thought that this was a sweet deal: Maintenance and 100-hour inspections based on Tach and revenue on Hobbs.
 
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