Tach Hour Meter

Interesting, the last trip the tach registered more time than the hobbs
 
When the engine is not running and the keys are in your pocket.
 
Did they teach that technical terminology in A&P school Duncan?
 
tmyers said:
So your Hobbs and tach always read the same?
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I haven't done a careful check (there is an offset because the tach was replaced) but they should be pretty close. There will be small differences because the hobbs runs off the key and the tach starts counting when the engine starts. The hobbs used to run right off the master so it would tick away when the master was on for things like a transponder check. :(
 
tmyers said:
At what rpm is does a tach hour meter record 1 to 1?
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At whatever cruise rpm it's calibrated for which should match whatever is recommended for the specific engine/aircraft it's being used on.

unless it's gone all cattywampus or something like that.
 
The Type Certificate Data Sheet (TCDS) or the maintenance manual should list the equivalence rpm for the plane. It can be quite different between various manufacturers and various models, but those planes with a common TCDS should be all the same.

-Skip
 
The only thing I've ever seen listed on a TCDS is static rpm.
 
Cattywampus is a tried and true technical term. Wonky just doesn't sound right.

My normal cruise is 24 square and redline is 2700. My Hobbs runs off of oil pressure, wonder if the switch is not staying engaged.
 
Capt. Geoffrey Thorpe said:
the hobbs runs off the key and the tach starts counting when the engine starts. The hobbs used to run right off the master so it would tick away when the master was on for things like a transponder check. :(
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Your tach runs at "normal speed" continuously when the engine is running? My experience is that they are proportional to power. Certainly if I spent two hours on the hobbs doing pattern work (especially full stop landings) I'd expect to see far less than two hours on the tach time.
 
dukeblue219 said:
Your tach runs at "normal speed" continuously when the engine is running? My experience is that they are proportional to power. Certainly if I spent two hours on the hobbs doing pattern work (especially full stop landings) I'd expect to see far less than two hours on the tach time.
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AFaIK, all mechanical tachs (including electric tachs that run from a tach generator) accumulate time at a rate proportional to RPM (e.g. twice as fast at 2000 RPM vs 1000 RPM) and all have a specific RPM where "tach time" runs like a Swiss watch (i.e. 1hr on the tach takes one our of "real" time. That RPM varies on different airplane models and if the RPM reading is in error, the "clock time" indicated RPM will be off as well (the time is proportional to actual engine RPM not indicated RPM).

Electronic tachs (e.g. Horizon, G1000, Auracle, EDM 930, etc) operate differently. Most accumulate "clock time" (i.e. 1hr tach = 1hr on the clock) above some RPM and don't accumulate at all below that RPM.
 
Gismo is right. It all depends on what you want to use the time for. The "TIME IN SERVICE" for maintenance is technically the time the aircraft is in the air. All of these things based on engine RPMs are going to be approximations, but the FAA says they don't care as long as you're consistent.

I don't even have "tach time" in my plane. The tach(s) have no hour meters. There is a Hobbs which is connect to the landing gear. This is probably as close to the FAA defintion of TIS as you're going to get.
 
I think this might be creeping a bit... I don't think Tim is really debating what to use for tracking and maintenance purposes. See post #4: Interesting, the last trip the tach registered more time than the hobbs.
This actually happened to me the last time I flew this plane. Usually the hobb is more than the tach (as is mostly the case.) But it just happened on this trip the tach was "significantly" higher than the hobbs, actually almost .5 higher than I really flew.
 
I only time I've had that happen was when the hobbs meter actually broke. Yes, it was on a rental airplane. Yes, I bought lottery tickets that night. :D

For the tach to read more than the hobbs, you'd have to be running at a higher rpm than the 1:1 ratio. So if the 1:1 was 2300, and you ran at 2500 for long enough, the tach could be higher. The problem is, though, all that taxiing and descending at low rpm causes the tach to read really slow. I suppose if your hangar was right at the end of the runway, and you did no runup, and took off and flew at 2700 rpm, the tach could be more than the hobbs, but it's not usually the case in "normal" flying.

Also consider that when started, the hobbs may have just ticked over to the next number on the last flight, where the tach might have been just seconds away from ticking over. That could make a small difference I suppose.
 
Interestingly enough one of the items replaced at last Annual was the pressure switch for the HOBBS meter. The darn thing cost $200 + install, so it should be a gold plated one. We will have to watch it over the next few trips to see what it is doing.

Appreciate the input we will get it figured out.
 
Weather permitting it will be flying this weekend I will note the relative times.
 
Infant mortality of the Hobbs switch? Connections not happy having been disturbed?

Just spit balling, seems you have some sharp A&Ps ;)
 
dukeblue219 said:
Your tach runs at "normal speed" continuously when the engine is running? My experience is that they are proportional to power. Certainly if I spent two hours on the hobbs doing pattern work (especially full stop landings) I'd expect to see far less than two hours on the tach time.
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Yes.

Obviously, you have a different tach than I do. Probably a mechanical tach - those will wind up the time as a function of engine revolutions.
 
My Cherokee was about 2375 rpm and the hobbs/engine meter are are 1:1.

It could be different from someone who louder s on the ground a bit longer and long taxis and wait times at big airports from others who get in, taxi to run up and take off.





tmyers said:
Cattywampus is a tried and true technical term. Wonky just doesn't sound right.

My normal cruise is 24 square and redline is 2700. My Hobbs runs off of oil pressure, wonder if the switch is not staying engaged.
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You can usually trace a issue back to the last item touched to discover the real problem. I checked the times from the last two months and in all but the last flight the hobbs was more than the tach. I will check the hobbs against a timer on the next couple flights since it is based on oil pressure it shouldn't be that hard to verify, if my pea brain can remember to do it.
I do trust the A&P that did the work, just wasn't that happy with their billing practices. I feel I have a couple A&P's now that have my back and that is a good feeling, plus those I have met here that make good sounding boards for me.
 
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