Hour Meter

xlr82v2 · May 1, 2008

"Hobbs" time is by the actual clock... it's an electric hourmeter, either powered off of the ignition switch, or an oil pressure switch.

Tach time is based off of a certain number of engine revolutions. At "rated" rpm, .1 hour is 6 minutes, 1 hour is 60 minutes. If you run the engine faster than rated speed, then 1 hour on the hourmeter will be somewhat less than 1 hour "clock" time. And, by the same token, if you are running the engine slower than rated rpm, then 1 hour tach time will be somewhat longer than 1 hour "clock" time.

Back when I was learning to fly, the Cessna 152's had both Hobbs meters, and hourmeters on the tachometer as well. We learned that if you kept the idle speed low enough during taxi, that the hobbs meter wouldn't run (the hobbs meter was powered by an oil pressure switch). So, at the beginning of taxi, you would build up as much speed as you safely could, then pull back to slow idle for as long as possible, to try to prevent the Hobbs meter from running...who wants to pay for taxi time? You paid for the airplane according to Hobbs time, but all of the maintenance on the airplanes was done according to tach time... Tach time was almost always .2 hours less than Hobbs time. Pretty good racket, huh?!

yekrut · May 1, 2008

Interesting stuff. I have an L39 and was wondering about this not to long ago. Sometimes it appeared I was on the tractor longer than the meter suggested. I wonder if the L39 is a tach meter like the B26?

N80 · May 1, 2008

My tractor is definitely tach time. I can do a lot of low rpm type work and it hardly moves. This seems odd to me. I can beat the machine pretty hard, and strain the engine a lot at under 2000 rpm. So it seems to me that the hour meter does not provide a good indication of what the tractor has been doing. But, I don't guess real clock hours would either.

KeithInSpace · May 1, 2008

N80 said:
...does not provide a good indication of what the tractor has been doing.

I think the way to think about this is that oil doesn't break down by itself. It needs heat and friction. Each time a combustion event takes place, a little bit of the oil is fouled. Less combusion events mean less oil is fouled, no matter how hard the engine is 'pushed'. Similarly, 1,000 RPMs generates significantly less friction and heat than 3,000 RPMs, further increasing the (relative) life of the oil.

Just as similarly, running the tractor at an RPM HIGHER than the magical 540 PTO RPM would put more stress on the oil, thereby hastening the hour meter's passage of "time".

xlr82v2 · May 1, 2008

N80 said:
My tractor is definitely tach time. I can do a lot of low rpm type work and it hardly moves. This seems odd to me. I can beat the machine pretty hard, and strain the engine a lot at under 2000 rpm. So it seems to me that the hour meter does not provide a good indication of what the tractor has been doing. But, I don't guess real clock hours would either.

Nope, tach time is just an indication of how many times the crankshaft has spun around. Hobbs time is just an indication of how long either the engine has been running, or how long the key has been turned on.

If I had to pick between the 2, I'd say that tach time is an overall better indicator of when the tractor needs to be serviced. But neither of the 2 types gives ANY indication of how hard/easy the tractor has been worked... just "hours".