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Tach Time vs Hobbs - Old School

HFS

Well Known Member
In the old days, before the advent of all of the electronic information gathering devices we have today, we relied on mechanical "trickery" to help slow down the hands of time. Most of us used the rise of oil pressure on startup to start the Hobbs - nothing new there.

But, on the mechanical tach/hour meter side, you could get tachs that logged hours at two different rates. For a fixed pitch propeller, 2500 rpm was considered the norm for cruise; and, for a CS propelle,r 2300 rpm was the accepted standard.

Now, if you wanted to "fudge" a little bit on the rate at which your tach hours were logged; and, ran your engine at a lower rpm in cruise, you got the benefit of slower hours logged - not much, but some.

On my (ancient) Rocket I have the 2500 rpm tach/hour meter, but only run the engine in cruise at 2200 - effectively slowing down the accumulation of hours by about 12%. Sleight of hand, maybe, but - oh well.

YMMV - and I am sure it does.

I see Mitchell still makes mechanical tachs that actually go up to 3000 rpm cruise - even more deception!

HFS
HR II S/N 002
 
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In the old days, before the advent of all of the electronic information gathering devices we have today, we relied on mechanical "trickery" to help slow down the hands of time.

The EFISes do the same thing...only can set the cruise RPM in the setup menus.

Cruise RPM is used when calculating tach time. Tach time is a measure of engine time normalized to a cruise RPM. If you spend one hour at your cruise RPM, tach time will increment one hour. If you spend 1 hour at 1/2 your cruise RPM, tach time will only increase by 1/2 hour. Tach time is defined as TIME x (CURRENT RPM / CRUISE RPM).
Hobbs time is a simple timer that runs whenever the oil pressure is above 15 PSI or the engine is above 0 RPM.
- Dynon SV installation manual
 
YTMV (Your Times May Vary)

I wouldn't describe it as "trickery", or "deception" - but there certainly is "fudge" involved. The FAA defines Flight Time as beginning when the aircraft first moves for the purpose of flight, "OUT of the chocks". Flight
time ends when back IN the chocks. Hobbs does a good job of capturing this in most instances. One exception would be where an instructor gets paid by Hobbs time, and does ground school in the airplane parked with the engine running...
Time In Service is the FAA term for defining the time accumulated on the aircraft for maintenance purposes, and begins when the aircraft takes to the air and terminates when the aircraft is back on the ground. Since taxi operations occur at low RPM (normally), the Tach Time approximates Time In Service closely enough to satisfy the FAA.
Two different times, for two different purposes.
To be most accurate, and eliminate the "fudge", one would need to log the OUT, OFF, ON, and IN times - as used to be done in large fleet operations. (maybe the airlines still do, was a long time ago for me)
(Sorry if this sounds like me preaching, not my intent)
My RV-6, for example, only turns 2400RPM WOT, so cruise is 2100 or 2200 RPM. Tach Time shows a bit over 800, while the Hobbs is over 1100. Both are correct.
 

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To be most accurate, and eliminate the "fudge", one would need to log the OUT, OFF, ON, and IN times - as used to be done in large fleet operations. (maybe the airlines still do, was a long time ago for me)

Still do at my job. We call it "start taxi", "up", down" and "engines off" and have to write it down manually. Most airlines track all this remotely through electronic means.
 
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