The tach is GEARED to the engine crankshaft. At some nominal RPM the
tack will read one hour in one clock hour. If you run the engine at
an excessive RPM then the tach will run fast. If you run at 60% power
with a fixed pitch prop then the tach should read slower than a
clock.
The hobb's is a clock that is started and stopped by a engine oil
pressure switch on most aircraft.

Tach hours are really the number of crankshaft revolutions times a
fixed scale factor which is the gear ratios from the crankshaft to the
counter wheels.

It sounds like you are running full throttle all the way since my tach
always reads slower than the hobbs but then I have to pay for the
maintenance so I take care of the engine.



On Sun, 21 Dec 2003 02:04:05 GMT, John Roncallo
wrote:

Hello

Im trying to figure out the Tach on our clubs Archer is screwing us. I
just flew down to the Centenial of Flight FFA from Planville CT. Round
trip with all diversions and ATC rerouts was 975nm. The tach said 12.4
hr. I never checked the Hobbs but I do know that for this plane the Tach
usually runs faster than the hobbs on cross countries. Is this normal. I
also did another flight previously at 65% power from Meriden MMK to
Williamsburg W94 in 4.6 Tach time. My watch said 3.9. Also based on the
4.6 my fuel burn was only 8 GPH.

Is there any text book way to check this. Our FBO seems to feel that
calibrating the Tach is a big deal. I'm currently thinking of just
replacing the Tach without tring to calibrate. Right now it looks to me
as if we just replaced and engine at 1700 hr thinking it had 2000 hr.

Thoughts ideas?