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?

Is the tach like the speedometer in a car? Flexible shaft turns an
aluminum cup to run the needle, and a worm gear run by the shaft
counts up the "hours"?

If so, let's say you set the tach for 2500 rpm and mark the time.
Ignore the gear reduction between the crankshaft and the tach. After
the crankshaft has gone around 150,000 times, it is supposed to be one
hour later.

But your tach is telling you that it's actually 1.18 (4.6/3.9) hours
later. Seems to me, that's the same as telling you that your
crankshaft has made 177,000 revolutions. Eg. that your actual rpm was
2950.

That's unlikely on an Archer, but an error in rpm is something you
could check easily with a strobe and it probably ought to be your next
step.

Don