On Jul 13, 1:51*am, Ron Wanttaja wrote:
Dan wrote:
*Neither being a lawyer nor having seen one on TV I would wonder if
resetting an hour meter without putting an entry into the log would
equivalent a false entry.

I agree. *When I put the temporary tach in, I noted the reading of the
previous tach and the current reading of the new one. *Had an annual in
the interim, and the A&P computed the actual time based on the two tach
readings.

I know how many hours the temporary tach recorded, and plan on adding
that amount of time to the old tach before installation. *Appropriate
log entries will be made, but I'd like to get back to reading the
aircraft total time directly rather than have to compute it for every
log entry.

I don't think I'm alone in that regard...if you look at the listings for
replacement mechanical tachs on the Aircraft Spruce page, you'll see
they can be set to whatever value at a slight additional charge.

Ron Wanttaja

The tach case is crimped around the faceplate. I took an old
one apart by uncrimping it and taking out those two screws on the
back. Took the tach mechanism out of the case, sawed a great chunk out
of the case, and put it back together as a teaching aid to demonstrate
the magnetic-drag type of tach. Students get a kick out of the rest of
the demo, which is to use an old rare-earth hard-drive magnet and a
strip of .032" aluminum that passes through the gap between the magnet
faces. Aluminum isn't magnetic until it passes through a magnetic
field, whereupon a current is generated in the aluminum, and that
current generates an opposing field that interacts with the magnet's
field to make the aluminum resist movement through the magnet. The
tach has a small bar magnet that spins in the aluminum cup that drives
the indicator needle. Tachs start to underread as the magnet ages and
weakens.

Dan