"Kyle Boatright" wrote in message
. ..

"Bill Daniels" bildan@comcast-dot-net wrote in message
. ..
Tell ATC that you aren't 200' low, they're 200' too high (Actually,
I've encountered 200' differences when switched to a new ATC sector -
somebody's not calibrated.)

Seriously, if you set your altimeter to the local AWOS/ATIS/ASOS does it
read the published field elevation? If not, the altimeter needs
calibration. If so, it's not the instrument.

The altimeter was reworked a couple of years ago because it got "sticky".
It came back with a yellow tag and a test card indicating it was
calibrated within VFR limits. Since the altitude discrepancy problem
existed before the altimeter rebuild and has persisted since, I'm guessing
the Altimeter is good.

It's a good bet that a yellow tagged altimeter is OK but I'd still check it
against field elevation. In fact, it's a good idea to do it each time you
do a pre-flight check. Just set the local altimeter pressure and see if the
altimeter indicates field elevation.

The most likely problem is a leak in the static system. If you're up to it
there's a simple check. Get a large plastic hypo syringe and carefully "T"
it into the static tubing behind the panel using a fish tank tubing "T" and
Tygon tubing - make sure all fittings are leak free. Cover the static ports
with electrical tape to seal them off. SLOWLY inject air with the syringe
until the altimeter reads 1000 feet lower than field elevation. Stop
injecting air and watch the altimeter. If there's a leak, the altimeter
will slowly return to field elevation. The ROC will indicate the rate of
the leak.

Realize that the leak can be inside an instrument. I've seen several ROC
instruments with internal leaks (Usually the glass gasket is bad which can
be determined with a soap solution.) but the ASI or even the altimeter
itself can be the culprit. If there is a leak, look for bubbles with soap
solution until you find where it is.

BTW, there's an old glider pilot trick to sealing Tygon tubing to instrument
nipples. Go to a veternary supply store and buy a bag of the rubber
bands(green O-rings) used to castrate sheep (I'm not kidding!) These are
just the right size to slip over the outside of the Tygon after it is pushed
onto the instrument nipple. Put the O-ring over the tubing before pushing
it onto the instrument nipple then roll it over the nipple. It makes a
reliable gas-tight fit even when the Tygon gets old and yellowed.

Bill Daniels