"Bill Daniels" bildan@comcast-dot-net wrote in message
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"Kyle Boatright" wrote in message
. ..

"Bill Daniels" bildan@comcast-dot-net wrote in message
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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.

I should have mentioned before that the altimeter is close to the field
elevation when on the ground. Well within VFR limits.

One interesting test would be a low altitude pass down the runway at cruise
power and speed. If I'm 50' above the field and the altimeter shows field
level + 50' (or thereabouts) what would that tell me?

A basic question: If there was a problem with the static system, wouldn't
that cause the same discrepancy in both the altimeter and transponder?


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.

I was trying to come up with a technique to do this, and this one sounds
like a winner. There is a location in the baggage compartment where it is
easy to get to the static tubing...


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

Thanks for the ideas.

KB