Blind encoders are just that.. Blind, they do not care what the current
proper "altimeter setting" is.

ATC decoders know the altimeter settings for a given area and automatically
apply the correction before displaying.
I've watched entire digital radar displays report everyone 300ft off
instantaneously as the altimeter setting table was loaded into the system, a
momentary blink when there was no corrections, and then the updates as the
new setting corrections were applied to the ModeC data received.

ATC will issue the altimeter setting to the pilot in the event the
"displayed" altitude does not agree with the pilot reported altitude just in
case the pilot has miss set or not set the altimeter for the correct
pressure. In such cases normally about a 200-300ft difference, the pilot
dials in the correct pressure and then either reports a new altitude that
may agree with the reported altitude, or descents/climbs to the reported
altitude and the ModeC now agrees.

It appears that the Garmin 3xx series transponder may have an error in it's
sensor, a leak in the static line to the sensor or something creating a
vacuum near the sensor as it was reading about 700-800ft to high.

You state that the altimeter agreed with the ASOS setting on the ground. At
ground level and not moving (ground speed = zero) does the reported FL
altitude display agree? If they agree, then I would suspect that the static
line might be off the back of the encoder transponder and that in-flight the
encoder is sensing cabin pressure and not outside static pressure. In flight
in C-172s, the inside cabin is slightly lower pressure than outside due to
vacuum effects of a leaky fuselage.

I'm somewhat surprised, if the pilot is part owner and flies this aircraft
regularly, she does not know the operation of her transponder. She can
select STBY, ALT and put in the code, but she does not understand it's
operation.
A nice thing about those transponders.. the VFR button will automatically
load 1200.

BT


"Viperdoc" wrote in message
...
I have flown with a couple of Garmin 327's and now have two 330's. The
pressure altitude readout on the transponder comes (as I recall) from the
blind encoder. It is obviously not connected to the altimeter setting.

So, the encoder could be culprit for the mis reported altitude. I suppose
there could be a difference in altitudes of 700 feet if the day was very
non-standard (such as very cold, high pressure, etc).