[Steve]

What would be the likely culprit(s) to cause my altimeter to indicate 300 feet too low? The GTX-327's pressure altitude display agrees with the altimeter (-300 feet while on the ground at sea level). The airspeed indicator works fine.

Particulars:
Van's airspeed indicator
GTX-327 xpndr
ACK encoder
airplane is hangared
no visible blockage on static ports and uncovered pitot tube
all systems worked fine for 436 hours until yesterday

I have a couple of ideas in mind but I wanted additional input from the group before I drive 40 miles to the hangar.

 

[Walt]

The only thing that will cause both the alt and encoder to have an error is a bockage in the static system somewhere before they 'T' off to the respective units. I would look for a slug of water at a low point somewhere.

The encoder is referenced to 29.92, setting the alt to 29.92 is the only time the altimeter and encoder will match.