ALTDEV ON DSCNT DUE TO WRONG ALTIMETER SETTING ON CAPT'S SIDE.

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FK100 ACFT WITH EFIS IN DSCNT TO CROSS BILIT INTXN AT 11000 FT FOR DCA. APCH QUESTIONED ALT. CAPT SIDE SHOWED 11750 FT DSNDING AND 3 NM FROM BILIT. MY SIDE (FO) SHOWED 900 FT HIGHER. CAPT LEVELED AT 11000 FT ON HIS ALTIMETER; XCHKED BOTH ALTIMETER SETTINGS AND CAPT'S WAS SET AT 28- 29 NOT 29-80 AS IT SHOULD HAVE READ. CAPT THEN LEVELED AT 11000 FT AND WAS THEN GIVEN FURTHER DSCNT. I AM SURE THAT 29.80 HAD BEEN SET IN BOTH LCD WINDOWS. ALSO THE PREVIOUS ALTIMETER SETTING WAS 29.78 WHICH WOULD ONLY REQUIRE A SLIGHT TURN OF THE KNOB. ON THE GND WE TURNED THE KNOB TO SEE HOW MUCH YOU WOULD HAVE TO TURN IT TO REACH FROM 29.80 TO 28.90 AND IT TOOK ABOUT 5-6 FULL SPINS OF THE KNOB. WE BOTH FEEL THAT THIS WAS NOT THE CAUSE; BUT THAT THE LCD CHANGED OR SKIPPED FROM A 9 TO AN 8. THERE WAS NO TFC CONFLICT IN XING BILIT 1000 FT HIGH AND THE CAPT DID RPT THE PROB TO MAINT IN DCA. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: FLC HAD ACFT CHANGE AFTER THIS INCIDENT AND WERE NOT ABLE TO FOLLOW UP AT THE TIME. RPTR STATES THAT CAPT CALLED DISPATCH AT END OF DAY TO DETERMINE WHAT MAINT HAD FOUND. APPARENTLY THEY COULD FIND NOTHING ABNORMAL. DISTRESSING TO FLC AS THEY HAD TRIED TO RECREATE THE CONDITION AND ONE WOULD HAVE TO TURN TOO MANY REVOLUTIONS TO NOT BE AWARE OF THE EXTREME SETTING CHANGE. RPTR HAS EXPERIENCED SUCH GLITCHES OF THE LED SWITCHING TO THE NEXT DIGIT INADVERTENTLY BUT NEVER WITH THE ALTIMETER. HE HAS NOT HEARD OF THIS HAPPENING TO ANY OTHER FLC OR ANY OTHER ACFT. THIS ACFT IS EQUIPPED WITH A SYS WHEREBY ONE PUSHES A BUTTON WHEN FL180 ON CLB OR DSCNT TO RESET THE ALTIMETER TO SETTING PLACED IN LOWER AREA. THIS SETTING IS PART OF THE DSCNT CHKLIST AND FLC KNOWS THEY PLACED CORRECT SETTING BECAUSE OTHER ALTIMETER AND STANDBY ALTIMETER HAD THE CORRECT NUMBERS.