B727 DSNDS BELOW GS AT PIT; PA; WHEN FLT INSTS GIVE FALSE; CONTRADICTING INDICATION. APCH WAS COMPLETED IN VFR CONDITIONS AFTER CTLR ASSISTANCE.

Narrative

DSNDING FROM ALT ENRTE FROM PHL ARPT TO PIT ARPT; CTLR ASSIGNED HDG TO INTERCEPT LOC TO RWY 32 AT PIT AND CLRED US TO 'MAINTAIN 4000 FT UNTIL ESTABLISHED ON THE LOC.' BOTH NAV SYS WORKED PROPERLY ENRTE AND WE HAD FLOWN SEVERAL INST APCHS IN THE SAME AIRPLANE WITHOUT PROBS THE PREVIOUS MORNING. CAPT'S INSTS INDICATED LOC WAS 'ALIVE' AND CAPT TURNED TO JOIN LOC AS ASSIGNED. MY LOC WAS NEAR FULL SCALE WHEN OTHER LOC WAS 1/2 SCALE OUT. AS WE GOT CLOSER TO THE ARPT; MY LOC CAME CLOSER TO BEING CTRED. I POINTED OUT THE DISCREPANCY AND SINCE ACFT IS EQUIPPED WITH A HUD; THE CAPT CHKED THE HUD AND HIS LOC -- THEY BOTH AGREED; AND MY LOC WAS SLOWLY CTRING. APCH PLATE CALLS FOR A DSCNT TO 2500 FT AT 9.3 DME AND GS CROSSES 4000 FT AT 9.3 DME. GS ON CAPT'S SIDE WAS CTRED AT 9.X DME; GS ON MY SIDE INDICATED WE WERE 1/2 DOT LOW. CAPT INTERCEPTED THE GS AND BEGAN HIS DSCNT. FE AND I FELT SOMETHING WAS 'WRONG' AND CHKED EVERYTHING -- FREQS; HDGS; DME'S; WARNING FLAPS -- BUT EVERYTHING APPEARED NORMAL EXCEPT MY INSTS STILL SHOWED 1/2 DOT LOW AND SLIGHTLY R OF COURSE. AS I GRABBED THE MIKE TO ASK CTLR TO CONFIRM OUR POS; THE APCH CTLR CALLED AND ASKED WHY WE WERE BELOW 4000 FT BEFORE BEING ESTABLISHED ON LOC. WE RESPONDED THAT OUR INSTS INDICATED WE WERE IN FACT ESTABLISHED ON THE LOC. CTLR SAID WE WERE 4 TO 5 MI E OF COURSE. THIS ENTIRE SEQUENCE TOOK LESS THAN 1 MIN BUT WE WERE NOW 3300 FT. CTLR GAVE US A NEW HDG TO INTERCEPT THE LOC AND AT THE SAME TIME I WAS ABLE TO SPOT THE ARPT. WE CONTINUED THE APCH VISUALLY WITHOUT INCIDENT. ON SHORT FINAL USING VASI; MY LOC WAS CTRED; GS WAS CTRED BUT ERRATIC. CAPT'S LOC SHOWED FULL SCALE TO THE R AND GS WAS 1 DOT HIGH. ON THE RAMP WE CHKED INDICATIONS. MY LOC AND GS SHOWED WARNING FLAPS (THE CORRECT READING); CAPT'S GS AND LOC FLAGS WERE NOT VISIBLE; LOC SHOWED LESS THAN FULL SCALE WHILE GS SHOWED ALMOST RIGHT ON. BOTH SYS WERE WRITTEN UP AND ACFT WAS GNDED UNTIL REPLACEMENT PARTS WERE FLOWN IN. MECH ALSO FOUND WATER STANDING AND DRIPPING INTO THE E&E BAY. BEFORE LEAVING PHL WE WERE DEICED TWICE.