Narrative:

Descending from altitude en route from phl airport to pit airport, controller assigned heading to intercept localizer to runway 32 at pit and cleared us to 'maintain 4000 ft until established on the localizer.' both navigation system worked properly en route and we had flown several instrument approachs in the same airplane without problems the previous morning. Captain's instruments indicated localizer was 'alive' and captain turned to join localizer as assigned. My localizer was near full scale when other localizer was 1/2 scale out. As we got closer to the airport, my localizer came closer to being centered. I pointed out the discrepancy and since aircraft is equipped with a HUD, the captain checked the HUD and his localizer -- they both agreed, and my localizer was slowly ctring. Approach plate calls for a descent to 2500 ft at 9.3 DME and GS crosses 4000 ft at 9.3 DME. GS on captain's side was centered at 9.X DME, GS on my side indicated we were 1/2 DOT low. Captain intercepted the GS and began his descent. Flight engineer and I felt something was 'wrong' and checked everything -- frequencys, headings, DME's, warning flaps -- but everything appeared normal except my instruments still showed 1/2 DOT low and slightly right of course. As I grabbed the microphone to ask controller to confirm our position, the approach controller called and asked why we were below 4000 ft before being established on localizer. We responded that our instruments indicated we were in fact established on the localizer. Controller said we were 4 to 5 mi east of course. This entire sequence took less than 1 min but we were now 3300 ft. Controller gave us a new heading to intercept the localizer and at the same time I was able to spot the airport. We continued the approach visually without incident. On short final using VASI, my localizer was centered, GS was centered but erratic. Captain's localizer showed full scale to the right and GS was 1 DOT high. On the ramp we checked indications. My localizer and GS showed warning flaps (the correct reading), captain's GS and localizer flags were not visible, localizer showed less than full scale while GS showed almost right on. Both system were written up and aircraft was grounded until replacement parts were flown in. Mechanic also found water standing and dripping into the east&east bay. Before leaving phl we were deiced twice.

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Original NASA ASRS Text

Title: 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.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.