ACR LTT RECEIVES FALSE GLIDE SLOPE INDICATIONS ON APCH RESULTING IN CONTROLLED FLT TOWARD TERRAIN.

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WE WERE ON AN ILS APCH TO RWY 5R IN PVD. CONDITIONS WERE AN INDEFINITE 400; 1 MI; RAIN AND FOG. MODERATE TURB WAS EXPERIENCED CONTINUOUSLY ON THE APCH ALONG WITH SOME STRONG UP AND DOWNDRAFTS. BOTH NAV RECEIVERS IDENTED OK (PLEASE BE AWARE THAT IN THE LTT; THEY SHARE A COMMON ANTENNA). I WAS FLYING THE APCH; THE COPLT WAS BACKING ME UP BY SCANNING BOTH INSTRUMENTS AND KEEPING A VISUAL SCAN FOR THE RWY ENVIRONMENT. APPROX 500' AGL MY G/S BEGAN A SLOW BUT STEADY FLY DOWN INDICATION. DUE TO THE TURB THE VSI WASN'T MUCH HELP IN SHOWING A TREND; SO I REDUCED PWR TO MAINTAIN WHAT I THOUGHT WAS THE PROPER GLIDE PATH. THE G/S NEEDLE CONTINUED ITS DOWNWARD TREND THEN WENT TO A FULL FLY UP INDICATION. THE COPLT'S G/S NEEDLE NEVER MOVED FROM ENTER (ON GLIDE) THROUGHOUT THE INCIDENT. THE COPLT NOTICED THE DISCREPANCY AT ABOUT THE SAME TIME WE HAD GND CONTACT AND VISUAL ON THE RWY. NEEDLESS TO SAY WE WERE QUITE LOW AND HE WE CONTINUED WITH NO CORRECTIONS WE PROBABLY WOULD HAVE HIT THE TREES SHORT OF THE RWY. NO NAV FLAGS WERE OBSERVED ON EITHER INSTRUMENT THROUGHOUT THE APCH AND A POSITIVE IDENT WAS ALWAYS AUDIBLE. THE OM XING HT WAS ALSO NOTED AND WAS CORRECT WHEN WE STARTED THE APCH. PLEASE NOTE ALSO THAT IN OUR LTT; THE CAPT CANNOT SEE THE LEFT SIDE OF THE COPLT'S HSI SO ANY G/S XCHKS ON AN APCH HAVE TO BE DONE BY THE COPLT. IT'S IMPOSSIBLE FOR THE CAPT TO SEE IT UNLESS HE LEANS OVER OR SLIDES HIS SEAT BACK. THE CAUSE OF THE PROB WAS WATER GETTING INTO THE ANTENNA AND CABLE SPLICE. THE MECHS ALSO STATED THAT HE NOTICED THIS HAPPENS FAIRLY OFTEN ON THE LTT. I DON'T KNOW WHAT ELSE WE AS THE CREW COULD HAVE DONE BUT WE FEEL IT SHOULD BE BROUGHT TO THE ATTN OF OTHER CREWS. THAT WAY THEY'LL KNOW YOU CAN GET A G/S FAILURE WITH NO OFF FLAG OR LOSS OF IDENT. ALSO THAT DEPENDING ON THE INSTRUMENT LAYOUT; ONLY 1 PLT MAY BE IN A POS TO NOTICE AN INSTRUMENT DISCREPANCY. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING: VERIFIED THAT AS PF CHASED HIS G/S WHEN IT GAVE HIM FLY DOWN COMMAND; THE PNF G/S SHOWED THEY WERE STILL ON G/S. BOTH G/S INDICATORS WERE GIVING ERRONEOUS INFO. THIS HAS HAPPENED TO HIM AGAIN ON ANOTHER OCCASION AND HE HAS HEARD THAT IT HAS HAPPENED TO OTHERS. HAS ALSO HAD CASE WHERE LOC INDICATIONS ALSO GAVE FAULTY COMMANDS ALL OCCURRED IN RAIN. HAS CONFIRMED WITH MECHS THAT COMMON LOC-G/S ANTENNA AND THAT THE SEALS ARE ALLOWING WATER TO GET INTO ANTENNA. WATER IS ALSO GETTING INTO THE SIGNAL SPLITTER FOR THE 2 RECEIVERS AT THIS POINT. HE SAID THIS MODEL LTT LEAKS LIKE A SIEVE AND OTHER SEALS ALSO LEAK IN RAIN. COULD NOT CONFIRM IF ACFT MFG HAS BEEN MADE AWARE OF PROB.