AIR TAXI CARGO ACFT ON APCH HAS GS NEEDLE STICK IN CTRED POS. ALTDEV RESULTS.

Narrative

ON AUG/XA/96; I WAS APCHING JFK ON RADAR VECTORS; AFTER GETTING ESTABLISHED ON THE ILS FOR RWY 31R; I WAS CLRED FOR THE APCH. I HAD ONLY BEEN ON THE LOC FOR MAYBE 15 SECONDS BEFORE I GOT GS INDICATION. GS NEEDLE MOVED FROM THE BOTTOM TOWARDS THE 'DIAMOND;' (THAT INDICATES ON THE GS) AND STOPPED ON THE 'DIAMOND.' ASSUMING I WAS ON THE GS I STARTED MY DSCNT TOWARDS DH. INTERCEPT ALT WAS 1500 FT AND I WAS COMING UP ON 1000 FT WHEN JFK TWR GAVE ME AN ALT ALERT AND ADVISED ME TO CLB BACK TO 1500 FT. FROM THIS POINT EVERYTHING WORKED NORMALLY; LNDG UNEVENTFULLY AT JFK. UPON RETURNING HOME ON AUG/XB/96; THE PIC'S GS WAS ACTING UP AGAIN; THIS TIME WE USED THE SIC'S HSI AND EVERYTHING WAS NORMAL. WHEN AVIONICS CHKED THE GS THEY FOUND THE GS NEEDLE PHYSICALLY STICKING ON THE 'DIAMOND' WHICH SYMBOL IS A PLASTIC PIECE PROTRUDING THE GS 'SCALE;' (NOT PAINTED MARKS). THIS WAS LEAVING NO GS WARNING FLAGS TO INDICATE A PROB; JUST A 'PICTURE PERFECT' ILS WITH A NOT SO PERFECT POS OUTCOME! THIS PROB HAS IN NEWER ACFT BEEN CORRECTED BY A COMPARABLE WARNING SYS; BUT IN THE AGING FLEET OF CHARTER ACFT; THIS IS NORMALLY NOT INSTALLED. ALL THIS ENDED WELL THANKS TO JFK TWR VIGILANCE AND NOT TOO BAD WX. HAD WE BEEN IN NO RADAR ENVIRONMENT OR WX DOWN TO MINIMUMS; I CAN ONLY IMAGINE THE POTENTIAL OUTCOME. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATES THE ACFT WAS A MERLIN IV; METRO. THE UNIT WAS REMOVED AND SENT TO THE MANUFACTURER FOR REPAIR. AS RPTR STATES THE NEWER INSTS DO NOT HAVE THE PROTRUDING MARK BUT INSTEAD HAVE A PAINTED INSIGNIA. THE ACFT WAS GNDED AND A LENDER UNIT INSTALLED WHILE THE REMOVED INST WAS SENT TO THE MANUFACTURER. THE UNIT IS A COLLINS HSI #105. UNIVERSAL PARTS NUMBER IS 7876385-002. SERIAL #591. RPTR FEELS STRONGLY THAT THE AVIATION COMMUNITY SHOULD KNOW ABOUT THIS PROB.