EMB170 FLT CREW REPORTS RADAR ALTIMETER FAILURE THAT CAUSES LOUD LANDING GEAR WARNING THAT CANNOT BE SILENCED. THE AUTOPILOT FAILS TO HOLD ALTITUDE DURING THE APPROACH BUT ONCE THE GEAR IS LOWERED THE WARNING IS SILENCED.

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UPON ARR INTO ATL; ABOUT 7000 MSL AND 20 MI OUT; THE AURAL WARNING UNIT BEGAN THE 'LNDG GEAR' CALLOUT WHICH WAS UNABLE TO BE SILENCED. NOTE: THE CAPT'S RADIO ALTIMETER READ '0' WHILE THE FO'S WAS FUNCTIONING NORMALLY. WE WERE ON WITH ATLANTA APCH AND RECEIVING RADAR VECTORS TO FINAL. THE CALLOUT WAS SO LOUD; WE WERE HAVING TROUBLE SIMPLY HEARING APCH. THE CTLR GAVE US THE CLRNC OF 5 MI (APPROX) FROM VINNI; TURN L HDG 070 DEGS TO REINTERCEPT THE LOC; CROSS VINNI AT 7000 FT MSL; CLRED FOR THE ILS PRM RWY 9R APCH. I READ BACK THE CLRNC AND WE COMMANDED THE AUTOPLT TO DO THE TURN; SUBSEQUENTLY ARMING THE APCH. THE AUTOPLT CAPTURED THE LOC AND GS APPROX 3 MI FROM VINNI; AND WE CONFIRMED IT. WE THEN SHIFTED OUR ATTN TO TRY TO SILENCE THE 'LNDG GEAR' CALL SINCE WE WERE STILL APPROX 19 MI OUT. HOWEVER; THE AUTOPLT BEGAN A DSCNT CONTRARY TO THE TRUE GS EVEN THE GS WAS CAPTURED AND WE ENDED UP XING VINNI 6300 FT. THE CTLR INFORMED US OF THIS; AND WE TURNED OFF THE AUTOPLT AND HAND FLEW THE ACFT BACK ONTO GLIDEPATH; MEANWHILE; THE 'LNDG GEAR' CALL IS STILL PLAYING. ABOUT 15 MI OUT; WE JUST DECIDED TO LOWER THE LNDG GEAR TO ATTEMPT TO GET THE HORN TO SILENCE. IT WORKED AND WE CONTINUED THE APCH WITHOUT FURTHER INCIDENT. I FULLY BELIEVE THAT THERE ARE 2 REASONS THAT THERE WAS ANY SORT OF ALTDEV. FIRST OFF; THE ERRONEOUS 'LNDG GEAR' WAS NOT ONLY VERY DISTRACTING; BUT MADE IT EXTREMELY HARD TO COMMUNICATE TO ATC AND AMONGST OURSELVES; THUS PROVIDING A HUGE DISTR. SECONDLY; THE CTLR GAVE US A LATE VECTOR TO INTERCEPT THE ILS WHICH WAS SLIGHTLY OFF THE NORM; JUST ADDING TO THE SITUATION. I STILL HAVE NO IDEA WHY THE AUTOPLT DSNDED; WHILE HAVING CAPTURED THE GS; BELOW THE GS. I DO HOWEVER THINK THAT HAD WE NOT BEEN EXTREMELY DISTR BY THE ERRONEOUS 'LNDG GEAR' CALL WE WOULD HAVE IMMEDIATELY SEEN THE ALTDEV. I ALSO BELIEVE AFTER TALKING WITH MAINT THAT THE DEV AND AUTOPLT PROBS WERE A DIRECT RESULT OF A THE RADAR ALTIMETER; THE PROBABLE CAUSE FOR THE 'LNDG GEAR' CALLOUT.