DEGRADATION OF AUTOFLT SYS DURING APCH RESULTS IN LESS THAN DESIRABLE LNDG.

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LOCATION: RWY 17 THRESHOLD. WX AT SCL RPTED AS 400 METERS AND SET UP FOR CAT III APCH. ALL PROCS WERE PERFORMED AS PER COMPANY OPS MANUAL. ON OUR FIRST APCH DUAL LAND GREEN WAS ACHIEVED AT APPROX 1500 RADIO ALTIMETER. AT APPROX 500 RADIO ALTIMETER AUTOPLT REVERTED TO SINGLE LAND. A GAR WAS PERFORMED AND A BRIEFING WAS DONE FOR CAT II CONDITIONS WITH AN AUTOLAND PLANNED AS PER OPS MANUAL DUAL LAND GREEN WAS AGAIN RECEIVED; BUT CHANGED TO SINGLE LAND AT APPROX 500 FT AGL. ALL WAS NORMAL UNTIL APPROX 100 FT RADIO ALTIMETER AND NOSE PITCHED DOWN. AUTOPLT WAS DISCONNECTED; BUT NOT ENOUGH TIME TO KEEP THE ACFT FROM HITTING THE RWY. ACFT CONTACTED THE RWY WITH MAIN GEAR FIRST AND BOUNCED; WE WOULD HAVE HIT WITH THE NOSE GEAR IF WE DID NOT REACT. THIS IS THE SECOND TIME THIS HAS OCCURRED IN THIS ACFT WITH ME ALONE. FIRST WAS WITH A CHK AIRMAN IN FRANKFURT AND NOW IN SANTIAGO. THIS AUTOPLT PROB NEEDS TO BE URGENTLY ADDRESSED OR WE ARE GOING TO DAMAGE ONE OF OUR ACFT. IN BOTH INSTANCES THE AUTOPLT DOWNGRADED IN THE APCH FROM DUAL LAND TO SINGLE LAND. BOTH CASES HAD A RADIO ALTIMETER INOP WITH NO INDICATIONS FROM THE COCKPIT OTHER THAN THE SINGLE LAND INDICATION. OUR BOOK STATES WE CAN MAKE AN AUTOLAND WITH SINGLE LAND AUTOPLT. WE HAVE TO HAVE 2 RADIO ALTIMETERS AT THE BEGINNING OF THE APCH; BUT CAN LOSE ONE AND STILL CONTINUE. IT IS MY OPINION THAT THIS ACFT SHOULD NOT BE ALLOWED TO DO AN AUTOLAND WHEN THE AUTOPLT DEGRADES FROM DUAL LAND TO SINGLE LAND. NEITHER RADIO ALTIMETER HAD AN 'X' INDICATED IN THE WINDOW. I FEEL THAT AUTOLANDS SHOULD BE DISCONTINUED UNTIL THIS ISSUE IS SETTLED. SUPPLEMENTAL INFO FROM ACN 489263: DURING THE LNDG PHASE OF A SINGLE LAND APCH TO RWY 17 SCL AT APPROX 80; THE ACFT PITCHED FORWARD REQUIRING THE CAPT TO TAKE CTL. THE APCH WAS NORMAL THROUGH 100 FT AGL EXCEPT FOR THE FO'S RADIO ALTIMETER WHICH CYCLED OUT OF VIEW AT 800 AND BACK AT 600. DURING THE PITCH FORWARD AND SUBSEQUENT RECOVERY THE ACFT'S ORAL 50; 40; 30; 20; 10; CALLS WERE OMITTED.