CORP LTT OVERSHOT ALT IN DESCENT.

Narrative

25 NM SE OF SJC WHILE INTERCEPTING THE SJC RWY 30L LOC AND DSNDING TO MAINTAIN 10000' ON AUTOPLT; THE ACFT DSNDED THROUGH THE ASSIGNED ALT. DURING THE DSCNT THE CREW WAS REVIEWING THE APCH PLATE FOR THE UPCOMING ILS APCH. WHEN ATTN WAS AGAIN FOCUSED TO THE INSTRUMENT PANEL; THE ACFT WAS DSNDING THROUGH 8600'. ALT WAS IMMEDIATELY CORRECTED AND ATC WAS ADVISED OF OUR AUTOPLT MALFUNCTION. NO OTHER TFC WAS INVOLVED AT THE TIME OF THE OCCURRENCE. THE PROB AROSE FROM A MALFUNCTIONING ALT PRESELECTOR (MORE TO FOLLOW ON THE HARDWARE); AND THE CREWS TOTAL TRUST IN ITS ABILITY TO OPERATE PROPERLY. THE PROB WAS DISCOVERED AS SOON AS THE ATTN RETURNED TO THE PANEL AFTER OTHER COCKPIT DUTIES. CORRECTIVE ACTION WAS TAKEN TO BRING THE ACFT TO ITS ASSIGNED ALT AS SOON AS THE PROB WAS DISCOVERED. THE ACFT USES A VNAV COMPUTER/CTLR TO PROVIDE ALT ALERTING; ALT PRESELECT AND VNAV MODES. OUR ACFT HAS HAD TO REPLACE 5 OF THESE UNITS SINCE THE ACFT WAS DELIVERED NEW FROM THE FACTORY ONLY 9 MONTHS AGO. THE FAILURE OF THE UNIT TO OPERATE PROPERLY PLACED THE ACFT AND CREW (NO PAX ABOARD) IN A POTENTIALLY DANGEROUS SITUATION. THE CREW WAS PROVIDED WITH NO AURAL/VISUAL 1000' ABOVE/BELOW WARNING; AND THE AUTOPLT FAILED TO CAPTURE AS THE FLT DIRECTOR/AUTOPLT WAS PROGRAMMED TO DO. THE NUMBER OF FAILURES WE HAVE EXPERIENCED WITH THIS UNIT SUGGESTS THAT AN FAA MALFUNCTION AND DEFECT RPT BE SUBMITTED RE THIS UNIT. THE LESSON TO BE LEARNED FROM THIS EXPERIENCE IS THAT AS COCKPITS BECOME MORE SOPHISTICATED; AND OURS HAS FULL EFIS AND FMS; PLTS WILL HAVE TO MAKE MORE OF AN EFFORT TO SUPERVISE THE ADVANCED AUTOMATION AND BE READY TO TAKE OVER MANUALLY IF AND WHEN NEEDED. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING: THE RPTR HAS FILED A DEFECT MALFUNCTION WITH THE FAA AND BEEN IN CONTACT WITH THE ACFT MFR. THEY ARE AWARE OF THE PROB AND ARE WORKING TO SOLVE. SINCE THE UNIT WAS PLACED INTO SERVICE; IN 136 OF THIS MODEL ACFT; THERE HAVE BEEN 138 FAILURES IN THE 3 YR PERIOD. ONLY 3 OF THE ACFT HAVE THE COMPLETE EFIS CRT SYSTEM INCLUDING THE ONE IN RPTED INCIDENT. THE SUBJECT ACFT HAS HAD 6 FAILURES OF THE ALT PRESELECT UNIT WITH SVC TIME VARYING FROM 10 HRS TO A MAX OF 67 HRS FOR AVERAGE SVC LIFE ON AROUND 30 HRS. MFR HAS ISOLATED THE INTEGRATED CIRCUIT IN THE UNIT THAT HAS FAILED AND NOW TRYING TO ASCERTAIN IF BAD CIRCUITS FROM VENDOR OR IF HEAT IS CAUSING PROB. THEY HAVE JUST INSTALLED HEAT TAPES ON THIS ACFT TO ASCERTAIN HEAT LEVEL. RPTR STATED NO FOLLOW UP FROM FAA ON ALT BUST SO FIGURES SNITCH NOT ACTIVATED AND NO PROBS CAUSED.