AN EMB-145 FLC OVERSHOT THEIR ALT WHEN THEY WERE UNAWARE THAT THE AUTOPLT HAD DISCONNECTED AND FAILED TO LEVEL THE ACFT AT 14000 FT. THE AUTOPLT DESIGN WAS CRITICIZED FOR NOT HAVING A WARNING SYS ALERT WHEN DISCONNECT OCCURS OVER 2500 FT AGL. BOTH PLTS WERE FILLING OUT CUSTOMS FORMS IN DSCNT.

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WE WERE FLYING FROM HALIFAX; NOVA SCOTIA (YHZ); TO NEWARK (EWR) ON A STAR AND GIVEN A DSCNT CLRNC TO 14000 FT. I'M NOT SURE TODAY IF THIS WAS THE ALT ATC GAVE US; BUT IT WAS AN EVEN MIDDLE-TEEN ALT AND I WAS SURE OF THE ALT ON THE DAY OF THIS INCIDENT -- THAT WAS NOT THE ISSUE. WHILE DSNDING ROUTINELY; IT WAS THE FO'S LEG AND I WAS FILLING OUT OUR CREW DECLARATION CUSTOMS FORM. I NOTICED THAT MY FO WAS ALSO FILLING OUT HER CUSTOMS FORM; SO I WOULD OCCASIONALLY LOOK UP TO MONITOR OUR FLT SIT. THE AUTOPLT WAS ON AND DSNDING INITIALLY; BUT HAD SOMEHOW DISENGAGED WITHOUT US KNOWING WHY. THE AUTOPLT AURAL WARNING ANNOUNCING DISENGAGEMENT ONLY OCCURS BELOW 2500 FT AGL AND NOT AT ANY OTHER ALTS ABOVE THAT. I HAVE ALREADY EXPERIENCED SEVERAL UNANNOUNCED AND UNKNOWN DISENGAGEMENTS OF THE AUTOPLT DURING CLBS; CRUISE; AND DSCNTS AND CAUGHT THEM DURING NORMAL MONITORING OF THE AUTOPLT WHILE IT WAS FLYING. BECAUSE OUR DSCNT WAS SHALLOW AND BECAUSE WE WERE BOTH FILLING OUT OUR CUSTOMS FORMS; NO ONE NOTICED WE HAD DSNDED THROUGH OUR ASSIGNED ALT UNTIL WE WERE 500 FT BELOW IT. MY FO CORRECTED PROMPTLY AND NO TFC WAS ON TCASII. BEING A SATURDAY; IT WAS A LIGHT TFC DAY ON THE ATC FREQ. ZBW DIDN'T MENTION THE ALTDEV. I STILL DON'T KNOW WHY THE AUTOPLT DISENGAGED. I'M CONCERNED THAT THIS PRIMUS 1000 DOES NOT ANNOUNCE DISENGAGEMENT ABOVE 2500 FT. I HAVE TALKED TO SEVERAL CREW MEMBERS WHO HAVE EXPERIENCED SIMILAR UNEXPLAINED DISENGAGEMENTS. IN THE FUTURE; I WILL PAY CLOSER ATTN TO MONITORING THE AUTOPLT SO THAT A DEV DOES NOT OCCUR IN THE FUTURE AS A RESULT OF AN UNEXPLAINED AND UNANNOUNCED DISENGAGEMENT AND SUPERVISE MY FO'S MORE CLOSELY DURING AUTOFLT. A SIMPLE CHANGE IN THE PRIMUS 1000 SOFTWARE WOULD PROBABLY SOLVE THIS ANNOUNCEMENT PROB. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR INDICATED THAT HE HAS HAD ANOTHER DISCONNECT INCIDENT SINCE HIS INITIAL RPT ON THE EMB145 AUTOPLT PROB. THE COMPANY HAS BEEN INFORMED OF THE INADEQUATE WARNING SYS AND HE IS INTENDING TO INFORM HIS AIRLINE UNION SAFETY COMMITTEE. THE ONLY WARNING THAT THE CREW RECEIVES FROM AN AUTOPLT DISCONNECT INFLT ABOVE 2500 FT IS WHEN THE LITTLE LIGHT GOES OUT IN THE CORNER QUADRANT OF THE AUTOPLT 'PUSH TO ENGAGE' BUTTON. ONE MUST BE VERY ALERT TO THIS LIGHT BEING LIT AND IT IS DIFFICULT AT BEST TO NOTE ANY CHANGE IN THE STATUS OF THE AUTOPLT.