A TFC CTL SIT DEVELOPS AFTER A BOMBARDIER GLOBAL EXPRESS CPR FLT SUFFERS A HDG TRACK DEV DURING AN UNDETECTED AUTOPLT DISCONNECT 5 MI FROM SFO; CA.

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WE WERE ON THE ARR INTO SFO AND WERE ASSIGNED THE QUIET BRIDGE ARR TO RWY 28R. THE FMS WAS SET UP TO FOLLOW THE ARR TO MENLO INTXN AND THEN A 330 DEG HDG TO INTERCEPT THE 095 DEG RADIAL OF THE SFO VOR. SOMETIME PRIOR TO MENLO; THE AUTOPLT HAD DISENGAGED AND THE AUTOTHROTTLES WERE NOT RESPONDING TO THE DSCNT SPD OF 210 KTS WHICH WAS ENTERED AS INSTRUCTED BY APCH CTL. THE CTLR CAME BACK TO US AND ASKED WHAT COURSE WE WERE FOLLOWING AND WHAT SPD WE WERE AT. THIS IS WHEN WE NOTICED THE ACFT WAS AT 230 KTS AND BASICALLY HOLDING A 290 DEG HDG. THIS IS A NEW ACFT AND HAS IN THE PAST HAS HAD THE AUTOPLT DISCONNECT WITHOUT WARNING; BUT IT HAS BEEN CAUGHT. THIS TIME DUE TO THE WORKLOAD IN LOOKING FOR TFC WHICH WE WERE TO FOLLOW; IT WENT UNNOTICED WE WERE RECLRED TO LAND ON RWY 28L BY APCH AND THEN WHEN SWITCHED TO THE TWR WE WERE RECLRED AGAIN TO LAND ON RWY 28R. TO OUR KNOWLEDGE THERE WAS NEVER A CONFLICT INVOLVED; ONLY THAT OUR COURSE AND SPD WAS NOT APPROPRIATE FOR THE RWY WHICH WE WERE ASSIGNED. OUR MAINT PEOPLE ARE WORKING WITH THE MANUFACTURER TO DETERMINE WHAT IS CAUSING THIS DISENGAGEMENT WITHOUT WARNING. AT PRESENT; ALL OUR PLTS HAVE BEEN INSTRUCTED TO MONITOR THE SYS CAREFULLY WHENEVER A SLIGHT HDG CHANGE FOLLOWING A FIX IS PERFORMED BY THE AUTOPLT AS THIS IS WHEN THE SYS SEEMS TO BE MOST LIKELY TO DISENGAGED. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR IDENTED THE ACFT AS A BOMBARDIER GLOBAL EXPRESS TYPE; ONE OF THE FIRST OF ABOUT 35 IN SVC WORLD WIDE. THE ACFT IS A COMPUTER DRIVEN ACFT WITH THE COMPUTERS SELF TESTING EACH OTHER ON A CONTINUOUS BASIS. HE CALLED THIS 'AN ACFT IN EVOLUTION.' THE AUTOFLT SYS IS PRODUCED BY HONEYWELL; A MODEL 8400 SERIES. HE SAID THAT THE PROB STARTED WITH THE LAST UPGRADE DENOTED AS A 'BLOCK 2' UPGRADE. THE AUTOPLT DISCONNECT OCCURS WHENEVER THE HDG CHANGE IS 10 DEGS OR LESS. THE AUTOPLT DISCONNECT WARNING WILL COME ON DURING A 'NORMAL' DISCONNECT BUT WHEN ASSOCIATED WITH THIS KIND OF PROBLEMATIC EQUIP DISCONNECT; THE AUTOPLT WARNING IS NOT TRIGGERED.