A BA3200 FO'S DISCOURSE ON THE INADEQUACIES OF THEIR FMS AS INSTALLED AND OPERATED IN THEIR ACFT. THE CREW HAD EXPERIENCED AN OVERSHOOT OF THEIR TURNING POINT IN THEIR STAR PROC DUE TO A MALFUNCTION IN THE FMS.

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I BELIEVE SEVERAL FACTORS ACTED CUMULATIVELY TO CAUSE THIS DEV. DISTR -- CERTAINLY THE HIGH WORKLOAD JUST PRIOR TO THE APCH PHASE PLAYED A ROLE. CHKLISTS; APCH BRIEFINGS; CALLS TO OUR COMPANY AND PAX BRIEFINGS. IN OUR ACFT; THE FMS IS MOUNTED TOWARD THE R EDGE OF THE CTR INST PANEL. FROM A NORMAL SITTING POS; THE FO'S VIEW OF MOST OF THE FMS DISPLAY IS OBSCURED BY THE L HALF OF HIS OR HER CTL WHEEL. OUR COMPANY TEACHES US TO USE THE FLT DIRECTOR WHENEVER FOLLOWING AN FMS PATH. HOWEVER; IN THIS CASE; THE FMS DID NOT CYCLE TO THE NEXT WAYPOINT AND THEREFORE THE FLT DIRECTOR STEERING COMMANDS DID NOT CHANGE. LACK OF MOVING MAP DISPLAY. WITHOUT IT; PROBS SUCH AS THE ONE WE ENCOUNTERED ARE NOT READILY; IF AT ALL; APPARENT. PRIOR TO TURNING OVER A WAYPOINT; THE FMS DISPLAYS THE NEW COURSE AND DISTANCE TO THE NEXT WAYPOINT IN PARENTHESES NEXT TO THE CORRESPONDING DATA FOR THE ACTIVE WAYPOINT. HOWEVER; WITHIN APPROX 30 SECONDS OR 2 MI PRIOR TO XING THE WAYPOINT; THIS INFO DISAPPEARS AND THE FLT DIRECTOR STARTS TO ISSUE STEERING COMMANDS TOWARD THE OUTBOUND COURSE. AN INTERNAL COMPANY MEMO ANNOUNCED THAT THIS SOFTWARE BUG IS BEING FIXED IN A FUTURE MODIFICATION. UNDER MOST CONDITIONS; WHEN THE FMS REACHES THE END OF THE LOADED RTE OR WILL NOT CYCLE TO A LATER WAYPOINT; IT GIVES A 'NO AUTO LEG CHANGE' MESSAGE ON THE 'SYS MESSAGES' PAGE. FOR REASONS UNEXPLAINED; WE DID NOT RECEIVE SUCH A MESSAGE OR A BLINKING MESSAGE LIGHT. I ALSO OBSERVED THE FMS ON A SUBSEQUENT FLT THAT DAY AND NOTICED THAT THE 'NO AUTO LEG CHANGE' MESSAGE WAS NOT APPEARING AT THE END-OF-RTE. THIS LEADS ME TO BELIEVE THAT THE SOFTWARE CHANGE MAY HAVE OCCURRED PRIOR TO THE ISSUANCE OF THE COMPANY MEMO TO THAT EFFECT. SINCE THE MEMO APPEARED 2 DAYS AFTER OUR INCIDENT; I MAY NOT HAVE HAD CURRENT AND COMPLETE INFO ON THE PROPER FUNCTIONING OF THE FMS. NOTE: THIS RPT REFERS TO VARIOUS SPECIFIC FUNCTIONS OF THE FMS INSTALLED ON OUR ACFT. THE UNIT IN QUESTION IS AN ALLIED SIGNAL GNS-XLS FMS.