AN A310 SHORT CUT GFS WHEN GIVEN GFS DIRECT HEC. THE DIRECT ROUTING REQUIRED A TURN OF GREATER THAN 90 DEGS.

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FLT FROM LAS TO LAX. FILED AND CLRED VIA OASYS 2 DEP GFS TRANSITION. (FMS EQUIP A310 AIRBUS). AFTER GFS DIRECT HEC; J9 LAX. DURING CLB WHILE APPROX 10 MI BEFORE GFS VOR; ACFT STARTED A 5 DEG ANGLE OF BANK TO MAKE TRANSITION TO FLY GFS TO HEC. AFTER 20 DEGS OF HDG CHANGE IT WAS APPARENT ACFT WAS NOT GOING TO OVERFLY GFS WHILE MAKING THE TURN. ACFT WAS TAKEN OUT OF NAV AND ACFT HDG ADJUSTED TO FLY TOWARD GFS. CTR HAD GIVEN US SPD RESTR AND AMENDED ALT IN CLB JUST PRIOR TO ACFT DEPARTING FROM COURSE. SHORTLY AFTER; CTR CLRED US DIRECT CIVET FOR THE CIVET ARR TO LAX AND ASKED IF WE WERE OVERHEAD GFS AT THIS TIME. WE ACKNOWLEDGED WE WERE OVERHEAD GFS AT THIS TIME. (BY THIS TIME WE WERE SLIGHTLY R OF COURSE CORRECTING; APPROX GFS 320 DEGS 7 DME; ALT AT TIME WAS FL260; CLBING.) IT IS MY BELIEF WE WERE NEVER MORE THAN 4 NM OFF COURSE; BUT IF ACFT WAS ALLOWED TO FLY THE TURN COMPLETELY; WE WOULD HAVE BEEN. APPROX 5 MINS LATER; CTR CALLED WITH A PHONE NUMBER TO CALL WHEN WE LANDED. I CALLED ZLA ON ARR TO LAX AND TALKED TO CTLR WHO JUST WANTED TO KNOW ABOUT AIRBUS NAV FUNCTION. HE SAID THEY HAVE HAD MANY AIRBUS 320'S WITH FMS EQUIP CUT CORNERS ON TURNS AND GET OUT OF AIRSPACE AND WAS JUST TRYING TO IDENT A POSSIBLE PROB WITH NAV SOFTWARE. I TOLD HIM WHAT HAD HAPPENED AND EXPLAINED IT TO BE A PROB WHEN A LARGE HDG CHANGE NEEDS TO BE MADE; THE FMS AND ACFT ARE PROGRAMMED FOR A VERY SHALLOW 5 DEG BANK AND IT MUST LEAD TURN 10-15 MINS BEFORE FIX TO COMPLETE TURN. NOT A GOOD PROGRAM.