Narrative:

On kepec one arrival to las, I was hand-flying the aircraft due to continuous light to occasionally moderate turbulence, this in an attempt to provide a somewhat smoother ride for the passenger since the autoplt does not have a 'smooth ride' function. I was also visually scanning for other traffic in the area. First officer was completing the descent/in-range checklists. I saw other traffic climbing out of las in the distance, and determined that the traffic would not be a factor to us. After a few moments, I sensed that we should have turned by now. Looking down to the FMS, I realized that, for some reason, the flight director had not indicated the turn, but rather acted as if selected to heading mode. I immediately started the left turn. A few moments later, ATC told me to turn left to heading 340 degrees. I queried my first officer whether he had pushed any flight director selector knobs or done anything to the FMS. He was baffled and did not understand what could have happened. After landing, I contacted las approach, spoke with a very cooperative controller, and explained what had happened. During my approach, I received an error message on the FMS and I proceeded to write it up as a squawk. I presently have our company's maintenance department investigating the affected FMS and conducting any necessary repairs. I see this event as an excellent reminder not to trust the modern FMS system 100%, but rather to maintain a continuous and very careful eye on the equipment especially in terminal areas.

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Original NASA ASRS Text

Title: GLF2 CREW HAD A TRACK DEV WHILE FLYING THE KEPEC RNAV ONE ARR AT LAS.

Narrative: ON KEPEC ONE ARR TO LAS, I WAS HAND-FLYING THE ACFT DUE TO CONTINUOUS LIGHT TO OCCASIONALLY MODERATE TURB, THIS IN AN ATTEMPT TO PROVIDE A SOMEWHAT SMOOTHER RIDE FOR THE PAX SINCE THE AUTOPLT DOES NOT HAVE A 'SMOOTH RIDE' FUNCTION. I WAS ALSO VISUALLY SCANNING FOR OTHER TFC IN THE AREA. FO WAS COMPLETING THE DSCNT/IN-RANGE CHKLISTS. I SAW OTHER TFC CLBING OUT OF LAS IN THE DISTANCE, AND DETERMINED THAT THE TFC WOULD NOT BE A FACTOR TO US. AFTER A FEW MOMENTS, I SENSED THAT WE SHOULD HAVE TURNED BY NOW. LOOKING DOWN TO THE FMS, I REALIZED THAT, FOR SOME REASON, THE FLT DIRECTOR HAD NOT INDICATED THE TURN, BUT RATHER ACTED AS IF SELECTED TO HEADING MODE. I IMMEDIATELY STARTED THE L TURN. A FEW MOMENTS LATER, ATC TOLD ME TO TURN L TO HEADING 340 DEGS. I QUERIED MY FO WHETHER HE HAD PUSHED ANY FLT DIRECTOR SELECTOR KNOBS OR DONE ANYTHING TO THE FMS. HE WAS BAFFLED AND DID NOT UNDERSTAND WHAT COULD HAVE HAPPENED. AFTER LNDG, I CONTACTED LAS APCH, SPOKE WITH A VERY COOPERATIVE CTLR, AND EXPLAINED WHAT HAD HAPPENED. DURING MY APCH, I RECEIVED AN ERROR MESSAGE ON THE FMS AND I PROCEEDED TO WRITE IT UP AS A SQUAWK. I PRESENTLY HAVE OUR COMPANY'S MAINT DEPT INVESTIGATING THE AFFECTED FMS AND CONDUCTING ANY NECESSARY REPAIRS. I SEE THIS EVENT AS AN EXCELLENT REMINDER NOT TO TRUST THE MODERN FMS SYS 100%, BUT RATHER TO MAINTAIN A CONTINUOUS AND VERY CAREFUL EYE ON THE EQUIP ESPECIALLY IN TERMINAL AREAS.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.