Narrative:

We were cleared for the glasr 3 arrival into sea and to expect the ILS runway 16R approach. The A320 software is such that when you select an arrival on the FMGC, it will automatically select an approach 'via' (an FMGC routing from the arrival to final approach course) to the selected runway. When I reviewed the leg distances and headings on the FMGC, everything appeared correct. However, once on the arrival, between jaksn and hethr, with the autoflt engaged, the aircraft turned to a 250 degree heading at sea 19 DME versus 10 DME. When checking the leg distances, the 19 DME turn point was not idented by distance from sea and we failed to catch this mileage error. The approach controller did catch the turn however, and gave us vectors to the ILS. I have since contacted our fleet captain and he is now aware of this gotcha, and will be discussing this with the manufacturer. In the future, I will back up our FMGC with actual raw data.

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Original NASA ASRS Text

Title: A320 ACFT ON STAR ARR FMGC INPUTS WERE CHKED, THE ACFT WAS ON AUTOFLT, BUT IT TURNED TO THE NEXT POINT TURN 9 MI TOO SOON. RPTR BELIEVES IT WAS A SOFTWARE GLITCH AND CONTACTED HIS CHIEF PLT WHO WILL CONTACT THE MANUFACTURER. THOUGH THE FLC DIDN'T CATCH THE EARLY TURN IMMEDIATELY, THE APCH CTLR DID AND RECLEARED THEM.

Narrative: WE WERE CLRED FOR THE GLASR 3 ARR INTO SEA AND TO EXPECT THE ILS RWY 16R APCH. THE A320 SOFTWARE IS SUCH THAT WHEN YOU SELECT AN ARR ON THE FMGC, IT WILL AUTOMATICALLY SELECT AN APCH 'VIA' (AN FMGC ROUTING FROM THE ARR TO FINAL APCH COURSE) TO THE SELECTED RWY. WHEN I REVIEWED THE LEG DISTANCES AND HEADINGS ON THE FMGC, EVERYTHING APPEARED CORRECT. HOWEVER, ONCE ON THE ARR, BTWN JAKSN AND HETHR, WITH THE AUTOFLT ENGAGED, THE ACFT TURNED TO A 250 DEG HDG AT SEA 19 DME VERSUS 10 DME. WHEN CHKING THE LEG DISTANCES, THE 19 DME TURN POINT WAS NOT IDENTED BY DISTANCE FROM SEA AND WE FAILED TO CATCH THIS MILEAGE ERROR. THE APCH CTLR DID CATCH THE TURN HOWEVER, AND GAVE US VECTORS TO THE ILS. I HAVE SINCE CONTACTED OUR FLEET CAPT AND HE IS NOW AWARE OF THIS GOTCHA, AND WILL BE DISCUSSING THIS WITH THE MANUFACTURER. IN THE FUTURE, I WILL BACK UP OUR FMGC WITH ACTUAL RAW DATA.

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.