Narrative:

The large transport FMS has a navigation problem en route that has been reported through my company but nothing has been done about it. With the FMS properly programmed it does not always make turns en route which comply with the required accuracy and technique to satisfy ATC, even though this FMC and aircraft are FAA certified. It happens at many different places but the las direct gfs direct hec route seems to be one of the worst. With an inbound track of 157 degrees into gfs, the FMS starts a 5-7 degree bank turn when 17 mi away from the gfs fix to roll out on course outbound (238 degree track) to hec. If allowed to continue, the aircraft will pass 6-7 mi abeam gfs, far off the airway. There is no apparent reason why the FMS won't use a larger bank angle as it will do it most other places. By going 'intercept leg' from gfs to hec the FMS will pass about 3 mi abeam gfs which is satisfactory but you shouldn't have to 'trick' the FMS to do a good job. It's bad when you have an FAA certified FMS that you cannot use as it was certified to be used because it won't provide standard and accepted accuracy at turn points. Asked the ZLA controllers who work the gfs area, I know they have seen a lot of this happen with the large transport. Callback conversation with reporter revealed the following: reporter seems convinced that there is a problem with the software in the FMS. He cited other places where the same problem seems to happen repeatedly. He said his air carrier has come out with a notice that crews should use the 'overfly' mode rather than the 'intercept leg' mode at certain fixes. He reiterated that the FMS seems to begin the turn early and then limits its bank angle to 5-7 degrees. Once again, this seems to happen only at certain fixes.

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Original NASA ASRS Text

Title: FMS MAKES EARLY TURNS RESULTING IN NAV ERROR AT SOME PARTICULAR FIXES.

Narrative: THE LGT FMS HAS A NAV PROB ENRTE THAT HAS BEEN RPTED THROUGH MY COMPANY BUT NOTHING HAS BEEN DONE ABOUT IT. WITH THE FMS PROPERLY PROGRAMMED IT DOES NOT ALWAYS MAKE TURNS ENRTE WHICH COMPLY WITH THE REQUIRED ACCURACY AND TECHNIQUE TO SATISFY ATC, EVEN THOUGH THIS FMC AND ACFT ARE FAA CERTIFIED. IT HAPPENS AT MANY DIFFERENT PLACES BUT THE LAS DIRECT GFS DIRECT HEC RTE SEEMS TO BE ONE OF THE WORST. WITH AN INBOUND TRACK OF 157 DEGS INTO GFS, THE FMS STARTS A 5-7 DEG BANK TURN WHEN 17 MI AWAY FROM THE GFS FIX TO ROLL OUT ON COURSE OUTBOUND (238 DEG TRACK) TO HEC. IF ALLOWED TO CONTINUE, THE ACFT WILL PASS 6-7 MI ABEAM GFS, FAR OFF THE AIRWAY. THERE IS NO APPARENT REASON WHY THE FMS WON'T USE A LARGER BANK ANGLE AS IT WILL DO IT MOST OTHER PLACES. BY GOING 'INTERCEPT LEG' FROM GFS TO HEC THE FMS WILL PASS ABOUT 3 MI ABEAM GFS WHICH IS SATISFACTORY BUT YOU SHOULDN'T HAVE TO 'TRICK' THE FMS TO DO A GOOD JOB. IT'S BAD WHEN YOU HAVE AN FAA CERTIFIED FMS THAT YOU CANNOT USE AS IT WAS CERTIFIED TO BE USED BECAUSE IT WON'T PROVIDE STANDARD AND ACCEPTED ACCURACY AT TURN POINTS. ASKED THE ZLA CTLRS WHO WORK THE GFS AREA, I KNOW THEY HAVE SEEN A LOT OF THIS HAPPEN WITH THE LGT. CALLBACK CONVERSATION WITH REPORTER REVEALED THE FOLLOWING: RPTR SEEMS CONVINCED THAT THERE IS A PROB WITH THE SOFTWARE IN THE FMS. HE CITED OTHER PLACES WHERE THE SAME PROB SEEMS TO HAPPEN REPEATEDLY. HE SAID HIS ACR HAS COME OUT WITH A NOTICE THAT CREWS SHOULD USE THE 'OVERFLY' MODE RATHER THAN THE 'INTERCEPT LEG' MODE AT CERTAIN FIXES. HE REITERATED THAT THE FMS SEEMS TO BEGIN THE TURN EARLY AND THEN LIMITS ITS BANK ANGLE TO 5-7 DEGS. ONCE AGAIN, THIS SEEMS TO HAPPEN ONLY AT CERTAIN FIXES.

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.