Narrative:

In cruise; at FL270 along J51; 100 NM south of fak; the FMS had lost itself with respect to its position. A message flagged us 'check position' meaning check position; where I updated the position via the update position function on the FMS. I had to do this about 3 different times. There was not a need to report the FMS as failed because I could successfully update the position. I believe on the 3RD time this occurred we were on a pretty large intercept angle to reintercept the airway when ATC asked us our heading; to which I had replied we were rejoining the airway and that I was having a little trouble with the FMS navigation and that I was correcting it. I do not know the exact distance we were off course; but I do know it was less than 10 mi for certain; but I am pretty sure that it was 5 NM or less. My navigation screen was zoomed out to the 80 NM scale making small distances difficult to determine. When I got the FMS position corrected I zoomed in on the navigation screen and it only showed me about 3 NM off course. I asked ATC if there were any issues or problems with our navigation problem; and he said no; and thanked me for asking. No other issues occurred under this problem. This problem could have been avoided if all turbine part 121 carriers with FMS were required to have GPS to aid the FMS in pinpoint accuracy navigation.

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Original NASA ASRS Text

Title: A CL65 CAPT DEVIATED FROM THE CLEARED COURSE WHEN HIS FMS MALFUNCTIONED.

Narrative: IN CRUISE; AT FL270 ALONG J51; 100 NM S OF FAK; THE FMS HAD LOST ITSELF WITH RESPECT TO ITS POS. A MESSAGE FLAGGED US 'CHK POS' MEANING CHK POS; WHERE I UPDATED THE POS VIA THE UPDATE POS FUNCTION ON THE FMS. I HAD TO DO THIS ABOUT 3 DIFFERENT TIMES. THERE WAS NOT A NEED TO RPT THE FMS AS FAILED BECAUSE I COULD SUCCESSFULLY UPDATE THE POS. I BELIEVE ON THE 3RD TIME THIS OCCURRED WE WERE ON A PRETTY LARGE INTERCEPT ANGLE TO REINTERCEPT THE AIRWAY WHEN ATC ASKED US OUR HDG; TO WHICH I HAD REPLIED WE WERE REJOINING THE AIRWAY AND THAT I WAS HAVING A LITTLE TROUBLE WITH THE FMS NAV AND THAT I WAS CORRECTING IT. I DO NOT KNOW THE EXACT DISTANCE WE WERE OFF COURSE; BUT I DO KNOW IT WAS LESS THAN 10 MI FOR CERTAIN; BUT I AM PRETTY SURE THAT IT WAS 5 NM OR LESS. MY NAV SCREEN WAS ZOOMED OUT TO THE 80 NM SCALE MAKING SMALL DISTANCES DIFFICULT TO DETERMINE. WHEN I GOT THE FMS POS CORRECTED I ZOOMED IN ON THE NAV SCREEN AND IT ONLY SHOWED ME ABOUT 3 NM OFF COURSE. I ASKED ATC IF THERE WERE ANY ISSUES OR PROBS WITH OUR NAV PROB; AND HE SAID NO; AND THANKED ME FOR ASKING. NO OTHER ISSUES OCCURRED UNDER THIS PROB. THIS PROB COULD HAVE BEEN AVOIDED IF ALL TURBINE PART 121 CARRIERS WITH FMS WERE REQUIRED TO HAVE GPS TO AID THE FMS IN PINPOINT ACCURACY NAV.

Data retrieved from NASA's ASRS site as of May 2009 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.