Narrative:

IRS's were aligned at the gate in sna. Route was checked by both pilots, no discontinuities noted. IRS's indicated a proper alignment with zero ground speed. We were cleared via the back bay 2 departure takeoff and climb were normal. We were established on a 175 degree heading. We cleaned up the aircraft and continued climbing at 210 KTS. Departure gave us a heading change of approximately 20 degrees to the left for traffic. Some time later we were cleared direct to bhose. Bhose was selected as the active waypoint, executed, and l-nav engaged. The FMS indicated a course of 082 degrees to bhose. Since the captain's flight director was inoperative, the first officer selected navigation on his HSI switch in order to confirm his flight director was centered and indicating on course. As we approached bhose, the first officer switched back to VOR mode, tuned in sxc and the 061 degree radial. He indicated that we appeared south of course. The FMS indicated a crossing of bhose and then commanded a course change of approximately 062 degrees to the next waypoint. At that time, the #2 VOR showed us at sxc 055 degrees/44 degrees bhose is about sxc 061 degrees/42 degrees. Soon thereafter center called and asked if we were on the departure. We responded that the FMS showed us on course but the VOR showed us south of course. He said he painted us 4 mi south of course. He then gave us a heading change to 050 degrees for traffic. Once we cleared the traffic. We were cleared direct blythe. Blythe was selected and l-nav engaged. The course to blythe was crosschecked on the #2 VOR. The FMS course and the VOR course agreed. Shortly thereafter we had time to check the position reference page. We remember the following: FMC position N33 degrees 34. -- West 117 degrees. IRS left N33 degrees 39. -- West 117 degrees. IRS right north 33 degrees 38. -- West 117 degrees. It seems the aircraft was navigating off an erroneous FMC position since departure. Since both vors were tuned to isna, I couldn't get a good update, or the navigation data base could have an error entered or maybe 1 of the navaids it was updating off wasn't where the computer thought it should be. I believe this happened to a crew going into atl once. FMS navigation was normal the remainder of the flight.

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Original NASA ASRS Text

Title: TRACK DEV.

Narrative: IRS'S WERE ALIGNED AT THE GATE IN SNA. RTE WAS CHKED BY BOTH PLTS, NO DISCONTINUITIES NOTED. IRS'S INDICATED A PROPER ALIGNMENT WITH ZERO GND SPD. WE WERE CLRED VIA THE BACK BAY 2 DEP TKOF AND CLB WERE NORMAL. WE WERE ESTABLISHED ON A 175 DEG HDG. WE CLEANED UP THE ACFT AND CONTINUED CLBING AT 210 KTS. DEP GAVE US A HDG CHANGE OF APPROX 20 DEGS TO THE L FOR TFC. SOME TIME LATER WE WERE CLRED DIRECT TO BHOSE. BHOSE WAS SELECTED AS THE ACTIVE WAYPOINT, EXECUTED, AND L-NAV ENGAGED. THE FMS INDICATED A COURSE OF 082 DEGS TO BHOSE. SINCE THE CAPT'S FLT DIRECTOR WAS INOP, THE FO SELECTED NAV ON HIS HSI SWITCH IN ORDER TO CONFIRM HIS FLT DIRECTOR WAS CTRED AND INDICATING ON COURSE. AS WE APCHED BHOSE, THE FO SWITCHED BACK TO VOR MODE, TUNED IN SXC AND THE 061 DEG RADIAL. HE INDICATED THAT WE APPEARED S OF COURSE. THE FMS INDICATED A XING OF BHOSE AND THEN COMMANDED A COURSE CHANGE OF APPROX 062 DEGS TO THE NEXT WAYPOINT. AT THAT TIME, THE #2 VOR SHOWED US AT SXC 055 DEGS/44 DEGS BHOSE IS ABOUT SXC 061 DEGS/42 DEGS. SOON THEREAFTER CTR CALLED AND ASKED IF WE WERE ON THE DEP. WE RESPONDED THAT THE FMS SHOWED US ON COURSE BUT THE VOR SHOWED US S OF COURSE. HE SAID HE PAINTED US 4 MI S OF COURSE. HE THEN GAVE US A HDG CHANGE TO 050 DEGS FOR TFC. ONCE WE CLRED THE TFC. WE WERE CLRED DIRECT BLYTHE. BLYTHE WAS SELECTED AND L-NAV ENGAGED. THE COURSE TO BLYTHE WAS XCHKED ON THE #2 VOR. THE FMS COURSE AND THE VOR COURSE AGREED. SHORTLY THEREAFTER WE HAD TIME TO CHK THE POS REF PAGE. WE REMEMBER THE FOLLOWING: FMC POS N33 DEGS 34. -- W 117 DEGS. IRS L N33 DEGS 39. -- W 117 DEGS. IRS R N 33 DEGS 38. -- W 117 DEGS. IT SEEMS THE ACFT WAS NAVIGATING OFF AN ERRONEOUS FMC POS SINCE DEP. SINCE BOTH VORS WERE TUNED TO ISNA, I COULDN'T GET A GOOD UPDATE, OR THE NAV DATA BASE COULD HAVE AN ERROR ENTERED OR MAYBE 1 OF THE NAVAIDS IT WAS UPDATING OFF WASN'T WHERE THE COMPUTER THOUGHT IT SHOULD BE. I BELIEVE THIS HAPPENED TO A CREW GOING INTO ATL ONCE. FMS NAV WAS NORMAL THE REMAINDER OF THE FLT.

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.