Narrative:

All procedures and INS checks were normal during preflight and after takeoff. The INS coordinates for present position were inserted by the first officer and verified by both the captain and the second officer. All the waypoint coordinates were inserted by the first officer and verified by the captain from a different document. All distances between waypoints were checked by both the captain and the first officer. After takeoff, 2 geographical position checks were made: 1) on the true north radial of upolu point VOR (73 NM), and 2) on the 030 degree right/159 NM (ebber) of the hilo VOR. Both INS navigation systems were accurate at both of these fixes. The flight continued normal in all respects along route R577, with all normal INS checks and plots, until a point east of etnic. At this point the aircraft was being flown by the first officer using the #2 INS system for steering. On a routine check of the x-track error, the captain noted and informed the first officer and second officer of a 1.5 mi error on the #1 system, increasing. We compared all functions of both INS systems and were unable to determine which system was in error. After referring to the procedures in the flight manual, we continued in normal operation until the x-track error increased to 10 NM. At this point (approximately escro), we used the handbook procedure and flew a course averaging the x-track difference of the 2 systems. Sfo ATC was notified that we had a malfunctioning INS and that our position was in doubt. In addition, airlines dispatch and maintenance were notified and a captain's report completed. Upon first painting by lax radar, approaching edsel, we were informed that we were 30 NM north of course. It was obvious at this time that the #2 INS system was malfunctioning. The INS post-flight distance check at lax showed the #1 system with a 7 NM error and the #2 system with a 67 NM error.

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Original NASA ASRS Text

Title: INS MALFUNCTION CAUSES OVER-WATER TRACK DEVIATION IN NON RADAR ENVIRONMENT.

Narrative: ALL PROCS AND INS CHKS WERE NORMAL DURING PREFLT AND AFTER TKOF. THE INS COORDINATES FOR PRESENT POS WERE INSERTED BY THE F/O AND VERIFIED BY BOTH THE CAPT AND THE S/O. ALL THE WAYPOINT COORDINATES WERE INSERTED BY THE F/O AND VERIFIED BY THE CAPT FROM A DIFFERENT DOCUMENT. ALL DISTANCES BTWN WAYPOINTS WERE CHKED BY BOTH THE CAPT AND THE F/O. AFTER TKOF, 2 GEOGRAPHICAL POS CHKS WERE MADE: 1) ON THE TRUE N RADIAL OF UPOLU POINT VOR (73 NM), AND 2) ON THE 030 DEG R/159 NM (EBBER) OF THE HILO VOR. BOTH INS NAV SYSTEMS WERE ACCURATE AT BOTH OF THESE FIXES. THE FLT CONTINUED NORMAL IN ALL RESPECTS ALONG ROUTE R577, WITH ALL NORMAL INS CHKS AND PLOTS, UNTIL A POINT E OF ETNIC. AT THIS POINT THE ACFT WAS BEING FLOWN BY THE F/O USING THE #2 INS SYS FOR STEERING. ON A ROUTINE CHK OF THE X-TRACK ERROR, THE CAPT NOTED AND INFORMED THE F/O AND S/O OF A 1.5 MI ERROR ON THE #1 SYS, INCREASING. WE COMPARED ALL FUNCTIONS OF BOTH INS SYSTEMS AND WERE UNABLE TO DETERMINE WHICH SYS WAS IN ERROR. AFTER REFERRING TO THE PROCS IN THE FLT MANUAL, WE CONTINUED IN NORMAL OPERATION UNTIL THE X-TRACK ERROR INCREASED TO 10 NM. AT THIS POINT (APPROX ESCRO), WE USED THE HANDBOOK PROC AND FLEW A COURSE AVERAGING THE X-TRACK DIFFERENCE OF THE 2 SYSTEMS. SFO ATC WAS NOTIFIED THAT WE HAD A MALFUNCTIONING INS AND THAT OUR POS WAS IN DOUBT. IN ADDITION, AIRLINES DISPATCH AND MAINT WERE NOTIFIED AND A CAPT'S RPT COMPLETED. UPON FIRST PAINTING BY LAX RADAR, APCHING EDSEL, WE WERE INFORMED THAT WE WERE 30 NM N OF COURSE. IT WAS OBVIOUS AT THIS TIME THAT THE #2 INS SYS WAS MALFUNCTIONING. THE INS POST-FLT DISTANCE CHK AT LAX SHOWED THE #1 SYS WITH A 7 NM ERROR AND THE #2 SYS WITH A 67 NM ERROR.

Data retrieved from NASA's ASRS site as of August 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.