Narrative:

On a flight from seattle to honolulu we encountered problems with our omega navigation system. The widebody transport we were flying has a dual omega installation as the sole means of overwater navigation. Shortly after passing sedar gateway outbnd the #1 omega went into ambiguity. It went into and out of the ambiguity mode several times during the flight, sometimes coming out of ambiguity by itself and at other times by crew action following the procedures given in the omega operating manual. The difference in distance between the 2 units was 4 mi or less for the first 4 hours of the flight. Signal strength from several ground stations was good. However the system accuracy (auxiliary 4) remained at 4-6, which I consider only fair from past experience. Approximately 1 hour out of hnl both omegas went into ambiguity and they diverged to a greater degree, approximately 10 mi apart. At this point we notified hnl center of 'navigation unreliability,' and asked for radar identify. A few mins later we were idented and were approximately 10-15 mi from our indicated position. Center then gave us vectors (included a warning area) and the flight landed normally. At the ramp the omegas were found to be 15 and 14 mi off, respectively. From past experience operating omegas throughout the world, and on this particular route for the past 3 yrs, it is evident that omegas are affected not only by the time of day and position over the earth's surface, but by the various seasons as well. On this route (sea-hnl) reliability has been virtually 100% during the spring, summer and fall. However, during the winter, instances such as described above have been quite common. One other thought: beyond our arrival gateway was a warning area. Not being cognizant of a navigation problem here could have posed another problem in addition to being off course. In summary, by following proper procedures (outlined in the afm section on navigation and the omega operating manual) we were able to avoid a large off course excursion, make ATC aware of our predicament and not create a conflict with other aircraft. Callback conversation with reporter revealed the following: conversation revealed that the procedure tried was the clear ambiguity procedure, which would momentarily clear the ambiguity, but then it would reappear. Reporter did not try deselecting norway, which is very subject to polar disturbances from sunspot activity in winter months and he apparently never heard of doing this nor the problem caused by polar disturbances. This procedure is apparently not in this operations manual.

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Original NASA ASRS Text

Title: ACR WDB TRACK DEVIATION ON PACIFIC TRACK SYSTEM.

Narrative: ON A FLT FROM SEATTLE TO HONOLULU WE ENCOUNTERED PROBS WITH OUR OMEGA NAV SYS. THE WDB WE WERE FLYING HAS A DUAL OMEGA INSTALLATION AS THE SOLE MEANS OF OVERWATER NAV. SHORTLY AFTER PASSING SEDAR GATEWAY OUTBND THE #1 OMEGA WENT INTO AMBIGUITY. IT WENT INTO AND OUT OF THE AMBIGUITY MODE SEVERAL TIMES DURING THE FLT, SOMETIMES COMING OUT OF AMBIGUITY BY ITSELF AND AT OTHER TIMES BY CREW ACTION FOLLOWING THE PROCS GIVEN IN THE OMEGA OPERATING MANUAL. THE DIFFERENCE IN DISTANCE BTWN THE 2 UNITS WAS 4 MI OR LESS FOR THE FIRST 4 HRS OF THE FLT. SIGNAL STRENGTH FROM SEVERAL GND STATIONS WAS GOOD. HOWEVER THE SYS ACCURACY (AUX 4) REMAINED AT 4-6, WHICH I CONSIDER ONLY FAIR FROM PAST EXPERIENCE. APPROX 1 HR OUT OF HNL BOTH OMEGAS WENT INTO AMBIGUITY AND THEY DIVERGED TO A GREATER DEGREE, APPROX 10 MI APART. AT THIS POINT WE NOTIFIED HNL CENTER OF 'NAV UNRELIABILITY,' AND ASKED FOR RADAR IDENT. A FEW MINS LATER WE WERE IDENTED AND WERE APPROX 10-15 MI FROM OUR INDICATED POS. CENTER THEN GAVE US VECTORS (INCLUDED A WARNING AREA) AND THE FLT LANDED NORMALLY. AT THE RAMP THE OMEGAS WERE FOUND TO BE 15 AND 14 MI OFF, RESPECTIVELY. FROM PAST EXPERIENCE OPERATING OMEGAS THROUGHOUT THE WORLD, AND ON THIS PARTICULAR ROUTE FOR THE PAST 3 YRS, IT IS EVIDENT THAT OMEGAS ARE AFFECTED NOT ONLY BY THE TIME OF DAY AND POS OVER THE EARTH'S SURFACE, BUT BY THE VARIOUS SEASONS AS WELL. ON THIS ROUTE (SEA-HNL) RELIABILITY HAS BEEN VIRTUALLY 100% DURING THE SPRING, SUMMER AND FALL. HOWEVER, DURING THE WINTER, INSTANCES SUCH AS DESCRIBED ABOVE HAVE BEEN QUITE COMMON. ONE OTHER THOUGHT: BEYOND OUR ARR GATEWAY WAS A WARNING AREA. NOT BEING COGNIZANT OF A NAV PROB HERE COULD HAVE POSED ANOTHER PROB IN ADDITION TO BEING OFF COURSE. IN SUMMARY, BY FOLLOWING PROPER PROCS (OUTLINED IN THE AFM SECTION ON NAV AND THE OMEGA OPERATING MANUAL) WE WERE ABLE TO AVOID A LARGE OFF COURSE EXCURSION, MAKE ATC AWARE OF OUR PREDICAMENT AND NOT CREATE A CONFLICT WITH OTHER ACFT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING: CONVERSATION REVEALED THAT THE PROC TRIED WAS THE CLEAR AMBIGUITY PROC, WHICH WOULD MOMENTARILY CLEAR THE AMBIGUITY, BUT THEN IT WOULD REAPPEAR. RPTR DID NOT TRY DESELECTING NORWAY, WHICH IS VERY SUBJECT TO POLAR DISTURBANCES FROM SUNSPOT ACTIVITY IN WINTER MONTHS AND HE APPARENTLY NEVER HEARD OF DOING THIS NOR THE PROB CAUSED BY POLAR DISTURBANCES. THIS PROC IS APPARENTLY NOT IN THIS OPS MANUAL.

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.