Narrative:

After our departure from seattle in the dc-10 we were cleared direct to our oceanic gateway, hemlo. To get a more precise position I elected to fly the published radial to the gateway instead of using the omega. I chose the #2 VOR. The #1 VOR CDI indicated the radial to be 4 degrees to the left, within tolerance for the vors. As it turned out it was probably correct, and so, we were about 9 mi off to start with. Both omegas showed a disparity at hemlo. I performed a precision update while the captain elected to leave his alone. I continued to navigation using the #2 omega. Upon obtaining radar contact, ZHN notified us that we were 34 mi off course. Shortly thereafter my omega showed an ambiguity. The japan station has been off the air for maintenance during oct. This definitely had an effect on my units, as the geometry of the remaining stations is poor, and we had been having omega problems all month long. In reality I don't think it is possible to navigation reliably along the sea-hnl route with japan OTS, using omegas only. Some days are better than others, with such things as solar activity, time of day, and antenna condition affecting performance. Even if we had 'split the difference' between the omegas, the pilot's time honored method of dealing with such problems, we would have still been 28 mi off, since the #1 omega was 20 mi in error as well. Perhaps the best course of action would have been to be rerted to a more southerly track, where pattern geometry is better, or to turn back. Without 2 good omegas we probably shouldn't be out in oceanic airspace. Going 'by the book' would have prevented the problem in this case.

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Original NASA ASRS Text

Title: OMEGAS DRIFTED ACFT 34 MI OFF COURSE. HDG TRACK POS DEV.

Narrative: AFTER OUR DEP FROM SEATTLE IN THE DC-10 WE WERE CLRED DIRECT TO OUR OCEANIC GATEWAY, HEMLO. TO GET A MORE PRECISE POS I ELECTED TO FLY THE PUBLISHED RADIAL TO THE GATEWAY INSTEAD OF USING THE OMEGA. I CHOSE THE #2 VOR. THE #1 VOR CDI INDICATED THE RADIAL TO BE 4 DEGS TO THE L, WITHIN TOLERANCE FOR THE VORS. AS IT TURNED OUT IT WAS PROBABLY CORRECT, AND SO, WE WERE ABOUT 9 MI OFF TO START WITH. BOTH OMEGAS SHOWED A DISPARITY AT HEMLO. I PERFORMED A PRECISION UPDATE WHILE THE CAPT ELECTED TO LEAVE HIS ALONE. I CONTINUED TO NAV USING THE #2 OMEGA. UPON OBTAINING RADAR CONTACT, ZHN NOTIFIED US THAT WE WERE 34 MI OFF COURSE. SHORTLY THEREAFTER MY OMEGA SHOWED AN AMBIGUITY. THE JAPAN STATION HAS BEEN OFF THE AIR FOR MAINT DURING OCT. THIS DEFINITELY HAD AN EFFECT ON MY UNITS, AS THE GEOMETRY OF THE REMAINING STATIONS IS POOR, AND WE HAD BEEN HAVING OMEGA PROBS ALL MONTH LONG. IN REALITY I DON'T THINK IT IS POSSIBLE TO NAV RELIABLY ALONG THE SEA-HNL RTE WITH JAPAN OTS, USING OMEGAS ONLY. SOME DAYS ARE BETTER THAN OTHERS, WITH SUCH THINGS AS SOLAR ACTIVITY, TIME OF DAY, AND ANTENNA CONDITION AFFECTING PERFORMANCE. EVEN IF WE HAD 'SPLIT THE DIFFERENCE' BTWN THE OMEGAS, THE PLT'S TIME HONORED METHOD OF DEALING WITH SUCH PROBS, WE WOULD HAVE STILL BEEN 28 MI OFF, SINCE THE #1 OMEGA WAS 20 MI IN ERROR AS WELL. PERHAPS THE BEST COURSE OF ACTION WOULD HAVE BEEN TO BE RERTED TO A MORE SOUTHERLY TRACK, WHERE PATTERN GEOMETRY IS BETTER, OR TO TURN BACK. WITHOUT 2 GOOD OMEGAS WE PROBABLY SHOULDN'T BE OUT IN OCEANIC AIRSPACE. GOING 'BY THE BOOK' WOULD HAVE PREVENTED THE PROB IN THIS CASE.

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.