Narrative:

Descending into teb. Given direct mugzy intersection to cross at 6000 ft (normal). On the way to mugzy we were given a 180 degree heading for traffic. We were in light rain at the time. The controller then gave us direct stw VOR. We punched in stw and it gave us about a 085 degree heading. After 2 mins the controller asked us if we were going direct stw, we said yes we are. He then turned us on a 150 degree heading for stw VOR. I used our backups at this point to then confirm our position. Stw VOR was 12.5 mi south of us on a 170 degree heading. VLF omega was off. My guess about 62 mi from my calculations. We were in showery precipitation which often causes VLF omegas to go to dead reckoning mode. But this one did not. Therefore we had no warning that the system was that far off. I will from now on check and make sure that every instrument is pointing the same way and doublechk the VLF position every few mins. Callback conversation with reporter revealed the following information: aircraft type LR35. Reporter captain wasn't sure what happened to cause such a gross navigation error. The system 'comparator' is supposed to compare the universal GPS information to the VLF omega, and if the omega is out of limits, it should provide a warning to the flight crew -- 4 mins after the error is compared and, if the error continues to 6 mins, it is supposed to go into dr, dead reckoning, mode. Normally, reporter says, they would have noticed the great error between the FMS omega sooner had they been on a specific VOR radial, but being on heading vectors and then cleared direct to a VOR, they didn't catch it until the controller questioned them. Reporter says he estimates their omega system was 62 mi off.

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Original NASA ASRS Text

Title: OMEGA VLF ERROR IN NAV SYS SHOULD HAVE ALERTED FLC OF ERROR, BUT DIDN'T. ESTIMATE OMEGA WAS 62 MI OFF AND FLC DIDN'T CATCH IT DUE TO HDG VECTORS AND NO CONSTANT POINT TO CHK THE NAV ACCURACY.

Narrative: DSNDING INTO TEB. GIVEN DIRECT MUGZY INTXN TO CROSS AT 6000 FT (NORMAL). ON THE WAY TO MUGZY WE WERE GIVEN A 180 DEG HDG FOR TFC. WE WERE IN LIGHT RAIN AT THE TIME. THE CTLR THEN GAVE US DIRECT STW VOR. WE PUNCHED IN STW AND IT GAVE US ABOUT A 085 DEG HDG. AFTER 2 MINS THE CTLR ASKED US IF WE WERE GOING DIRECT STW, WE SAID YES WE ARE. HE THEN TURNED US ON A 150 DEG HDG FOR STW VOR. I USED OUR BACKUPS AT THIS POINT TO THEN CONFIRM OUR POS. STW VOR WAS 12.5 MI S OF US ON A 170 DEG HDG. VLF OMEGA WAS OFF. MY GUESS ABOUT 62 MI FROM MY CALCULATIONS. WE WERE IN SHOWERY PRECIPITATION WHICH OFTEN CAUSES VLF OMEGAS TO GO TO DEAD RECKONING MODE. BUT THIS ONE DID NOT. THEREFORE WE HAD NO WARNING THAT THE SYS WAS THAT FAR OFF. I WILL FROM NOW ON CHK AND MAKE SURE THAT EVERY INST IS POINTING THE SAME WAY AND DOUBLECHK THE VLF POS EVERY FEW MINS. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: ACFT TYPE LR35. RPTR CAPT WASN'T SURE WHAT HAPPENED TO CAUSE SUCH A GROSS NAV ERROR. THE SYS 'COMPARATOR' IS SUPPOSED TO COMPARE THE UNIVERSAL GPS INFO TO THE VLF OMEGA, AND IF THE OMEGA IS OUT OF LIMITS, IT SHOULD PROVIDE A WARNING TO THE FLC -- 4 MINS AFTER THE ERROR IS COMPARED AND, IF THE ERROR CONTINUES TO 6 MINS, IT IS SUPPOSED TO GO INTO DR, DEAD RECKONING, MODE. NORMALLY, RPTR SAYS, THEY WOULD HAVE NOTICED THE GREAT ERROR BTWN THE FMS OMEGA SOONER HAD THEY BEEN ON A SPECIFIC VOR RADIAL, BUT BEING ON HDG VECTORS AND THEN CLRED DIRECT TO A VOR, THEY DIDN'T CATCH IT UNTIL THE CTLR QUESTIONED THEM. RPTR SAYS HE ESTIMATES THEIR OMEGA SYS WAS 62 MI OFF.

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.