Narrative:

We were cleared to cross psp at FL190 bands at 14000 ft. After crossing psp we were surprised by the controller's comment that our clearance was to cross psp at FL190 as though we had not done so. We acknowledged his statement. Later we discovered an error in the FMS that might have indicated our position slightly early. No corrective action was needed. We called to investigate this possible error and they confirmed most aircraft equipped with VLF omega are experiencing these problems. Callback conversation with reporter revealed the following information: reporter says the controller wasn't happy with their crossing clearance, but they were only 1.8 mi from psp, so it wasn't a lot. But, reporter says they were off a few hundred ft so that may have been more the issue. As to the accuracy of the FMS, it uses VLF, basically LORAN and that navigation inputs to the FMS which also has other inputs including GPS. When the omega inputs are erroneous, the system is designed to alert the flight crew by a blinking light that says 'sensors miscompare.' then, the system is supposed to disconnect the erroneous input. Reporter's company had received a memo which is included in the report about the shutting down of the annapolis VLF and moving it to hawaii. That could have been causing navigation problems, but reporter says even after deselecting the annapolis VLF, they still had errors. Normally, the flight crew switch selects the FMS on the PF CDI and the PNF on the VOR. The error was so small that the first officer, in this incident, didn't think anything was unusual, but reporter says that the omega system error wasn't functioning correctly and disconnecting, and as a result, was dragging the other system into error. They had contacted lear corporation avionics shop about the VLF problem and they said to deselect the bad VLF station, but reporter says that didn't correct it. Now, reporter says, at the first sign of error, flag, he disconnects the VLF omega. Aircraft LR35.

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Original NASA ASRS Text

Title: VLF OMEGA NAV ERROR CAUSED FMS ERROR AND FLC WAS OFF SLIGHTLY ON XING RESTR. RPTR CAPT SAYS THEY WERE ONLY 1 PT 8 MI FROM XING FIX AND ALSO A FEW HUNDRED FT. VLF STATION SHUTDOWN HAS AFFECTED NAV ACCURACY OF VLF NAV EQUIP.

Narrative: WE WERE CLRED TO CROSS PSP AT FL190 BANDS AT 14000 FT. AFTER XING PSP WE WERE SURPRISED BY THE CTLR'S COMMENT THAT OUR CLRNC WAS TO CROSS PSP AT FL190 AS THOUGH WE HAD NOT DONE SO. WE ACKNOWLEDGED HIS STATEMENT. LATER WE DISCOVERED AN ERROR IN THE FMS THAT MIGHT HAVE INDICATED OUR POS SLIGHTLY EARLY. NO CORRECTIVE ACTION WAS NEEDED. WE CALLED TO INVESTIGATE THIS POSSIBLE ERROR AND THEY CONFIRMED MOST ACFT EQUIPPED WITH VLF OMEGA ARE EXPERIENCING THESE PROBS. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR SAYS THE CTLR WASN'T HAPPY WITH THEIR XING CLRNC, BUT THEY WERE ONLY 1.8 MI FROM PSP, SO IT WASN'T A LOT. BUT, RPTR SAYS THEY WERE OFF A FEW HUNDRED FT SO THAT MAY HAVE BEEN MORE THE ISSUE. AS TO THE ACCURACY OF THE FMS, IT USES VLF, BASICALLY LORAN AND THAT NAV INPUTS TO THE FMS WHICH ALSO HAS OTHER INPUTS INCLUDING GPS. WHEN THE OMEGA INPUTS ARE ERRONEOUS, THE SYS IS DESIGNED TO ALERT THE FLC BY A BLINKING LIGHT THAT SAYS 'SENSORS MISCOMPARE.' THEN, THE SYS IS SUPPOSED TO DISCONNECT THE ERRONEOUS INPUT. RPTR'S COMPANY HAD RECEIVED A MEMO WHICH IS INCLUDED IN THE RPT ABOUT THE SHUTTING DOWN OF THE ANNAPOLIS VLF AND MOVING IT TO HAWAII. THAT COULD HAVE BEEN CAUSING NAV PROBS, BUT RPTR SAYS EVEN AFTER DESELECTING THE ANNAPOLIS VLF, THEY STILL HAD ERRORS. NORMALLY, THE FLC SWITCH SELECTS THE FMS ON THE PF CDI AND THE PNF ON THE VOR. THE ERROR WAS SO SMALL THAT THE FO, IN THIS INCIDENT, DIDN'T THINK ANYTHING WAS UNUSUAL, BUT RPTR SAYS THAT THE OMEGA SYS ERROR WASN'T FUNCTIONING CORRECTLY AND DISCONNECTING, AND AS A RESULT, WAS DRAGGING THE OTHER SYS INTO ERROR. THEY HAD CONTACTED LEAR CORPORATION AVIONICS SHOP ABOUT THE VLF PROB AND THEY SAID TO DESELECT THE BAD VLF STATION, BUT RPTR SAYS THAT DIDN'T CORRECT IT. NOW, RPTR SAYS, AT THE FIRST SIGN OF ERROR, FLAG, HE DISCONNECTS THE VLF OMEGA. ACFT LR35.

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.