A B737 CREW; IN CRUISE AT FL330; WERE QUERIED BY ATC AS TO THEIR POS RELATIVE TO SWL VORTAC; THEN ISSUED A HEADING TO REJOIN THE FILED AIRWAY.

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ON FLT FROM FT LAUDERDALE INTL ARPT (FLL) TO LONG ISLAND MAC ARTHUR (ISP) WE REQUESTED A DIRECT RTE TO CALVERTON VORTAC (CCC; 117.20 MHZ) FROM ZDC. THE CTLR THEN ASKED OUR POS AND TO VERIFY OUR ASSIGNED ROUTING. WE RESPONDED WITH; 'J174 THEN J121 CCC ETC; TO ISP.' HE REMARKED ON OUR PRESENT POS AND REQUESTED A TURN TO REJOIN J121. HAVING FLOWN FAA FLT CHK PREVIOUSLY FOR 3 YRS; (WE) I FELT A MAJOR CONTRIBUTOR IN THIS DISCREPANCY IS THE UNRELIABILITY OF SNOW HILL VORTAC (SWL; 112.40 MHZ). WE NOTED OUR POS AS SLIGHTLY E OF J121 (NO MORE THAN 4 MI); HOWEVER; WE COMPLIED WITH THE CTLR'S INSTRUCTIONS. THE PERIODICITY FOR VOR FLT CHKS HAS OVER THE YRS INCREASED FROM 365 DAYS TO 1086 DAYS. THIS ALLOWS A FACILITY THE OPPORTUNITY TO BECOME LESS THAN ADEQUATE FOR NAV OVER A GREATER PERIOD OF TIME. USER COMPLAINTS ON THIS FACILITY ARE/HAVE BEEN UNADDRESSED ACCORDING TO OUR ATC QUERY. IN VIEW OF THIS PERHAPS A JOINT EFFORT BTWN PLTS (OF MANUAL/TRADITIONAL NAV ACFT) AND ATC CTLRS; FOR VIGILANCE AS TO CTRLINE/ON-COURSE VERIFICATION WOULD THWART ANY POSSIBLE OFF-COURSE; NAV ERROR PROBS FROM OCCURRING. SUPPLEMENTAL INFO FROM ACN 563101: I THINK THE MAJORITY OF CONTRIBUTION LIES WITH THE EVER PRESENT UNRELIABILITY OF SWL AS A NAVAID. WINDS HAD SHIFTED SIGNIFICANTLY TO THE W FROM THE PREVIOUS EVENING'S TRAVELS AS WELL; AND WERE UNUSUALLY STRONG. CREW AND CTLR VIGILANCE ARE ALSO AREAS WHICH LEAVE ROOM FOR IMPROVEMENT; AS ALWAYS; ESPECIALLY WERE NAV OFF OF SWL WITH TRADITIONAL NON GPS; NON FMS ACFT ARE CONCERNED. CALLBACK CONVERSATION WITH ARTCC SPECIALIST ON ACN 563100: A CONVERSATION WITH AN ARTCC SPECIALIST REVEALED THAT THE 'SNOW HILL' VORTAC DOES INDEED EXPRESS NAV WAVERING 3-5 MI E OF THE PROJECTED AIRWAY. MAINT ON THE FACILITY; PLANNED DURING NOV OF 2002; HAS BEEN RESCHEDULED FOR JAN OF 2003.