ACR LGT TRACK HDG DEV USING OMEGA NAV.

Narrative

ON A SCHEDULED AIRLINE FLT FROM PVD (PROVIDENCE; RI) TO RDU (RALEIGH-DURHAM; NC) WHILE CRUISING AT FL270; THE FOLLOWING SITUATION DEVELOPED. AFTER A COMPLEX DEP OUT OF PVD; WE WERE CLRED DIRECT TO NATLY INTXN (WHICH IS NOT PROGRAMMED INTO OUR OMEGA/VLF). WE ENTERED ITS COORDINATES AND PROCEEDED DIRECT TO NATLY. UPON ARR AT NATLY; THE OMEGA TURNED ON COURSE SBOUND DOWN THE AIRWAY (J174). DURING THE LATER PART OF THE CLBOUT; THE VOR INDICATIONS HAD BEEN VERY ERRATIC AND I SUSPECT SOMEONE IN THE CABIN WAS USING A CELLULAR TELEPHONE OR OTHER ILLEGAL RF EMITTER NOT APPROVED FOR USE AND I MENTIONED THIS FACT AS WE MIGHT WANT TO HAVE THE FLT ATTENDANTS CHK THIS IN THE CABIN. THE NOTAM; ISSUED WITH OUR FLT PLAN/RELEASE; DID NOT CONTAIN ANY VORTAC OUTAGES ALONG OUR RTE; BUT DID STATE THAT OMEGA POLAR PATH DISTURBANCES HAVE COMMENCED. RIGHT ABOUT THE TIME THE FLT ATTENDANT BROUGHT THE CAPT'S AND FLT ENGINEER'S LUNCHES AND MY CUP OF COFFEE; THE ACFT STARTED IN A SLOW L TURN; WHICH I NOTICED AFTER ABOUT 20-30 DEGS OF HDG CHANGE. I THEN DISENGAGED THE OMEGA AND COMMENCED A R TURN BACK TO THE AIRWAY; AND AS I DID SO CENTER ISSUED A 40 DEG R TURN TO REINTERCEPT THE AIRWAY AND TOLD US WE WERE 8.2 MI L OF CENTERLINE. WE CORRECTED BACK USING THE VOR WHICH WAS STILL SLIGHTLY 'SHAKY' EVEN THOUGH IT WAS WELL WITHIN USABLE RANGE AND HAD A CLR IDENT. THE OMEGA UNIT SHOWED NO MALFUNCTION LIGHTS OR INDICATIONS PRIOR TO OR DURING THIS TIME. IT WAS LOADED; PRE-DEP; VIA ACARS AT THE GATE AND ALL DOMESTIC PRE-DEP CHKS WERE PERFORMED; EACH WAYPOINTS AND COORDINATES WERE CHKED; AND AT THE ARR GATE AN ACCURACY CHK SHOWED A 13.2 NM ERROR; WHICH IS; OF COURSE UNACCEPTABLE FOR USE. I DO NOT KNOW IF RF EMITTERS IN THE CABIN EFFECT OMEGA IN ANY WAY. AS IS OFTEN THE CASE; THE OMEGA/ULF'S MANY WEAK POINTS ALL COME TOGETHER AND COUPLED WITH A SLOW HDG CHANGE POINT OUT AND REINFORCE THE NEED FOR JUST AS GREAT A VIGILANCE AS WHEN HAND FLYING VOR AIRWAYS!