NAV ERROR.

Narrative

PRIOR TO TAKING COMMAND OF A MD-82 FROM LGA TO RDU; MAINT WAS ON BOARD REPLACING A VLF/OMEGA NAV UNIT. THE PREVIOUS CREW WROTE A COMPLAINT IN THE ACFT LOG STATING THAT THE UNIT WAS UNRELIABLE FOR NAV DUE TO TRACK ERRORS IN EXCESS OF 30 NM. THE UNIT WAS REPLACED AND TESTED BY MAINT PERSONNEL. AFTER WITNESSING ITS TEST; I PROCEEDED TO INITIALIZE AND TEST ITS ACCURACY. IT TESTED TO BE WITHIN ACCEPTABLE LIMITS AND APPEARED TO OPERATE NORMALLY. SOON AFTER DEPARTING THE LGA TERMINAL AREA; WE WERE INSTRUCTED TO NAV FROM OUR PRESENT POS TO WHITE INTXN. WE PROCEEDED DIRECT TO THAT INTXN ACCURATELY. AFTERWARD I RECALL BEING VECTORED OFF COURSE FOR TFC. WE WERE THEN CLRED BACK ON COURSE. AT THIS TIME THE NAV SYS WAS BECOMING UNRELIABLE EVIDENCED BY A 'DR' ANNUNCIATOR. WE WERE OFF THE AIRWAY SYS AND FOUND IT DIFFICULT TO RETURN TO OUR COURSE WITH ACCURACY. WE WERE DRIFTING E OF COURSE NEAR A RESTR AREA NEAR THE PXT VOR. WE WERE VECTORED BACK ON COURSE BY ATC. WE RPTED THE FAILURE OF THE RNAV EQUIP AND ASKED AND RECEIVED VECTORS BACK TO THE AIRWAY SYS. BOTH VORS' WERE TUNED PROPERLY BUT WERE NOT STABLE; NOT ALLOWING FOR VERY ACCURATE NAV. BROOKE (BRV) VOR WAS PART OF THE COURSE AND WAS PARTICULARLY UNSTABLE. AFTER PASSING PXT VOR WE MISREAD OUR FILED FLT PLAN. WE WERE ON J191 WHEN WE SHOULD HAVE BEEN ON A SEPARATE RADIAL NOT DEPICTED ON THE CHARTS. ATC ASKED OUR CLRNC. WE STATED J191 TO HOPEWELL VOR. IT SHOULD HAVE BEEN A SEPARATE RADIAL TO INTERCEPT ANOTHER FROM HOPEWELL. WE WERE THEN RECLRED DIRECTLY TO HOPEWELL VOR. ALTHOUGH WE WERE NEVER MORE THAN A FEW MI OFF OUR AIRWAY; IT IS A VERY TIGHT AREA AS FAR AS AIRWAYS. WE WERE NOT IN CONFLICT WITH ANY OTHER TFC. CONTRIBUTING TO THE ERRORS WAS AN INACCURATE RNAV SYS IN COMPACTED AIRWAY SYS; AND UNSTABLE VOR INDICATIONS. MISREADING THE FILED CLRNC WAS A LACK OF ATTN TO THE DETAILS OF WHAT WAS FILED AND NOT WHAT WAS OBVIOUS ON THE CHART. ADDITIONALLY THERE WAS AN EBOUND XWIND IN EXCESS OF 120 KTS REQUIRING 30-40 DEGS CORRECTION ANGLES WHICH COMPLICATED THE SIT.