A CL65 FLT CREW EXPERIENCED AN AHRS FAILURE WHICH RESULTED IN A TRACK AND HDG DEV. THE ACFT TCAS PROVIDED A TA.

Narrative

IN CRUISE FLT; LEVEL AT FL270 IN THE VICINITY OF HVQ VOR; THE CREW RECEIVED AN 'EFIS COMPUTER MONITOR' CAUTION MESSAGE FOR HDG DISCREPANCY. THE APPROPRIATE CHKLIST PROCS WERE PERFORMED VIA THE ACFT'S QRH. THE FAULT CLRED; AND CREW VERIFIED THAT IT WAS THE CAPT'S HDG INDICATOR AT FAULT. APPROX 10 MINS LATER; THE FAULT REOCCURRED. AGAIN THE CHKLIST PROCS WERE PERFORMED. SHORTLY AFTER; THE CREW NOTICED THE FMS MAKING A DRASTIC L TURN TO RE-CAPTURE THE COURSE; DUE TO AN APPARENT FMS MAP SHIFT; CAUSING THE CREW TO BE UNKNOWINGLY OFF COURSE. THE PF IMMEDIATELY SWITCHED AUTOPLT FROM NAV TO HDG MODE TO BEGIN R TURN BACK TOWARDS PROPER HDG; AND IMMEDIATELY WENT MANUAL ('GREEN DATA') VOR NAV TO EXPEDITE RETURN TO AIRWAY. DURING THE TURN BACK ON COURSE; CREW RECEIVED A TCAS TA ALERT. THE CREW OBTAINED A VISUAL SIGHTING OF THE OTHER ACFT; MAINTAINED A VISUAL OF THE ACFT; AND DETERMINED THERE TO BE NO CONFLICT. ATC THEN QUERIED ABOUT THE COURSE DEV. CREW ADVISED THE CTLR OF THE APPARENT HDG FAULT AND THAT THE FMS WAS NO LONGER BEING USED FOR NAV DUE TO UNRELIABLE INFO. SINCE WE WERE NAVING BY BASIC VOR TO VOR NAV; ATC PROVIDED A SUITABLE HDG TO JOIN THE HVQ 095 DEG RADIAL. WE PROCEEDED TO THE DEST; IAD; VIA THE JASEN3 ARR; SAFELY WITHOUT INCIDENT. UPON ARR AT THE GATE; MAINT WAS CALLED AND THEY WERE INFORMED OF THE FAULT MESSAGES WE HAD EXPERIENCED. PRIOR TO THE FLT; MAINT HAD REPLACED THE #1 ALT/HDG REF SYS (AHRS).