Narrative:

In cruise flight, level at FL270 in the vicinity of hvq VOR, the crew received an 'EFIS computer monitor' caution message for heading discrepancy. The appropriate checklist procedures were performed via the aircraft's QRH. The fault cleared, and crew verified that it was the captain's heading indicator at fault. Approximately 10 mins later, the fault reoccurred. Again the checklist procedures were performed. Shortly after, the crew noticed the FMS making a drastic left 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 navigation to heading mode to begin right turn back towards proper heading, and immediately went manual ('green data') VOR navigation 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 aircraft, maintained a visual of the aircraft, and determined there to be no conflict. ATC then queried about the course deviation. Crew advised the controller of the apparent heading fault and that the FMS was no longer being used for navigation due to unreliable information. Since we were navigating by basic VOR to VOR navigation, ATC provided a suitable heading to join the hvq 095 degree radial. We proceeded to the destination, iad, via the JASEN3 arrival, safely without incident. Upon arrival at the gate, maintenance was called and they were informed of the fault messages we had experienced. Prior to the flight, maintenance had replaced the #1 altitude/heading reference system (ahrs).

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Original NASA ASRS Text

Title: 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).

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.