Narrative:

This flight was operated with the FMS on MEL. The flight operated normally with the MEL as planned up until the approach to [runway] xr at ZZZ. After intercepting the localizer to xr we noticed that the overlaid FMS course was significantly different than what the conventional nav was indicating. I asked ATC if they showed us south of course and they did. It was fairly significant and was very clear that our conventional nav was not accurate at all. We declared an emergency at this point. At that time we were at 6;000 feet MSL and broke off the approach. ATC gave us vectors; and we decided to try and see if we could line up with the localizer again. The second try it appeared to us and ATC that we were aligned with the localizer and the glideslope was accurate to the published altitudes. We planned for an ASR approach into our alternate if we would loose the conventional nav. We continued the approach believing that the conventional nav was working again; however when we popped out at approximately 400 feet we were lined up outside of the taxiway south of runway xr. We maneuvered to land on runway xr. I felt that the risk of going missed again without navigation; no VFR airports available and deteriorating weather conditions were a greater risk than landing on xr with some maneuvering we were able to land the aircraft without further incident.the threats were the FMS MEL; conventional nav failure; long landing; and low altitude maneuver in poor weather conditions. I do not believe that there was anything to be done that could have prevented this; however if I had more information to tell me that we were not aligned with the localizer on the second approach I would have gone missed early on and proceeded to ZZZ1 for the ASR approach.

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Original NASA ASRS Text

Title: An EMB-145 compass system malfunctioned during an IMC ILS approach with the FMS MEL'ed so a go-around was executed and the crew returned for a second approach which terminated near minimums slightly off the final approach course.

Narrative: This flight was operated with the FMS on MEL. The flight operated normally with the MEL as planned up until the approach to [Runway] XR at ZZZ. After intercepting the localizer to XR we noticed that the overlaid FMS course was significantly different than what the conventional nav was indicating. I asked ATC if they showed us south of course and they did. It was fairly significant and was very clear that our conventional nav was not accurate at all. We declared an emergency at this point. At that time we were at 6;000 feet MSL and broke off the approach. ATC gave us vectors; and we decided to try and see if we could line up with the localizer again. The second try it appeared to us and ATC that we were aligned with the localizer and the glideslope was accurate to the published altitudes. We planned for an ASR approach into our alternate if we would loose the conventional nav. We continued the approach believing that the conventional nav was working again; however when we popped out at approximately 400 feet we were lined up outside of the taxiway south of Runway XR. We maneuvered to land on Runway XR. I felt that the risk of going missed again without navigation; no VFR airports available and deteriorating weather conditions were a greater risk than landing on XR with some maneuvering we were able to land the aircraft without further incident.The threats were the FMS MEL; conventional nav failure; long landing; and low altitude maneuver in poor weather conditions. I do not believe that there was anything to be done that could have prevented this; however if I had more information to tell me that we were not aligned with the localizer on the second approach I would have gone missed early on and proceeded to ZZZ1 for the ASR approach.

Data retrieved from NASA's ASRS site as of July 2013 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.