Date: 201312
Local time of day: 1201-1800
Location: ZZZ.TRACON, US
Flight conditions: IMC
Light: Daylight
Aircraft: EMB ERJ 145 ER/LR
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Initial Approach
Airspace: Class B ZZZ
ATC / advisory: TRACON ZZZ
Aircraft component: Navigational Equipment and Processing
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Not Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Distraction; Workload; Troubleshooting; Confusion
Second reporter function: Pilot Flying; First Officer
Anomaly: Aircraft Equipment Problem Critical; Deviation - Track / Heading All Types
Detector: Person Air Traffic Control; Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Provided Assistance; Flight Crew Executed Go Around / Missed Approach; Flight Crew Landed in Emergency Condition; Flight Crew Took Evasive Action; General Declared Emergency
Contributing factors: Aircraft; Weather
Primary problem: Aircraft

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.


Second narrative: We operated to ZZZ. The airplane arrived late; we deiced and went to ZZZ uneventful until we were cleared for the ILS XR in ZZZ. After we were given a heading of I believe 120 and a decent from 7;000 to 4;000 we were cleared for the ILS XR. The autoplilot was ON and we intercepted the course. I noticed that the course we intercepted we were south of the overlay on the FMS; although the FMS was MEL'd I asked the Captain to ask ATC if we were south of course. They told us yes; which both the Captain and I agreed we needed to terminate the approach. ATC told us to stop at 6;000 and fly inbound. The Captain then declared an emergency and asked for another approach.

Second callback: The Reporter stated that he had not heard the cause of this anomaly. The Green Needles appeared to be operating normally but obviously were not. The localizer should not be capable of indicating ON Track when the aircraft is in fact not. In this case; there was enough time to maneuver to the runway. Another go-around and an attempt to fly to the nearest suitable alternate would have been very risky because of the fuel state.

Synopsis: 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.