B737-700
Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument
Flight Crew Last 90 Days 36
Human-Machine Interface
Aircraft Equipment Problem Less Severe; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance
Air Traffic Control Issued New Clearance; Flight Crew Overcame Equipment Problem; Flight Crew Returned To Clearance; Flight Crew Executed Go Around / Missed Approach
Aircraft; Equipment / Tooling
We initially attempted the ILS PRM (Precision Runway Monitor) Runway XXL approach into ZZZ. We subsequently went missed approach due to not having the landing environment in sight at the decision altitude. No abnormal navigation issues occurred during this approach. After executing the ATC climb out instructions; we were given vectors for the ILS PRM Runway XY as the weather was improving. We reprogrammed and re-briefed the approach for ILS Runway XY. We were given a vector of approximately 300 degrees to intercept the Runway XY Localizer and subsequently cleared for the approach. As we started to intercept the localizer; the lateral deviation indicator started acting erratically and thus the autopilot started banking left and right to attempt to regain the localizer. The Pilot Flying noticed immediately and attempted to correct back on localizer centerline. Simultaneously; the Pilot Monitoring notified ATC that we were having a navigation issue. The PRM monitor advised to correct back to centerline which we were doing. We were never deviated more than 1/2 dot either side of centerline. ATC then told us to discontinue the approach. We were then given a vector and told to maintain 3;000 ft. MSL. Furthermore; the Pilot Monitoring told ATC that we needed time to assess the problem with the navigation equipment. As were we on a downwind vector; the Pilot Monitoring noticed that the Pilot Flying didn't have the Localizer ID of ZZZZ displayed on his PFD (Primary Flight Display) while the Pilot Monitoring did; even though both VHF NAVs where tuned to the correct localizer frequency and course and we had no 'OFF' flags displayed. We both discussed the situation and elected that the Pilot Monitoring fly this approach as it appeared his VHF NAV was working correctly. We accomplished the ILS PRM Runway XY approach and landed uneventfully. During the approach; the new Pilot Monitoring noticed his lateral deviation indicator acting erratically again. After landing; the Captain called Maintenance and Dispatch to discuss the situation that had occurred. Since there was no navigation issue during the first approach (ILS XXL); we did suspect that we had a bad VHF NAV 1. Thus; even though we were getting good LOC (Localizer) and glideslope symbology while being vectored to final; neither of us noticed that the Pilot Flying did not have a good localizer ID until after being told to discontinue the approach. Another preventative measure that we discussed after landing was: since we had the ILS programmed in the FMC; we could have selected LNAV (Lateral Navigation) when the LOC was being erratic to give us good guidance to the runway centerline (while we were troubleshooting the issue) since PRM approaches were being conducted. This would have more than likely prevented the approach monitor from giving a deviation call and reduce the task overload that was already occurring with an erratic LOC; poor weather; etc. until we either told ATC that we could not continue the approach; or resolved the issue.
B737 First Officer reported a VHF/NAV 1 anomaly resulting in a missed approach.