B737 flight crew reported receiving a GPWS 'bank angle' warning while recovering from contradictory flight director guidance prior to joining localizer for an ILS approach. Flight crew executed a missed approach and landed safely.
Synopsis
B737 flight crew reported receiving a GPWS 'bank angle' warning while recovering from contradictory flight director guidance prior to joining localizer for an ILS approach. Flight crew executed a missed approach and landed safely.
Narrative
On approach for the ILS XXR into ZZZ my FO and I both had the ILS freq of XXX.X tuned with the correct identifier. He was PF with the Flight Director (FD) set to master on his side. We were given a vector and told to join the localizer; maintain 3;000 until crossing ZZZZZ fix. Based on the vector and route of flight and confirming with our Multi-function Display (MFD) nav display it should have been a left turn to join. We had VOR/LOC capture on the Flight Mode Annunciators (FMAs) and my FD was commanding a left turn as well as my localizer. However my maintenance system LOC and FD showed a right turn and thats what the aircraft started to do. I told him to disconnect once I realized it wasn't flying the correct path and had him correct back to localizer. Approach noticed immediately that we had overflown the final approach course and queried us. During the recovery and trying to assess the situation my FO descended approximately 300 ft and got the audible 'BANK ANGLE' call; I believe he kept the bank no more than 35-40 degrees and was trying to get us back to course as quickly as possible. My FD continued to show what appeared to be correct information; but it was in disagreement with my FOs display. We decided to ask approach for vectors off of the ILS and proceeded to request the RNAV XXR due to unreliable information. We ran through the charts; FMC and Flight Manual (FM) RNAV guide to set up. We briefed everything and re-accomplished the descent checklist. From there we were brought in for the RNAV appr and landed with no issues. We wrote up the Maintenance entry and had a discussion with maintenance at the gate. We are unsure of the cause and triple checked that we had everything programmed correctly.
Second reporter narrative
We were set up for the ILS XXR into ZZZ. As we intercepted the localizer we received conflicting information from the localizer and our flight directors. FO (pf) received a right turn to intercept and CA (pm) received a left turn to intercept. I (FO) turned off the autopilot and initiated a bank turn to the left to intercept the correct inbound course. We received a 'bank angle' as I believe I banked to about 30-35 degrees in an attempt to re-intercept; I corrected the bank once hearing the 'bank angle.' Once I corrected the aircraft bank and we realized the conflicting information we initiated a missed approach and told tower we'd need vectors to set up for another approach. We set up for the RNAV XXR and executed the approach and landing with no further issues.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.