B737 flight crew flying reported erroneous flight guidance information on approach.
Synopsis
B737 flight crew flying reported erroneous flight guidance information on approach.
Narrative
We were in heavy icing with no visibility at 5000 ft. when we were cleared for ILS XXC and were in the turn to join. The Captain (CA) saw that we were not capturing in our turn to join so I turned off the autopilot to complete the turn to capture the localizer. I noticed then that my flight director information was starting to give erroneous information. I asked the CA to please check my radio frequency for the localizer. Unfortunately; CA was giving me directions for heading and altitude at the same time; so they did not hear my request. CA side was working but they didn't realize that my side wasn't picking up the localizer and had erroneous information. As CA continued to give me info to fly; I started asking for CA to give me heading info as I was trying to keep the speed and altitude while going between both screens trying to fly raw data that was not accurate on my side. After bracketing the localizer a couple of times; I finally asked if they wanted to fly it. I realized that we were not seeing the same picture and CA side was working. After I handed over the plane; I looked down at the inputted localizer frequency on my side to double check to make sure it was a correct frequency. The display showed an ERR next to the correct frequency. I recycled the input and the frequency reactivated accurately. The CA flew the rest of the approach and safely landed the airplane. After we landed and arrived at the gate. I spoke with the CA and told them of the ERR on the localizer frequency. I also told CA what I was seeing on my screens and what the Flight Director was showing. CA did not realize the bad info I was getting. Their side had picked everything up correctly and CA was perplexed to why I wasn't capturing the localizer because no flags came up on the screens. CA also did not hear me ask them to double check my radio for the frequency. We determined that due to the heavy icing we were flying in; the antenna for the radio had probably frozen up and caused the ERR.
Second reporter narrative
This was the First Officer's (FO's) leg and autopilot B was engaged. We briefed getting Runway XXR in ZZZ as we were coming from the NW which would be the normal flow. The runway assignment ended up being XXC. The FMC was reprogrammed; and the radios were tuned to the new frequency. Our route of flight took us from the NW to the W to the S and then eventually E of the field. I mention this because we spent a lengthy amount of time at 5000 ft. in moderate rime icing conditions. Just prior to intercepting the localizer (with a good ID) I called out that the aircraft was not turning to intercept the final approach course. We passed through the final approach course. The FO turned off the autopilot and began to hand fly the aircraft. The FO called for a heading. I gave the FO guidance both vertically and horizontally to get back on course. We passed through the localizer again. I was confused at this point as my FMA indications and flight directors were giving me correct guidance. We varied above and below 5000 ft.; but I don't believe we exceeded more than 180 ft. off. Laterally we bracketed the course several times. I did not trouble shoot as I was focused on keeping us in the lateral confines of the approach course. I continued with course and altitude several times. ATC did advise us we were getting close to the no transgression zone but at that point we were already correcting toward the localizer. I responded that we may have to go around. Very shortly after the FO offered me the controls. I took over the aircraft and quickly got the aircraft back on track. The approach continued without issue breaking out of the clouds at about 3500 ft. Upon debrief we went over what had transpired. The FO explained that after I took over the aircraft they looked down and saw an error on the ILS radio tuning head and that after cycling the radios the ILS error went away and their indications went back to normal. That indication was not there before turning final. They also said that they called for me to 'check the radios'. I did not hear that. I heard no verbalization of any issue the FO had on their FMA indications during the approach. My indications were normal and the only thing that I saw happen was the aircraft did not turn to final on the localizer. I am unsure of what caused the ILS to have an error; but I will say I have never seen the ice accumulation to this extent in my career.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.