A flight crew reported they received an autopilot malfunction warning and lost the glide slope signal while on final approach and initiated a go-around.
Synopsis
A flight crew reported they received an autopilot malfunction warning and lost the glide slope signal while on final approach and initiated a go-around.
Narrative
Winds at 9;000 ft. were out of the southwest at 125 kts. CVG weather was marginal with gusty winds out of the West. The NOTAMs for ILS27 included alternate missed approach instructions which we used to edit the FMC. We were to climb to 2;000 ft. and turn left to FLM for holding at 3;000 ft. We briefed and talked about this difference which would most likely set us up for vectors from the south of the field. We were vectored for ILS27; holding 170 kts. until the marker. Configuration was F20 gear up. I held off on gear extension until glide slope capture which put our landing flap selection close to '1;000 ft. stable' threshold. I remember thinking that we had about 600 ft. for the flaps to move into position and complete the landing checklist. The remaining items of the event seemed compressed in time. Flaps were selected 25; speed brakes armed and landing checklist complete; and speed bug set for a wind correction of 161 kts. Just prior to 1;000 ft. an amber line strike out the GS pitch FMA. Shortly after the pitch bar on the FD biased out of view and EICAS message AUTOPILOT appeared. The raw data glideslope was missing.I do not believe that the Land 3 FMA annunciation changed; or at the least we didn't get a No AUTOLAND annunciation. My initial thought was that we were in an attitude stabilizing mode as there was a Aircraft Y in position on the runway. We then got a GPWS 'glideslope' aural alert in IMC flight conditions. I called and executed an immediate Go Around. Initial Tower instructions had us fly 290 heading and climb 3;000 ft. We were then given right turn to 360 when able and climb 4;000 ft. Starting the missed approach at 1;000 ft. AGL; 2;000 ft. MSL put us close to level off at 3;000 ft. and I recognized and compensated for this by holding the power levers from going full forward. Still the high rate at which the event took place coupled with hand flying; trying to get back to a place that we could re-automate; and accomplishing the go-around procedure while communicating with ATC made for a busy flight deck environment. We were able to engage the autopilot with the aircraft trimmed for the current flight path with about 1;000 ft. before leveling at 4;000 ft. I noticed that we were approaching 4;000 ft. with too much momentum and again manually decreased the thrust lever position. It didn't seem like the autopilot was going to even attempt a level off; much different than not exceeding a 1G level off; it was not pushing over much at all. I disconnected the autopilot and decreased the thrust levers to near idle as I attempted a manual level off but we were already climbing through our assigned altitude. The PM (Pilot Monitoring) called Tower telling them that we were not able 4;000 ft. and that we were already through the altitude. Tower immediately cleared us to 5;000 ft. which was uneventful.I believe that we were probable at around 4;300 ft. before the planes flight path was beginning to descend. We cleaned the aircraft to flaps 10 and 190 kts. due to the strong winds. We discussed the possibility of 5G interference with the glide slope as it wasn't a temporary fluctuation as you would expect with an aircraft in the critical area. I briefed and we discussed what the procedure would be if we encountered the same issues with the glide slope and how we could reconfigure for an IAN (Integrated Approach Navigation) approach. The second approach was uneventful with all systems working normally. ATC reported that the preceding aircraft reported no anomalies with the glideslope.Maintenance log reported Autopilot Caution and FD Bar Bias with ILS BEAM ERROR (FCC) 'NO ACTION REQUIRED'. I needed to focus more on the aircraft during the missed approach so as to anticipate the failed level off by the automation. The missed approach was a very busy time which was compounded by ATC changing headings and altitudes verses what was expected.
Second reporter narrative
We were conducting the ILS to Runway 27. The Captain was PF (Pilot Flying) and I was PM (Pilot Monitoring). Per the NOTAMs; there were alternate missed approach procedures. The Captain briefed the change and we setup the CDU to correspond to the alternate missed instructions. The winds were out of the west and gusting. Ceiling was OVC 008.While shooting the ILS; we were given 170 kts. to the FAF. That did not leave us much time; but we were able to configure and finish landing checklist before the 1000 ft. stable call. However before reaching 1000 ft.; I first noticed an EICAS message of 'AUTOPILOT' and called it out. Right after that I noticed the amber line through the GS FMA. The Captain had already noticed the GS FMA was out; and the pitch bar was gone; on the flight director. We were getting GPWS 'glideslope' annunciations and the glideslope indication on the PFD was flashing amber. I don't remember if the glideslope diamond was still showing or not. The Captain immediately called for the go around. We initiated the go-around and were given to fly a heading of 290 and climb to 4000 ft. There was traffic departing 27; prior to our arrival.As we were going around; we dipped below the clouds and I saw an Aircraft Y departing 27. That is why were given the heading of 290. As we were climbing; we turn the left autopilot back on. As the airplane was approaching 4000 ft.; I noticed the Captain had manually pulled back the power; but the airplane continued above 4000 ft. As we were passing 4000 ft.; I was talking to the Controller about the reason for the go-around. The Captain called it out and I immediately told her we were overshooting 4000 ft. She immediately gave us 5000 ft. The airplane looked like it was going to level off at about 4300 ft.; before returning to 4000 ft. But since she gave us 5000 ft.; we let it continue to climb. We discussed the possibility of changing to the IAN (Integrated Approach Navigation) approach; if we needed to. We were vectored around and shot the ILS approach again without and problems.After we parked; we were looking at the maintenance log. It showed 'ILS BEAM ERROR (FCC) 'NO ACTION REQUIRED'.' This showed it was the reason for the AUTOPILOT EICAS and FD BAR BIAS. We discussed if it was due to 5G or an aircraft in the critical area. I would think that if it was an aircraft in the critical area; the glideslope loss would have not lasted as long as it did. Not sure of the glideslope error was because of 5G; aircraft in critical area; or just lost the signal. I don't think we could have done anything different on the approach and initiating the go-around. As far as the altitude deviation; instead of explaining to the Controller the reason for the go-around; I should have told her to standby until we had leveled off.
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.