Date: 202201
Local time of day: 0001-0600
Location: DEN.Airport, CO
Aircraft: Commercial Fixed Wing
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Final Approach; Landing
Airspace: Class B DEN
ATC / advisory: Tower DEN
Aircraft component: Radio Altimeter
Component problem: Failed
Reporter organization: Air Carrier
Reporter function: First Officer; Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Anomaly: Aircraft Equipment Problem Less Severe
Detector: Automation Aircraft Other Automation; Person Flight Crew
When detected: In-flight
Result: Flight Crew Executed Go Around / Missed Approach
Contributing factors: Aircraft; Environment - Non Weather Related
Primary problem: Environment - Non Weather Related

Narrative: We were preparing to land in Denver; DEN. Its weather was showing 1/4 SM and VV001. We discussed with [the] FO (First Officer) and decided to brief the CAT II Approach checklist. There was no NOTAM affecting the CAT II approach in DEN about the 5G band interference. We reviewed the daily read and crew memo XX-XX about 5G interference. We tested the RA as the procedure required. RA test passed. We were assigned to descend via Runway 35L transition on [the] NIIXX 3 Arrival. We briefed the approach and we also briefed that in case of any RA interference; we could continue the CAT I Approach minimum if the reported RVR is equal to or greater than 1800 RVR. This was how we were trained in the CAT II training in [the] simulator. If the current visibility permits us to continue and land with CAT I minimum; no need to go missed even if we lost the CAT II compatibility. We received the approach clearance and switched to the Tower frequency. Tower advised us that the current visibility is 1800 RVR. We verified that if we will lose our CAT II compatibility; we will continue to CAT I minimums; which were 200 ft. DA(H). We were about 2 NM away from [the] FAF. I asked for gear down; flaps 22; and CAT II landing check. We checked the CAT II annunciator with the CAT II landing checklist; it was green. Autopilot was ON and armed for the approach. As we crossed the FAF; FO stated; 'Altitude checks; no flags.' We were about 1000 AGL - not sure about the exact altitude - and our RA turned into an amber color. As we previously briefed; I stated to continue the approach with CAT I approach minimums since we have enough visibility for [the] CAT I Approach. We were about 300 - 200 AGL and our autopilot failed when [we] were in IMC. It was very unexpected and I felt distracted. I called the go-around. I felt it was the safest decision. We climbed and maintained 7000 ft. as ATC instructed. There was only an Autopilot Fail message on EICAS as much as I remember. FO and I discussed trying to put the autopilot back on to see [if] it was inoperative or not. It was pretty reliable during the 2 hours flight and our overall experience in [the] Aircraft X autopilot rarely fails. We decided to put the autopilot back on. It was working fine. Eventually; we received vectors back to Runway 35R and landed with [the] ILS CAT I Approach. The situation did not repeat. When we got to the parking position. I tested [the] autopilot again; it was working how it was supposed to. I am not sure why we lost the RA and CAT II compatibility. As a crew; we thought it might be related to 5G band interference. I couldn't understand why our autopilot failed in the critical phase of the flight. Autopilot only failed one time in my more than 3 years of experience in Aircraft X. I can say that it is one of the most reliable systems in our plane. I believe it is a big safety concern to lose autopilot at a very low altitude in IMC. I don't know if somehow there is any connection between autopilot failure; 5G band interference; and RA in amber color; or if it is a different issue. I have discussed what happened in this flight with 5 other pilot friends in the company [and] we couldn't find any logical explanation. I believe it is a big safety concern. I recommend doing some studies and testing inflight to learn about 5G interference and our procedures in every aspect; and training our pilots accordingly. I also recommend CAT II operations be suspended temporarily.




Synopsis: Air carrier First Officer reported executing a go-around at DEN when the radio altimeter failed; followed by the failure of the autopilot. Reporter stated 5G interference may have been the possible cause of the failures.