Air carrier crew reported an aircraft autopilot malfunction resulting in the aircraft diving below the glide slope on the ILS approach in visual conditions. The Captain had the FO disconnect the autopilot; climb to the glide slope and continued to landing.
Synopsis
Air carrier crew reported an aircraft autopilot malfunction resulting in the aircraft diving below the glide slope on the ILS approach in visual conditions. The Captain had the FO disconnect the autopilot; climb to the glide slope and continued to landing.
Narrative
We were on the visual approach for PDX and it was a backed up ILS. The ILS had a beam error and the aircraft started diving towards the ground before the airplane should have started a decent. I responded to the incident to my FO as pilot monitoring this leg that he needed to kick off the autopilot and fly the airplane. And was trying to explain what happened while on the approach. The FO did kick off the autopilot but struggled to understand the full scope of what was going on and what was needed to fly the approach successfully. as I was trying to explain to him to just hand fly the visual when things with the automation doesn't go as planned he struggled maintaining the proper altitude. I finally just shut off all of the flight directors as it was giving us bad data for the approach. I instructed him to climb back to a proper altitude and restart the approach. The tower gave us an altitude warning and we explained we were having issues with our equipment on the approach and that we are correcting. After landing I mentioned to the tower that I believe we had gotten some bad glide slope data from the beam and asked if they were having issues with their equipment. They responded that everything checked out on their end. The only thing else that came to mind after the landing is that it was a plane crossing the beam and possibly holding in the beam to cause the error while we were landing. I also mentioned to the FO that if something happens on approach to always fly the airplane and kick off the automation. We wrote up in the aircraft logbook the error and the mechanic said this is a recurring issue there. He also showed us the beam error in the ECAS readout.As a result of the issues I worked with the FO the rest of the trip to assist him with visual approaches. I suggest at his next sim event that he receive some additional training on those visual approaches specifically.
Second reporter narrative
We were on visual backed up by the ILS in to PDX. The ILS system had a beam error and the aircraft started diving towards the ground before it was to start the descent. We both realize what was happening and I decided to kick off the auto pilot while the captain was telling me the same thing; The captain shut off the flight directors as they were giving us bad data as I was trying to get back to the proper altitude; I flew the approach and landed the aircraft with no problems. When we landed the captain ask the tower if they were having any kind of issues with their equipment. They responded that everything was OK on their end. We wrote up in the aircraft logbook the error and we mention it to the mechanic. He said this is a recurring issue there. He also showed us the error on the ECAS READ OUT.
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.