Air carrier flight crew reported avionics malfunction on approach.
Synopsis
Air carrier flight crew reported avionics malfunction on approach.
Narrative
We were on approach at 5000 ft. for 28C into ORD. Auto pilot was on and we were cleared to cross RZUKO at 5000 and cleared for the approach. We were about MEMAW on the approach tracking inbound from around SNTOE. Then all at once the glide slope which was at the top of our instrument; moved rapidly down below the horizon in a second or two. As it went through the horizon the auto pilot engaged it as it was armed for the approach. The aircraft now following the bad glide slope; rapidly descended out of 5000 ft. and started to dive and pick up speed as it following the rapidly descending glide path. I disengaged the auto pilot and attempted to gain control of the plane. I maneuvered the plane back up to 5000; as we weren't at RZUKO yet. I tried to slow the accelerating plane back down to our assigned speed. As this was happening we were getting closer to traffic ahead of us and ATC gave us a slower speed; we complied as quickly as we could. I got the aircraft back stabilized and slowed down to the new speed. I had the flight director and auto pilot reengaged as I continued the descent from 5000 as we were just crossing RZUKO. I continued the stabilized approach and landed without any further issues. There may have been an issue with our instrument or antennas. We had no indications in the flight deck during that flight or earlier that day of any issues. Also may have been some interference from Chicago's system; but nobody else seemed to have any issues at that time. When it did happen we were straight and level tracking inbound; we weren't defending or turning into it.
Second reporter narrative
On ILS at 5000 ft. glideslope become erroneous and causing the airplane to pitch down and increase airspeed while ATC instruct us to slow down. Captain disengage the autopilot and hand fly the airplane after occurrence.
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.