Flight crew reported autopilot descending below glideslope with false signals resulted in disconnect to rejoin the ILS. Captain uncertain whether this was ILS critical area incursion; or ground ILS equipment failure.
Synopsis
Flight crew reported autopilot descending below glideslope with false signals resulted in disconnect to rejoin the ILS. Captain uncertain whether this was ILS critical area incursion; or ground ILS equipment failure.
Narrative
When given our clearance for the ILS 27 into Cincinnati; CVG Approach gave us a heading and 3500 until JELDA cleared for the ILS 27. Subsequently I armed the approach. We were already at 3;500 with localizer and glide slope pointers indicating correct sensing. The PM had also successfully verified the ILS ID. I armed the approach after being cleared. LOC became alive and captured; GS became alive and captured. The aircraft began to descend. I failed to notice that the we were still outside of JELDA. Shortly after joining the GS and setting the missed approach altitude the GS 'flagged' and FD guidance was removed. I disconnected the autopilot. The runway at this point was already clearly in sight. Within seconds the GS came back and was indicating high above us. I noticed that we were still outside of JELDA and had descended below 3;500. I assumed manual control of the aircraft while the PM set the MCP back up to intercept the localizer and glide slope. The second intercept of the GS was successful with the altimeter indicating 2400 at SOAND. We are unsure if the anomaly in the GS was a result of a transmitter malfunction or perhaps an aircraft being near to the ILS critical area. In any case we were able to place the aircraft back on to the ILS and obtain stabilized approach criteria long before 1;000 feet.Cause : Expectation bias played a part. I am used to hearing 'maintain XXXX feet until ESTABLISHED cleared for the ILS'; not 'maintain XXXX feet until ABCD cleared for the ILS'. This led me into a false sense of security that when the glide slope came alive and captured that we were where we needed to be 'in space'. Knowing we would have a FAF/altitude call at SOAND to verify we are not on a false glide slope I believe that caused an omission of verifying my altitude prior to JELDA.Suggestions: In the future I will prioritize altitude into my scan at GS intercept and not wait until the FAF to start diagnosing whether or not I am on the 'correct GS'. I will also not take for granted that when the GS comes alive and captures that my situational awareness is given. Again I will keep my scan sharp at this point and look for potential threats to mitigate.
Second reporter narrative
During arrival into CVG for Runway 27 we were cleared to fly heading 300 and cross JELDA at 3500 ft. and cleared for the approach into Runway 27. This was a slightly different clearance as I was accustomed to because usually for and it's approach you're cleared to an altitude until established then cleared for the approach. This however just made me momentarily confused as at this time we started configuring with flaps/gear and around this time were wee told to contact Tower. My head was down for the most part configuring and switching frequencies and as I reached for the landing checklist the Captain stated that there was an autopilot error. I looked up and saw that the GS on the FMA had a red line through it and we were below 3500 ft. when the glide slope came back we were well below the glide slope. Exactly what happened next is a little blurry but I think I said you should stop descending. The Captain at this point had disconnected the autopilot and was hand flying trying to make sense of what just happened. I realized that the glide slope information was very wrong and asked the Captain if I could reset the flight directors and he said yes. I reset the directors and that fixed the flight director error and we adjusted to get back on glide slope. We were outside the final fix and were stable on the approach so we continued to a safe landing. Autopilot error or glide slope error. Possibly an aircraft crossing the ILS critical area.
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.