B737-800
Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Flight Crew Last 90 Days 163; Flight Crew Total 24488; Flight Crew Type 17308
Human-Machine Interface; Confusion
Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural Other / Unknown; Ground Event / Encounter Other / Unknown; Inflight Event / Encounter Weather / Turbulence
Flight Crew Became Reoriented; Flight Crew Overcame Equipment Problem
Airport
We were cleared to fly ILS24R LLROY transition. When we called Tower they told us cleared to land with multiple departures. I believe the last departure was a heavy MD-11 departing our runway from the north side of runway. Runway 24L was closed at the time. I believe the MD-11 departing caused the full scale deflection of our glideslope. It started approaching SHERK intersection and joining the glideslope transitioning from VNAV and LNAV to localizer and glideslope. First the glideslope went up then started rapidly down followed by autopilot disconnect. The First Officer attempted to follow the down glideslope not yet recognizing the problem. I pointed out the glideslope would take us below the 2;000 ft. crossing altitude at GROZA and told her to level off and ignore glideslope. We flew level until GROZA and then the glideslope resumed working and matched up our VNAV info and we elected to continue the approach. It was gusty and raining and snowing at the time and we were between layers at time of incident. We were stabilized from FAF GROZA inbound and broke out at approximately 1;000 ft. on final. The reason I am filing this report is because of what I think is a flaw in the system that would allow a heavy jet to interfere with the safety of a CAT I approach that goes down to 200 ft. AGL. The weather was not less then 800 ft. and 2 miles that would require protection of the critical area but this is unsafe. I would appreciate somebody to look into this. I talked to Tower and they believe they did nothing wrong. It is unusual to have a plane go on the runway from the north side of Runway 24R as most aircraft come from the terminal side. I had this happen before in VFR conditions on the same runway which probably helped in my understanding of the events.
Cleared ILS 24R CLE. Passing SHERK; glideslope suddenly plunged to show us well above glideslope. Disconnected autopilot and began aggressive descent toward glideslope. Had to then level off to meet GROZA above 2;000 ft. Erroneous glideslope info identified; then reestablished by GROZA. Normal approach after that.
B737 Flight Crew reported that the glide slope indicator moved erratically and the autopilot disconnected during approach and resulted in a go-around.