Air carrier flight crew reported getting too low on approach to ORD as they were receiving erratic glide slope signals; probably because of aircraft taxiing past the end of the runway.
Synopsis
Air carrier flight crew reported getting too low on approach to ORD as they were receiving erratic glide slope signals; probably because of aircraft taxiing past the end of the runway.
Narrative
Flying ILS 9C. At 4000 I noticed the GS signal moving abnormally fast from full up deflection to downward as we intercepted the GS. The movement was so fast the GS captured quickly and the aircraft pitched 5 degrees nose down (AP was on). The FO (First Officer) disconnected the AP but initially continued to follow the FD until I commanded him to pitch up and recover. Once stabilized flight was regained; I noticed an aircraft taxi south across the approach end of the runway on Taxiway Z. We both attributed the erratic GS to this aircraft. Once the aircraft cleared the runway; the signal appeared normal. We continued the approach in VMC. Approximately 2 miles from EZELL; I noticed we were approximately 2200 ft. MSL which is 200 ft. lower than we should have been when on the GS. I attempted to determine what was wrong and visually confirmed we were low by looking at the PAPIs. When we passed EZELL; I noticed we were only 1350 ft. AGL (200-300 lower than we should have been on the GS). I verbalized this to the FO who seemed distracted with the obvious irregularity. We extended the gear and flaps and were fully configured by 1000 ft. The FO disregarded the GS and flew the remaining approach visually. I again noticed another aircraft holding short of [Runway] 9C on Z Taxiway which I assumed caused the GS error. I reported this to ATC which seemed unconcerned and mentioned that an aircraft had not stopped in the correct location. Shortly after; another aircraft; who had heard our transmission; advised ATC that they experienced the same issue. Clearly this is a very unsafe situation and could have numerous consequences. In IMC; we would have gone around. It also may have resulted in a CFIT escape.
Second reporter narrative
On approach to Runway 9C at ORD; the autopilot was coupled to the ILS signal while conducting a visual approach. At approximately 15 miles from the runway (2 miles west of FRANN on IAP 9C) while VOR/LOC and ALT HOLD modes at 5;000 ft. MSL; the glide path signal abruptly started from the top of the case and appeared to approach glide path capture. About 1 dot above; the flight director indication and autopilot 'lurched' to capture the GS mode. It caused the airplane to pitch up abruptly and then pitch down after the autopilot captured the false signal. I immediately disengaged the autopilot and manually flew the airplane. The airplane was configured at flaps 5; gear up and 170 kts. assigned by ATC. I proceeded to manually fly the aircraft; visually to the runway and using PAPI guidance for glide path. At approximately 6 miles from the field (1 mile from EZELL) the localizer and glide path signals became unreliable again. We could see an airplane crossing the approach end of 9C from left to right. We passed EZELL at approximately 1;900 ft. AGL at flaps 5 gear up. I became temporarily distracted looking outside at the crossing traffic and flying the airplane using visual references outside; when the PM (Pilot Monitoring) stated configuration at approximately 1;400 ft. AGL. I called for gear down; flaps 15; and flaps 30; reference speed after that. The aircraft was fully configured and checklist completed at approximately 1;000 ft. AGL. The PM informed the Tower that we experienced a temporary loss of ILS signal while on final approach and they replied it was probably due to aircraft crossing the approach end of 9C on Taxiway Zulu. Another aircraft behind us on their approach to 9C said they experienced the same thing (temporary loss of the ILS signal). The landing was uneventful.
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.