B737-800
Flight Crew Air Transport Pilot (ATP)
Aircraft Equipment Problem Less Severe
Encountered a nose-low usual attitude while approaching ORD at night under VFR conditions. Autopilot engaged; approach mode in use; gear down flaps 15 and speed 170 knots. GS and LOC captured. Altitude between 2;500 feet and 3;000 feet. The glide slope rapidly deflected to full scale and the autopilot and trim followed. I disconnected the autopilot and began recovery as nose-low guidance on the HUD became active. We may have approached 20 degrees nose down pitch. Max speed was 197 knots so we didn't over speed the flaps. After recovery and stabilized I continued the approach. We were stable at 1;000 feet so I did not go around. On landing we noticed a line of heavy aircraft on the south side of 10L. Normally aircraft wait on the north side. I suspect GS interference by the heavies. Fatigue was a factor since we flew the red eye in the morning. Spent daylight hours in the hotel and then on the same day flew a night flight. When using 10L as only runway during late night ops; aircraft should wait on the north side of 10L.
We had just intercepted the localizer and glideslope to 10L; with the autopilot on; in VMC conditions; as per ATC's instructions and were on the approach in ORD; outside of BUGSE around CARLE. At first the aircraft was tracking the glideslope and localizer correctly but at approximately 2700 MSL the aircraft suddenly; unexpectedly and fairly aggressively pitched nose down. Both the Captain and myself at first thought this was a momentary pitch deviation and waited a couple of seconds for the autopilot to correct itself and return the aircraft to a normal flight path as it had in the past. But instead the autopilot did not correct itself and increased the downward pitch. As the downward pitch increased the Captain immediately disconnected the auto pilot and corrected the pitch attitude of the aircraft to a stable and normal flight path. We did not get the GPWS warning. At the same time this was happening the localizer was bouncing from one scale limit on the Primary Flight Display to the other. After the Captain corrected to the normal flight path we continued the approach; which was uneventful; to a successful landing. Upon landing the Captain remarked that according to his HUD; an unusual attitude had occurred which prompted me to suggest that we both file a [safety] report. The Captain took corrective action in a timely manner and at no time did either of us think or feel safety of the flight had been compromised. I suspect ATC/Tower/Ground had permitted an aircraft to taxi in front of the localizer and or glideslope antennas which caused the aggressive pitch deviation and the erroneous localizer information. I suggest that ATC be mindful and relay the information that the integrity of the signals may be compromised even in VMC or protect the antennas when aircraft are using it at night. Normally; when taxing aircraft inadvertently interfere with localizer and glide slope signal the auto pilot just has a momentary pitch deviation but this time it was much more aggressive and longer lasting.
With the autopilot engaged; crew intercepted the localizer and glideslope (GS) to 10L at ORD. At approximately 2;700 MSL the aircraft pitched nose down. The autopilot did not correct the deviation and increased the downward pitch. As the downward pitch increased the Captain disconnected the autopilot and corrected the pitch attitude to a stable flight path and landing. Crew suspects that there was ground aircraft interference with the ILS/GS transmitter signal.