Airbus Industrie Undifferentiated or Other Model
Confusion
Deviation - Altitude Overshoot; Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural Clearance
Air Traffic Control Issued Advisory / Alert; Flight Crew FLC Overrode Automation; Flight Crew Became Reoriented
Environment - Non Weather Related; Procedure
We were getting vectors from ORD approach for runway 28R.  We were given a clearance to intercept the localizer just outside ADAME. Approach was armed and everything proceeded as normal as the autopilot captured the localizer and glideslope.  The aircraft proceeded down the glideslope initially fine but suddenly and quite dramatically pitched down and the airspeed started racing toward max speed for flaps 2. I initially thought the aircraft was trying to catch the glideslope which it had let get a little below us but it was such a violent pitch down that I knew something was wrong. Suddenly the glide slope showed a full deflection downward.  At this point we knew that the aircraft was attempting to somehow capture a false glide slope.  I disconnected the autopilot and raised the nose to arrest the high descent rate and called for gear down to get the speed under control.  The flight directors were giving bad information so I called for them to be OFF.  All of this happened very quickly and at the same time and by the time I stopped the autopilots actions; we were about 300 feet below the required altitude at WILLT which is 2;200 feet.  I immediately climbed back up to 2;200 to cross WILLT. We were probably about 1-2 miles outside WILLT when we were at our lowest.  As we were climbing; approach told us they got a low altitude warning on us evidently because we were lower than the required altitude at WILLT.  Although there was a scattered layer of clouds between us and the airport; we were in VFR conditions the whole time. The airport was in sight before WILLT so I just continued the approach using the PAPI and vertical deviation indicator. It wasn't until we went to the tower frequency that we learned that a heavy had been cleared onto the runway and had most likely disrupted the glide slope signal. We reported the false glide slope to the tower.First I would like to point out that I have never seen the autopilot pitch down as quickly and violently as it did.  I am not sure why the airbus technology does not incorporate some kind of sensor that backs up the glideslope with GPS and alerts the pilots to possible false glideslopes.  I think if we had been IMC and at a lower altitude on the approach; we definitely would have had our hands full.Secondly I am quite sure that the heavy aircraft on the ground was responsible for the lost signal and false glideslope.  This is ATC's responsibility and although it is VFR; they should still be thinking about signals when they are clearing people for ILSs.
After intercepting the glideslope and localizer for Runway 28R at ORD; a Captain experienced a dramatic pitch over as the autopilot attempted to follow a rapidly descending glideslope indicator. The reporter disconnected the autopilot and commenced a climb back to the appropriate altitude at the same time the Tower issued a low altitude alert. Tower indicated that a heavy jet entering Runway 28R was the likely cause of the signal disruption.