Air carrier on the ILS 28R at ord experienced erratic glideslope indications that may have been caus...

Narrative:

Our flight was on approach to ord runway 28R. It was visual conditions at airport. The first officer was flying the approach that had rain occurring from shoreline to near willt. Approach control cleared us for the ILS and reported that the localizer was reported as a little erratic. We continued and localizer seemed to move some much like [runway] 4R localizer did at ord for some time. At 4;000 ft the first officer disconnected the autopilot to keep the localizer roll to a minimum. Prior to willt glideslope had some displacement; but seemed reasonable. The glideslope was stable as we approached willt and as we crossed willt with glideslope tracking right on I noticed our altitude over willt off by 200 ft. Had we continued to follow the glideslope guidance we would have got even lower on the approach. As it was we broke out and saw the airport and flew visually in from that point. As we were landing we heard an aircraft behind us commenting on glideslope indications as well. Once we got to ground we reported the anomaly. ATC indicated that they had a heavy on the south side of the east end of [runway] 28R that they were going to move soon and that they believed it was the cause of the beam error. Once at the gate I again called ground to suggest that a localizer only approach should be used until this could be addressed. The airport had reconfigured shortly before we arrived and there were a lot of aircraft lined up on the north side of the runway as well as the big one we saw on the south side. Don't know if that was a factor but it definitely appeared that we were getting some glideslope information that wasn't correct. It appeared all controllers had some knowledge of a problem but thought it was limited to the localizer only. Suggestions; it was busy on the approach and we do deal with signal strength issues but almost exclusively on localizer's or VOR's. First thought would be to go to an RNAV approach when these conditions exist or are expected to exist. The minimums are higher; but with weather like we had it would have been a non-event. I have seen this signal wavering on the other end of [runway] 28R and attributed it to a large aircraft at approach end of [runway] 10L. I am not totally convinced it is an obstruction situation but maybe something to do with ILS/localizer ground equipment.

Original NASA ASRS Text

Title: Air Carrier on the ILS 28R at ORD experienced erratic glideslope indications that may have been caused by a parked aircraft awaiting taxi to another runway.

Narrative: Our flight was on approach to ORD Runway 28R. It was visual conditions at airport. The First Officer was flying the approach that had rain occurring from shoreline to near WILLT. Approach Control cleared us for the ILS and reported that the LOC was reported as a little erratic. We continued and LOC seemed to move some much like [Runway] 4R LOC did at ORD for some time. At 4;000 FT the First Officer disconnected the autopilot to keep the LOC roll to a minimum. Prior to WILLT glideslope had some displacement; but seemed reasonable. The glideslope was stable as we approached WILLT and as we crossed WILLT with glideslope tracking right on I noticed our altitude over WILLT off by 200 FT. Had we continued to follow the glideslope guidance we would have got even lower on the approach. As it was we broke out and saw the airport and flew visually in from that point. As we were landing we heard an aircraft behind us commenting on glideslope indications as well. Once we got to ground we reported the anomaly. ATC indicated that they had a Heavy on the south side of the east end of [Runway] 28R that they were going to move soon and that they believed it was the cause of the beam error. Once at the gate I again called ground to suggest that a LOC only Approach should be used until this could be addressed. The airport had reconfigured shortly before we arrived and there were a lot of aircraft lined up on the north side of the runway as well as the big one we saw on the south side. Don't know if that was a factor but it definitely appeared that we were getting some glideslope information that wasn't correct. It appeared all controllers had some knowledge of a problem but thought it was limited to the LOC only. Suggestions; it was busy on the Approach and we do deal with signal strength issues but almost exclusively on LOC's or VOR's. First thought would be to go to an RNAV approach when these conditions exist or are expected to exist. The Minimums are higher; but with weather like we had it would have been a non-event. I have seen this signal wavering on the other end of [Runway] 28R and attributed it to a large aircraft at approach end of [Runway] 10L. I am not totally convinced it is an obstruction situation but maybe something to do with ILS/LOC ground equipment.

Data retrieved from NASA's ASRS site as of July 2013 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.

Attributes
ACN	1091977
Time
Date	201306
Local Time Of Day	1801-2400
Place
Locale Reference	ORD.Airport
State Reference	IL
Environment
Flight Conditions	IMC
Aircraft 1
Make Model Name	MD-80 Series (DC-9-80) Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Flight Phase	Initial Approach
Flight Plan	IFR
Person 1
Function	Captain Pilot Not Flying
Qualification	Flight Crew Air Transport Pilot (ATP)
Events
Anomaly	ATC Issue All Types Deviation - Altitude Excursion From Assigned Altitude Deviation - Procedural Published Material / Policy

Air carrier on the ILS 28R at ord experienced erratic glideslope indications that may have been caused by a parked aircraft awaiting taxi to another runway.