An air carrier crew reported that on a coupled ILS approach to ord runway 28 the aircraft pitched up...

Narrative:

We were on vectors for the ILS to runway 28 by chicago approach on a heading of 340; at 4000 ft; and assigned 210 KTS approx 15 miles southeast of the airport. We had preceding traffic about 10 miles ahead of us. Roughly 3 miles from adame; we were told to turn left to a 290 degree heading; maintain 4000 ft and cleared the ILS 28 approach; speed 170 KTS until willt (the FAF). Approach/land was armed and while rolling out to 290 degrees; we received a false glide slope indication showing us momentarily on glide slope followed by the glide slope indicator going to the top of the scale. This was followed almost immediately by the localizer coming alive. The autopilot responded very aggressively by pitching up to regain the glide slope resulting in the aircraft gaining 400 ft before the captain was able to manually override the autopilot and bring the aircraft back to our assigned altitude. We reported to approach control and tower of our deviation and also the erratic glide slope information. We were not aware of any ILS interference before commencing the approach and after reporting the incident to tower; another aircraft also reported the same problem with the glide slope. Tower reported that the ILS critical area was not being protected (due to the favorable weather conditions) during the entire approach; we were receiving a valid ILS identifier. Within probably a 10 second time frame we received the approach clearance; localizer becoming active; false glideslope; and airspeed instructions. I am now very aware of the fact that after the approach/land switch is selected and armed and the glide slope or localizer indicators meet the capture criteria; the flight director and autopilot will try to intercept the indicators whether the data is necessarily valid or not. I will make a better attempt in the future to be aware of 'ghost' indications during not only marginal weather; but during good weather as well.

Original NASA ASRS Text

Title: An air carrier crew reported that on a coupled ILS approach to ORD Runway 28 the aircraft pitched up and gained 400 FT because an aircraft taxied into the ILS protected area. The airport was VMC and ATC did not have to protect the ILS antenna.

Narrative: We were on vectors for the ILS to Runway 28 by Chicago approach on a heading of 340; at 4000 FT; and assigned 210 KTS approx 15 miles southeast of the airport. We had preceding traffic about 10 miles ahead of us. Roughly 3 miles from ADAME; we were told to turn left to a 290 degree heading; maintain 4000 FT and cleared the ILS 28 approach; speed 170 KTS until WILLT (the FAF). Approach/Land was armed and while rolling out to 290 degrees; we received a false glide slope indication showing us momentarily on glide slope followed by the glide slope indicator going to the top of the scale. This was followed almost immediately by the localizer coming alive. The autopilot responded VERY AGGRESSIVELY by pitching up to regain the glide slope resulting in the aircraft gaining 400 FT before the Captain was able to manually override the autopilot and bring the aircraft back to our assigned altitude. We reported to approach control and tower of our deviation and also the erratic glide slope information. We were not aware of any ILS interference before commencing the approach and after reporting the incident to tower; another aircraft also reported the same problem with the glide slope. Tower reported that the ILS critical area was not being protected (due to the favorable weather conditions) during the entire approach; we were receiving a valid ILS identifier. Within probably a 10 second time frame we received the approach clearance; localizer becoming active; false glideslope; and airspeed instructions. I am now very aware of the fact that after the Approach/Land switch is selected and armed and the glide slope or localizer indicators meet the capture criteria; the flight director and autopilot will try to intercept the indicators whether the data is necessarily valid or not. I will make a better attempt in the future to be aware of 'ghost' indications during not only marginal weather; but during good weather as well.

Data retrieved from NASA's ASRS site as of April 2012 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	914363
Time
Date	201010
Local Time Of Day	0001-0600
Place
Locale Reference	ORD.Airport
State Reference	IL
Environment
Flight Conditions	VMC
Light	Night
Aircraft 1
Make Model Name	Widebody Low Wing 3 Turbojet Eng
Operating Under FAR Part	Part 121
Route In Use	Vectors
Component
Aircraft Component	ILS/VOR
Person 1
Function	Pilot Not Flying First Officer
Experience	Flight Crew Last 90 Days 90 Flight Crew Total 12000 Flight Crew Type 2400
Person 2
Function	Captain Pilot Flying
Qualification	Flight Crew Air Transport Pilot (ATP)
Experience	Flight Crew Last 90 Days 50 Flight Crew Total 20000 Flight Crew Type 200
Events
Anomaly	ATC Issue All Types Deviation - Altitude Overshoot Deviation - Procedural Clearance Inflight Event / Encounter Loss Of Aircraft Control

An air carrier crew reported that on a coupled ILS approach to ord runway 28 the aircraft pitched up and gained 400 ft because an aircraft taxied into the ILS protected area. The airport was VMC and ATC did not have to protect the ILS antenna.