CL60 flight crew experiences altitude deviation when autoplt captures false glide slope.

Attributes
ACN	763694
Time
Date	200711
Local Time Of Day	1201 To 1800
Place
Locale Reference	airport : iah.airport
State Reference	TX
Altitude	msl single value : 5000
Environment
Flight Conditions	IMC
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : i90.tracon tower : iah.tower
Operator	common carrier : air carrier
Make Model Name	Challenger CL600
Operating Under FAR Part	Part 121
Navigation In Use	ils localizer only : 8r
Flight Phase	descent : approach
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
Experience	flight time last 90 days : 150 flight time type : 600
ASRS Report	763694
Events
Anomaly	aircraft equipment problem : less severe altitude deviation : excursion from assigned altitude other anomaly other anomaly
Independent Detector	other flight crewa
Resolutory Action	flight crew : overrode automation flight crew : regained aircraft control
Supplementary
Problem Areas	Navigational Facility Aircraft
Primary Problem	Navigational Facility

Narrative:

During an instrument approach in heavy rain and full IMC; we were assigned 5000 ft till established on the ILS runway 8R approach into iah. We were still approximately 12-15 mi from the airport when the localizer was captured and tracking. The GS was still intermittent at this point. The autoplt then seemed to capture the GS and lose it or capture a false GS. Either way; the autoplt initiated a climb to capture it. With no visual references I initiated an immediate power input to prevent a stall as the aircraft lost airspeed. I then realized it was climbing and turned off the autoplt to avoid any further climbing. The plane climbed to 5700 ft before I was able to correct the faulty GS capture. I leveled and upon advice of the captain; we turned on the autoplt again. It captured the GS and followed it down properly without incident.

Original NASA ASRS Text

Title: CL60 FLT CREW EXPERIENCES ALT DEV WHEN AUTOPLT CAPTURES FALSE GLIDE SLOPE.

Narrative: DURING AN INST APCH IN HVY RAIN AND FULL IMC; WE WERE ASSIGNED 5000 FT TILL ESTABLISHED ON THE ILS RWY 8R APCH INTO IAH. WE WERE STILL APPROX 12-15 MI FROM THE ARPT WHEN THE LOC WAS CAPTURED AND TRACKING. THE GS WAS STILL INTERMITTENT AT THIS POINT. THE AUTOPLT THEN SEEMED TO CAPTURE THE GS AND LOSE IT OR CAPTURE A FALSE GS. EITHER WAY; THE AUTOPLT INITIATED A CLB TO CAPTURE IT. WITH NO VISUAL REFS I INITIATED AN IMMEDIATE PWR INPUT TO PREVENT A STALL AS THE ACFT LOST AIRSPD. I THEN REALIZED IT WAS CLBING AND TURNED OFF THE AUTOPLT TO AVOID ANY FURTHER CLBING. THE PLANE CLBED TO 5700 FT BEFORE I WAS ABLE TO CORRECT THE FAULTY GS CAPTURE. I LEVELED AND UPON ADVICE OF THE CAPT; WE TURNED ON THE AUTOPLT AGAIN. IT CAPTURED THE GS AND FOLLOWED IT DOWN PROPERLY WITHOUT INCIDENT.

Data retrieved from NASA's ASRS site as of January 2009 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.