CL600 captain reports autoplt climbing aggressively 700 ft to capture GS at 5700 ft.

Attributes
ACN	762841
Time
Date	200711
Local Time Of Day	1201 To 1800
Place
Locale Reference	airport : iah.airport
State Reference	TX
Altitude	msl bound lower : 5000 msl bound upper : 5700
Environment
Flight Conditions	IMC
Weather Elements	Turbulence Rain
Light	Daylight
Aircraft 1
Controlling Facilities	tower : iah.tower
Operator	common carrier : air carrier
Make Model Name	Regional Jet CL65 Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Navigation In Use	ils localizer & glide slope : 8r
Flight Phase	descent : approach
Route In Use	approach : instrument precision
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Experience	flight time last 90 days : 120 flight time type : 500
ASRS Report	762841
Events
Anomaly	aircraft equipment problem : less severe altitude deviation : excursion from assigned altitude inflight encounter : turbulence non adherence : clearance
Independent Detector	other flight crewa
Resolutory Action	none taken : anomaly accepted
Supplementary
Problem Areas	Aircraft Flight Crew Human Performance
Primary Problem	Flight Crew Human Performance

Narrative:

At level flight at 5000 ft autoplt on. Localizer captured. GS was intermittent and autoplt made aggressive climb to capture GS. The GS was above our altitude. First officer was flying aircraft. He turned off the autoplt and recovered the aircraft. Aircraft climbed to 5700 ft. The GS subsequently came in strong and we monitored and continued the approach with no other problems. The autoplt appeared to be working properly when GS signal was strong (not red). I am not sure if the rain or perhaps someone momentarily blocking the signal caused this. Or perhaps it was that we were very far out on the approach. Tower did not mention the deviation to us. It occurred as a flap setting was applied which may have increased the balloon effect.

Original NASA ASRS Text

Title: CL600 CAPT RPTS AUTOPLT CLBING AGGRESSIVELY 700 FT TO CAPTURE GS AT 5700 FT.

Narrative: AT LEVEL FLT AT 5000 FT AUTOPLT ON. LOC CAPTURED. GS WAS INTERMITTENT AND AUTOPLT MADE AGGRESSIVE CLB TO CAPTURE GS. THE GS WAS ABOVE OUR ALT. FO WAS FLYING ACFT. HE TURNED OFF THE AUTOPLT AND RECOVERED THE ACFT. ACFT CLBED TO 5700 FT. THE GS SUBSEQUENTLY CAME IN STRONG AND WE MONITORED AND CONTINUED THE APCH WITH NO OTHER PROBS. THE AUTOPLT APPEARED TO BE WORKING PROPERLY WHEN GS SIGNAL WAS STRONG (NOT RED). I AM NOT SURE IF THE RAIN OR PERHAPS SOMEONE MOMENTARILY BLOCKING THE SIGNAL CAUSED THIS. OR PERHAPS IT WAS THAT WE WERE VERY FAR OUT ON THE APCH. TWR DID NOT MENTION THE DEV TO US. IT OCCURRED AS A FLAP SETTING WAS APPLIED WHICH MAY HAVE INCREASED THE BALLOON EFFECT.

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.