Narrative:

First officer was the PF. Aircraft autoplt was engaged entire leg from msp to mdw. During the time of occurrence, however, I'm not sure if FMS was engaged or altitude hold. We were cleared for the ILS runway 4R approach to mdw (in VMC conditions) and instructed to 'maintain 4000 ft until cadon, cleared for ILS runway 4R approach, contact tower FAF inbound.' approximately 2-3 mi from cadon, first officer called for flaps 5 degrees. Upon reaching back from the flap handle, I noticed the aircraft climbing through 4200 ft. I simultaneously told the first officer to check his altitude, disconnected the autoplt and returned to 4000 ft MSL. At no time was another aircraft observed in the area nor an alert given (or warning) by TCASII or ATC.

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Original NASA ASRS Text

Title: B737 ACFT WITH AUTOMATED FLT GUIDANCE SYS DEVIATED 200 FT ABOVE CLRED ALT BEFORE CAPT CORRECTED IT.

Narrative: FO WAS THE PF. ACFT AUTOPLT WAS ENGAGED ENTIRE LEG FROM MSP TO MDW. DURING THE TIME OF OCCURRENCE, HOWEVER, I'M NOT SURE IF FMS WAS ENGAGED OR ALT HOLD. WE WERE CLRED FOR THE ILS RWY 4R APCH TO MDW (IN VMC CONDITIONS) AND INSTRUCTED TO 'MAINTAIN 4000 FT UNTIL CADON, CLRED FOR ILS RWY 4R APCH, CONTACT TWR FAF INBOUND.' APPROX 2-3 MI FROM CADON, FO CALLED FOR FLAPS 5 DEGS. UPON REACHING BACK FROM THE FLAP HANDLE, I NOTICED THE ACFT CLBING THROUGH 4200 FT. I SIMULTANEOUSLY TOLD THE FO TO CHK HIS ALT, DISCONNECTED THE AUTOPLT AND RETURNED TO 4000 FT MSL. AT NO TIME WAS ANOTHER ACFT OBSERVED IN THE AREA NOR AN ALERT GIVEN (OR WARNING) BY TCASII OR ATC.

Data retrieved from NASA's ASRS site as of July 2007 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.