Narrative:

We were at 4;000 ft and given a climb to 6;000 ft. Autopilot 'B' was in use. During the climb at about 5;200 ft; autopilot went into altitude capture. At about 5;500 ft to 5;600 ft; the aircraft pitched up to about 20 degrees. The vvi went from 1;200 to almost 5;000 fpm. The autothrottles did not move and the airspeed dropped from 250 to 210 knots. I disconnected the autopilot and pushed the throttles up in an attempt to regain control of the aircraft. We leveled off at about 6;300 ft. The mode control for pitch was level change and speed was 250 knots when it happened. A few things that were also strange during this were that the captain was trying to delete the reduced climb and some of the keys on his FMC would not work. (Mainly the delete key.) we also had a TCAS failure at the same time. Once the aircraft was level at 6;000 ft; we tried to reengage the autopilot in 'B' mode but were unable. We tried both cmd and cws. Neither would engage. We engaged autopilot 'a' mode and flew the airplane to destination without further problems. Once there; we called maintenance and they MEL'd both the 'B' autopilot and the TCAS. Once we arrived in base; we asked for maintenance to come out and we explained everything that had happened. They had several personal there to troubleshoot the problems. We were both at a loss as to why the autopilot would pitch like this when it went into altitude capture.

Related reports:

Google
 

Original NASA ASRS Text

Title: B737-700 flight crew reports a vigorous autopilot pitch up as the aircraft is capturing the assigned altitude of 6;000 FT. The autopilot is quickly disconnected by the FP and the aircraft returned to 6;000 FT. Shortly before and during this event the Captain was testing the TCAS and deleting CLB-2 from the autothrust system.

Narrative: We were at 4;000 FT and given a climb to 6;000 FT. Autopilot 'B' was in use. During the climb at about 5;200 FT; autopilot went into Altitude Capture. At about 5;500 FT to 5;600 FT; the aircraft pitched up to about 20 degrees. The VVI went from 1;200 to almost 5;000 fpm. The autothrottles did not move and the airspeed dropped from 250 to 210 knots. I disconnected the autopilot and pushed the throttles up in an attempt to regain control of the aircraft. We leveled off at about 6;300 FT. The mode control for pitch was Level Change and speed was 250 knots when it happened. A few things that were also strange during this were that the Captain was trying to delete the reduced climb and some of the keys on his FMC would not work. (Mainly the delete key.) We also had a TCAS failure at the same time. Once the aircraft was level at 6;000 FT; we tried to reengage the autopilot in 'B' mode but were unable. We tried both CMD and CWS. Neither would engage. We engaged autopilot 'A' mode and flew the airplane to destination without further problems. Once there; we called Maintenance and they MEL'd both the 'B' autopilot and the TCAS. Once we arrived in base; we asked for Maintenance to come out and we explained everything that had happened. They had several personal there to troubleshoot the problems. We were both at a loss as to why the autopilot would pitch like this when it went into Altitude Capture.

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.