Narrative:

Approach advised to hold west of ZZZZ2. Autoplt a was being used; autoplt B was MEL'ed due to uncommanded pitch and heading changes. While attempting to turn outbound autoplt a was not responding to heading inputs. Autoplt was disconnected; the aircraft rapidly descended 400 ft below the assigned altitude. After re-trimming and climbing back to the assigned altitude autoplt a was reengaged. On the inbound turn to ZZZZ2 autoplt a again would not respond to heading inputs and rapidly descended 400 ft before I could disengage autoplt a. Once disengaged the aircraft would pitch up/down +/-1500 FPM. The aircraft was re-trimmed; however the autoplt was not reengaged. At this time approach cleared us for the ILS. Inside the FAF tower was contacted and we were cleared to land. Prior to landing and inside the OM; tower advised RVR 1600/1400/1600. The entire approach light system; runway approach end lights reils and markings were visible from 10 mi out. A normal landing was made. The following write-up was made in logbook: a autoplt would not hold altitude. Aircraft descends 400 ft below selected altitude in excess of 1500 FPM; then climbs 400 ft above selected altitude at 1500 FPM. When autoplt is disconnected aircraft descends at 1500 FPM until pitch trim can be re-trimmed for normal flight. Other write-ups for pitch and heading deviations were recorded on other log pages. Maintenance also referenced boeing service letter for right elevator power package.

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Original NASA ASRS Text

Title: B747-200 CAPT REPORTS APPARENT AUTOPILOT FAILURES THAT CAUSED HIM TO HAND FLY CAT II ILS ALTHOUGH ACTUAL VISIBILITY IS ABOVE CAT I MINIMUMS.

Narrative: APCH ADVISED TO HOLD W OF ZZZZ2. AUTOPLT A WAS BEING USED; AUTOPLT B WAS MEL'ED DUE TO UNCOMMANDED PITCH AND HDG CHANGES. WHILE ATTEMPTING TO TURN OUTBOUND AUTOPLT A WAS NOT RESPONDING TO HDG INPUTS. AUTOPLT WAS DISCONNECTED; THE ACFT RAPIDLY DSNDED 400 FT BELOW THE ASSIGNED ALT. AFTER RE-TRIMMING AND CLBING BACK TO THE ASSIGNED ALT AUTOPLT A WAS REENGAGED. ON THE INBOUND TURN TO ZZZZ2 AUTOPLT A AGAIN WOULD NOT RESPOND TO HDG INPUTS AND RAPIDLY DSNDED 400 FT BEFORE I COULD DISENGAGE AUTOPLT A. ONCE DISENGAGED THE ACFT WOULD PITCH UP/DOWN +/-1500 FPM. THE ACFT WAS RE-TRIMMED; HOWEVER THE AUTOPLT WAS NOT REENGAGED. AT THIS TIME APCH CLRED US FOR THE ILS. INSIDE THE FAF TWR WAS CONTACTED AND WE WERE CLRED TO LAND. PRIOR TO LNDG AND INSIDE THE OM; TWR ADVISED RVR 1600/1400/1600. THE ENTIRE APCH LIGHT SYS; RWY APCH END LIGHTS REILS AND MARKINGS WERE VISIBLE FROM 10 MI OUT. A NORMAL LNDG WAS MADE. THE FOLLOWING WRITE-UP WAS MADE IN LOGBOOK: A AUTOPLT WOULD NOT HOLD ALT. ACFT DSNDS 400 FT BELOW SELECTED ALT IN EXCESS OF 1500 FPM; THEN CLBS 400 FT ABOVE SELECTED ALT AT 1500 FPM. WHEN AUTOPLT IS DISCONNECTED ACFT DSNDS AT 1500 FPM UNTIL PITCH TRIM CAN BE RE-TRIMMED FOR NORMAL FLT. OTHER WRITE-UPS FOR PITCH AND HDG DEVS WERE RECORDED ON OTHER LOG PAGES. MAINT ALSO REFED BOEING SVC LETTER FOR R ELEVATOR PWR PACKAGE.

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.