A B747-400 entered an uncommanded 25-30 degree bank after a FMS direct to the next fix selection and...

Narrative:

While at FL350 we were cleared direct. Upon verification and execution of the direct; the aircraft commenced a 25 to 30 degree left bank away from our direct course with LNAV engaged. Selected heading select to correct back on course (heading bug was slightly right of course). Aircraft continued its left turn; not responding to the heading mode. I disconnected the autopilot and turned the aircraft back to our programmed course. Upon disconnecting the autopilot the nose descended as there was more pitch down trim than I suspected for cruise flight. Aircraft quickly descended to [between] 350 [and] 400 ft below our assigned altitude of FL350. Quickly corrected the deviation as ATC asked us to check our altitude. At this point we were back to our assigned altitude and course. Also had a brief TA warning with and aircraft below which indicated 800 ft on the nd. Re-engaged the autopilot once stabilized and proceeded on course. The rest of the flight automation performed reasonably well and we did not encounter this type of problem the rest of the flight. I wrote the event up in the logbook and briefed maintenance upon arrival. I was surprised by the pitch down forces of the aircraft once the autopilot was disconnected. I would suggest leaving the automation on longer to alleviate workload overload at the time.

Original NASA ASRS Text

Title: A B747-400 entered an uncommanded 25-30 degree bank after a FMS direct to the next fix selection and when the aircraft did not respond the Captain disengaged the aircraft then lost about 350 FT which triggered a TA from nearby traffic.

Narrative: While at FL350 we were cleared direct. Upon verification and execution of the direct; the aircraft commenced a 25 to 30 degree left bank away from our direct course with LNAV engaged. Selected Heading Select to correct back on course (heading bug was slightly right of course). Aircraft continued its left turn; not responding to the heading mode. I disconnected the autopilot and turned the aircraft back to our programmed course. Upon disconnecting the autopilot the nose descended as there was more pitch down trim than I suspected for cruise flight. Aircraft quickly descended to [between] 350 [and] 400 FT below our assigned altitude of FL350. Quickly corrected the deviation as ATC asked us to check our altitude. At this point we were back to our assigned altitude and course. Also had a brief TA warning with and aircraft below which indicated 800 FT on the ND. Re-engaged the autopilot once stabilized and proceeded on course. The rest of the flight automation performed reasonably well and we did not encounter this type of problem the rest of the flight. I wrote the event up in the logbook and briefed maintenance upon arrival. I was surprised by the pitch down forces of the aircraft once the autopilot was disconnected. I would suggest leaving the automation on longer to alleviate workload overload at the time.

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.

Attributes
ACN	1067991
Time
Date	201302
Local Time Of Day	0001-0600
Place
Locale Reference	ZZZ.ARTCC
State Reference	US
Environment
Flight Conditions	VMC
Light	Night
Aircraft 1
Make Model Name	B747-400
Operating Under FAR Part	Part 121
Flight Phase	Cruise
Flight Plan	IFR
Aircraft 2
Flight Phase	Cruise
Flight Plan	IFR
Component
Aircraft Component	Autoflight System
Person 1
Function	Captain Pilot Flying
Qualification	Flight Crew Air Transport Pilot (ATP)
Person 2
Function	Pilot Not Flying Captain Relief Pilot
Qualification	Flight Crew Air Transport Pilot (ATP)
Events
Anomaly	Aircraft Equipment Problem Less Severe Deviation - Altitude Excursion From Assigned Altitude Deviation - Procedural Clearance Deviation - Track / Heading All Types

A B747-400 entered an uncommanded 25-30 degree bank after a FMS direct to the next fix selection and when the aircraft did not respond the captain disengaged the aircraft then lost about 350 ft which triggered a TA from nearby traffic.