B747-400 first officer reported an altitude deviation resulted following the failure of the stabiliz...

Narrative:

During initial climb we received a 'stabilizer trim unschd' EICAS message; followed by a 'stabilizer trim 'stab trim 3' and shortly thereafter a 'stabilizer trim 2' EICAS message. The autopilot was engaged and appeared to be functioning normally. We were climbing to our assigned altitude of 3300m (10;800ft). Nearing our assigned altitude; while I was running the QRH items for a stabilizer trim unschd; I noticed the FMA's changed indicating the autopilot was in altitude capture mode and attempting to level at our assigned altitude; but the climb rate was too high to capture properly. The flight director showed that it needed nose down and the autopilot was not following it. The autopilot was disconnected by the pilot flying; and very high control forces were required to push the nose down. We ended up at 11;400 ft; and descended back to our assigned altitude 10;800 ft within 20-30 seconds. ATC never mentioned anything regarding the deviation to us.upon autopilot disconnect it was found that the stabilizer trim had failed completely; and the autopilot was unable to control the pitch due to the high forward pressure required on the control wheel. We completed the flight without the use of the autopilot; completed all QRH items; jettisoned fuel; and landed uneventfully. During the event the autopilot did not indicate to us that it was in a degraded mode; or unable to maintain pitch control.

Original NASA ASRS Text

Title: B747-400 First Officer reported an altitude deviation resulted following the failure of the stabilizer trim. The crew diverted uneventfully.

Narrative: During initial climb we received a 'STAB TRIM UNSCHD' EICAS message; followed by a 'STAB TRIM 'stab trim 3' and shortly thereafter a 'STAB TRIM 2' EICAS message. The autopilot was engaged and appeared to be functioning normally. We were climbing to our assigned altitude of 3300m (10;800ft). Nearing our assigned altitude; while I was running the QRH items for a STAB TRIM UNSCHD; I noticed the FMA's changed indicating the autopilot was in altitude capture mode and attempting to level at our assigned altitude; but the climb rate was too high to capture properly. The Flight Director showed that it needed nose down and the autopilot was not following it. The autopilot was disconnected by the pilot flying; and very high control forces were required to push the nose down. We ended up at 11;400 ft; and descended back to our assigned altitude 10;800 ft within 20-30 seconds. ATC never mentioned anything regarding the deviation to us.Upon autopilot disconnect it was found that the stabilizer trim had failed completely; and the autopilot was unable to control the pitch due to the high forward pressure required on the control wheel. We completed the flight without the use of the autopilot; completed all QRH items; jettisoned fuel; and landed uneventfully. During the event the autopilot did not indicate to us that it was in a degraded mode; or unable to maintain pitch control.

Data retrieved from NASA's ASRS site 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	1384345
Time
Date	201608
Local Time Of Day	0601-1200
Place
Locale Reference	ZZZZ.Airport
State Reference	FO
Environment
Light	Daylight
Aircraft 1
Make Model Name	B747-400
Operating Under FAR Part	Part 121
Flight Phase	Climb
Flight Plan	IFR
Component
Aircraft Component	Horizontal Stabilizer Trim
Person 1
Function	Pilot Not Flying First Officer
Qualification	Flight Crew Air Transport Pilot (ATP)
Events
Anomaly	Aircraft Equipment Problem Less Severe Deviation - Altitude Overshoot Deviation - Procedural Clearance

B747-400 first officer reported an altitude deviation resulted following the failure of the stabilizer trim. The crew diverted uneventfully.