A B747-400 IN CRUISE AT FL350 DECLARED AN EMER AND DIVERTED DUE TO A HARD OVER LOWER RUDDER STUCK AT 17 DEGS L. CAUSED BY FAILED PWR CTL UNIT CASTING.

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HARD-OVER LOWER RUDDER ON B747-400. IN CRUISE; SMOOTH AIR; STRAIGHT AND LEVEL; ACFT SUDDENLY VEERED TO L AND ENTERED SHARP BANK WITHOUT WARNING. 2ND CAPT WAS FLYING. HE IMMEDIATELY DISCONNECTED AUTOPLT AND USED FULL R RUDDER TO REGAIN CTL. HE DECLARED EMER AND HEADED FOR ANC. THEY SUMMONED ME AND OTHER FO BACK FROM BREAK; AND WE ALL BEGAN TO TROUBLESHOOT. I RELIEVED 2ND CAPT AND ASSUMED PF DUTIES. WE SOON DISCOVERED THERE WAS NOTHING IN THE CREW OPERATING MANUAL TO COVER THIS; SO WE GOT DISPATCH; MAINT CTL; AND OUR FLEET TRAINING CAPT ON A CONFERENCE CALL ON HF. WE DECIDED TO HEAD FOR ANC AND GO TO A LOWER ALT OVER COOK INLET AND GRADUALLY; CAREFULLY CONFIGURE TO LNDG CONFIGN A STEP AT A TIME; CHKING CONTROLLABILITY AND STABILITY AT EACH STEP. ONCE WE WERE CONVINCED ACFT WAS ACCEPTABLY CONTROLLABLE; WE HEADED FOR ANC. LANDED WITH NORMAL FLAPS BUT 'ABNORMAL FLAP' SPDS DUE TO CTL ISSUES. ALSO; DIFFERENTIAL PWR HELPED ON FINAL. OF COURSE; MUCH COORD DONE WITH ATC; FLT ATTENDANTS; COMPANY; AND MANY PA'S MADE TO PAX. LNDG WAS CHALLENGING; BUT SUCCESSFUL. WHAT WAS THE SINGLE MOST IMPORTANT FACTOR IN THE FAVORABLE CONCLUSION TO THIS 'NOTHING-IN-THE-BOOK' CRITICAL SIT? CRM! CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE SUDDEN VEER TO THE L WAS CAUSED BY THE FAILURE OF THE LOWER RUDDER PWR UNIT CASTING; CRACKING OUT AT A PLUG BOSS. THE RPTR SAID THE RUDDER WENT TO 17 DEGS L AND AS SPD WAS REDUCED AT LOWER ALTS THE RUDDER DEFLECTED TO 31 DEGS AT 180 KTS. THE RPTR STATED AT THIS TIME THE ONLY WARNING RECEIVED WAS A 'LOWER YAW DAMPER FAIL' WARNING. THE RPTR SAID IT TOOK 66 DEGS OF AILERON YOKE INPUT TO LEVEL THE ACFT AND ON LNDG APCH DIFFERENTIAL PWR WAS APPLIED AND HELPED ON FINAL. CALLBACK CONVERSATION WITH RPTR ON ACN 562362 REVEALED THE FOLLOWING INFO: THE RPTR STATED HE WAS THE CAPT FLYING THE AIRPLANE WHEN THE INCIDENT OCCURRED. THE RPTR SAID THE AIRPLANE WENT INTO A QUICK ROLL WITH A 35 DEG BANK; NOT A VIOLENT MANEUVER; BUT QUICK. THE RPTR STATED THE AUTOPLT WAS ENGAGED AND DID NOT DISCONNECT WHEN THE BANK EXCEEDED 30 DEGS. THE RPTR SAID THE FLT DATA RECORDER HAD REGISTERED A 34 DEG BANK AND THE ACFT REQUIRED 65% OF R AILERON. WITH THIS MUCH AILERON DEFLECTION; THE FLT SPOILERS WERE DEPLOYED. THE RPTR STATED THE RUDDER DEFLECTION WAS 17 DEGS IN 1 SECOND. HE RECALLED THAT THE SVC BULLETIN STATED THE VERT FIN WAS NOT DESIGNED FOR QUICK L AND R MOTION; SO HE AVOIDED THE APPLICATION OF QUICK R RUDDER. THE RPTR STATED THE ONLY WARNING RECEIVED IN THE COCKPIT WAS 'LOWER YAW DAMPER FAIL.' THE RPTR SAID THE CAUSE OF THE RUDDER DEFLECTION WAS FAILURE OF THE LOWER RUDDER PWR UNIT CASTING WHICH BROKE OUT AT A PLUG BOSS; ODDLY ENOUGH; WITH NO LOSS OF #2 HYD SYS FLUID OR PRESSURE. THE RPTR STATED THAT; ON THE GND; THE RUDDER DISPLACED THE FULL 31 DEGS.