A B747-200 on initial climb declared an emergency and diverted due to excessive rudder trim and unco...

Attributes
ACN	402416
Time
Date	199805
Day	Thu
Local Time Of Day	0601 To 1200
Place
Locale Reference	atc facility : nrt
State Reference	FO
Altitude	agl bound lower : 0 agl bound upper : 10000
Environment
Flight Conditions	IMC
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : nrt
Operator	common carrier : air carrier
Make Model Name	B747-100
Operating Under FAR Part	Part 121
Flight Phase	climbout : initial other other
Route In Use	departure other
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 200 flight time total : 22000
ASRS Report	402416
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : instrument pilot : commercial
Events
Anomaly	aircraft equipment problem : critical
Independent Detector	other flight crewa
Resolutory Action	flight crew : declared emergency other
Consequence	Other
Supplementary
Primary Problem	Aircraft

Narrative:

After takeoff from nrt, copilot (who was flying) noticed an abnormal amount of right rudder trim was required to maintain a trimmed condition (approximately 2 1/2 units of right rudder trim). During acceleration to en route climb speed out of 10000 ft, uncommanded flight control inputs were noted. These inputs became more intense as airspeed increased from 250 KIAS to 290 KIAS. No indications of equipment failure were evident in cockpit. Captain declared emergency and vectors to an uneventful landing were provided by narita approach control. After landing, it was discovered that both yaw damper computers had failed, and it was determined that those computer failures caused the uncommanded flight control inputs. The computers were replaced and the flight resumed with no repeat of problem. Callback conversation with reporter revealed the following information: the reporter stated the aircraft was a B747-200 and both upper and lower yaw dampers tested ok during the preflight check. The reporter said at no time during the flight did any yaw damper warning lights illuminate and the circuit breakers were checked and found closed. The reporter said the fix was the replacement of both yaw damper computers.

Original NASA ASRS Text

Title: A B747-200 ON INITIAL CLB DECLARED AN EMER AND DIVERTED DUE TO EXCESSIVE RUDDER TRIM AND UNCOMMANDED FLT CTL INPUTS CAUSED BY BOTH YAW DAMPER COMPUTER FAILURES WITH NO COCKPIT WARNINGS OR INDICATIONS.

Narrative: AFTER TKOF FROM NRT, COPLT (WHO WAS FLYING) NOTICED AN ABNORMAL AMOUNT OF R RUDDER TRIM WAS REQUIRED TO MAINTAIN A TRIMMED CONDITION (APPROX 2 1/2 UNITS OF R RUDDER TRIM). DURING ACCELERATION TO ENRTE CLB SPD OUT OF 10000 FT, UNCOMMANDED FLT CTL INPUTS WERE NOTED. THESE INPUTS BECAME MORE INTENSE AS AIRSPD INCREASED FROM 250 KIAS TO 290 KIAS. NO INDICATIONS OF EQUIP FAILURE WERE EVIDENT IN COCKPIT. CAPT DECLARED EMER AND VECTORS TO AN UNEVENTFUL LNDG WERE PROVIDED BY NARITA APCH CTL. AFTER LNDG, IT WAS DISCOVERED THAT BOTH YAW DAMPER COMPUTERS HAD FAILED, AND IT WAS DETERMINED THAT THOSE COMPUTER FAILURES CAUSED THE UNCOMMANDED FLT CTL INPUTS. THE COMPUTERS WERE REPLACED AND THE FLT RESUMED WITH NO REPEAT OF PROB. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE ACFT WAS A B747-200 AND BOTH UPPER AND LOWER YAW DAMPERS TESTED OK DURING THE PREFLT CHK. THE RPTR SAID AT NO TIME DURING THE FLT DID ANY YAW DAMPER WARNING LIGHTS ILLUMINATE AND THE CIRCUIT BREAKERS WERE CHKED AND FOUND CLOSED. THE RPTR SAID THE FIX WAS THE REPLACEMENT OF BOTH YAW DAMPER COMPUTERS.

Data retrieved from NASA's ASRS site as of July 2007 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.

A B747-200 on initial climb declared an emergency and diverted due to excessive rudder trim and uncommanded flight control inputs caused by both yaw damper computer failures with no cockpit warnings or indications.