During climb, rudder trim runs away causing severe yaw.

Attributes
ACN	213222
Time
Date	199204
Day	Wed
Local Time Of Day	1801 To 2400
Place
Locale Reference	airport : sfo
State Reference	CA
Altitude	msl bound lower : 23000 msl bound upper : 23000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zoa
Operator	common carrier : air carrier
Make Model Name	Medium Large Transport, Low Wing, 2 Turbojet Eng
Navigation In Use	Other Other
Flight Phase	climbout : intermediate altitude
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : commercial pilot : instrument
Experience	flight time last 90 days : 180 flight time total : 7000 flight time type : 900
ASRS Report	213222
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Events
Anomaly	aircraft equipment problem : critical other anomaly
Independent Detector	aircraft equipment other aircraft equipment : unspecified other flight crewa
Resolutory Action	flight crew : overcame equipment problem
Consequence	Other
Supplementary
Primary Problem	Aircraft
Air Traffic Incident	other

Narrative:

The captain was hand flying prior to level off, he trimmed the rudder then engaged the autoplt. The aircraft entered into a severe yaw and the autoplt kicked off. After checking for an engine failure, I looked at the rudder trim. It was moving to the far left. I quickly reversed the trim to center. I cannot say whether the trim knob ctring mechanism failed (I tested the ctring mechanism many times after landing and it centered every time), or the trim continued to move with a centered trim knob. Callback conversation with reporter revealed the following information. Climbing out of sfo, the captain who was new on the airplane, used the rudder trim just prior to engaging the autoplt. When aircraft yawed, reporter noticed 10 units of left rudder trim. He grabbed the trim knob and turned it completely to the right and trim moved to '0'. He is therefore unsure if, during the time trim was running to the left, the trim knob was in the left or center ('0') position. Air carrier proposes a squirt of lubricant on the knob as a fix.

Original NASA ASRS Text

Title: DURING CLB, RUDDER TRIM RUNS AWAY CAUSING SEVERE YAW.

Narrative: THE CAPT WAS HAND FLYING PRIOR TO LEVEL OFF, HE TRIMMED THE RUDDER THEN ENGAGED THE AUTOPLT. THE ACFT ENTERED INTO A SEVERE YAW AND THE AUTOPLT KICKED OFF. AFTER CHKING FOR AN ENG FAILURE, I LOOKED AT THE RUDDER TRIM. IT WAS MOVING TO THE FAR L. I QUICKLY REVERSED THE TRIM TO CTR. I CANNOT SAY WHETHER THE TRIM KNOB CTRING MECHANISM FAILED (I TESTED THE CTRING MECHANISM MANY TIMES AFTER LNDG AND IT CTRED EVERY TIME), OR THE TRIM CONTINUED TO MOVE WITH A CTRED TRIM KNOB. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO. CLBING OUT OF SFO, THE CAPT WHO WAS NEW ON THE AIRPLANE, USED THE RUDDER TRIM JUST PRIOR TO ENGAGING THE AUTOPLT. WHEN ACFT YAWED, RPTR NOTICED 10 UNITS OF L RUDDER TRIM. HE GRABBED THE TRIM KNOB AND TURNED IT COMPLETELY TO THE R AND TRIM MOVED TO '0'. HE IS THEREFORE UNSURE IF, DURING THE TIME TRIM WAS RUNNING TO THE L, THE TRIM KNOB WAS IN THE L OR CTR ('0') POS. ACR PROPOSES A SQUIRT OF LUBRICANT ON THE KNOB AS A FIX.

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.