B757 runaway rudder trim.

Attributes
ACN	296292
Time
Date	199502
Day	Tue
Local Time Of Day	1201 To 1800
Place
Locale Reference	airport : lax
State Reference	CA
Altitude	agl bound lower : 0 agl bound upper : 0 msl bound lower : 0 msl bound upper : 0
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Operator	common carrier : air carrier
Make Model Name	B757-200
Operating Under FAR Part	Part 121
Navigation In Use	Other Other
Flight Phase	ground other : taxi
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time total : 18000 flight time type : 3500
ASRS Report	296292
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : atp
Events
Anomaly	aircraft equipment problem : critical other anomaly other
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:

On taxi out of lax, I turned the rudder control knob momentarily to center rudder trim. The copilot, a min later, noticed the rudder trim knob had not rectred and it had stuck in the 'nose right position.' the rudder trim had gone full right trim, 17 degrees. We re-zeroed the trim and both of us made a mental note to be careful on the next flight. All went well until 1000 ft altitude on final at slc. The copilot needed some slight right trim, the knob stuck in the full right position and began to drive the rudder. He felt the rudder pedals move and quickly looked at the trim and trim knob. It was going through 7 degrees of right trim. He re-centered the knob and then zeroed the trim. Had we not caught the trim problem on takeoff at lax, the takeoff would have bee interesting! The same can be said for the landing in slc. The aircraft was a B757. Callback conversation with reporter revealed the following information: reporter stated that this was the first time that he had experienced a runaway rudder trim in his 5 yrs and approximately 3500 hours of operating the B757. He later learned through his employee union chairman that the trim control knob ctring spring had broken causing the trim control to remain connected to the electrical inputs for continuing to drive the trim motor after the flight crew released the control knob instead of re-ctring the control knob as designed. Therefore, it had an affect of causing a 'runaway' trim. The ctring spring was replaced. He further stated that this was the only known incident reported through the mechanical interruption discrepancy reporting system to this date!

Original NASA ASRS Text

Title: B757 RUNAWAY RUDDER TRIM.

Narrative: ON TAXI OUT OF LAX, I TURNED THE RUDDER CTL KNOB MOMENTARILY TO CTR RUDDER TRIM. THE COPLT, A MIN LATER, NOTICED THE RUDDER TRIM KNOB HAD NOT RECTRED AND IT HAD STUCK IN THE 'NOSE R POS.' THE RUDDER TRIM HAD GONE FULL R TRIM, 17 DEGS. WE RE-ZEROED THE TRIM AND BOTH OF US MADE A MENTAL NOTE TO BE CAREFUL ON THE NEXT FLT. ALL WENT WELL UNTIL 1000 FT ALT ON FINAL AT SLC. THE COPLT NEEDED SOME SLIGHT R TRIM, THE KNOB STUCK IN THE FULL R POS AND BEGAN TO DRIVE THE RUDDER. HE FELT THE RUDDER PEDALS MOVE AND QUICKLY LOOKED AT THE TRIM AND TRIM KNOB. IT WAS GOING THROUGH 7 DEGS OF R TRIM. HE RE-CTRED THE KNOB AND THEN ZEROED THE TRIM. HAD WE NOT CAUGHT THE TRIM PROB ON TKOF AT LAX, THE TKOF WOULD HAVE BEE INTERESTING! THE SAME CAN BE SAID FOR THE LNDG IN SLC. THE ACFT WAS A B757. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATED THAT THIS WAS THE FIRST TIME THAT HE HAD EXPERIENCED A RUNAWAY RUDDER TRIM IN HIS 5 YRS AND APPROX 3500 HRS OF OPERATING THE B757. HE LATER LEARNED THROUGH HIS EMPLOYEE UNION CHAIRMAN THAT THE TRIM CTL KNOB CTRING SPRING HAD BROKEN CAUSING THE TRIM CTL TO REMAIN CONNECTED TO THE ELECTRICAL INPUTS FOR CONTINUING TO DRIVE THE TRIM MOTOR AFTER THE FLC RELEASED THE CTL KNOB INSTEAD OF RE-CTRING THE CTL KNOB AS DESIGNED. THEREFORE, IT HAD AN AFFECT OF CAUSING A 'RUNAWAY' TRIM. THE CTRING SPRING WAS REPLACED. HE FURTHER STATED THAT THIS WAS THE ONLY KNOWN INCIDENT RPTED THROUGH THE MECHANICAL INTERRUPTION DISCREPANCY RPTING SYS TO THIS DATE!

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.