Narrative:

Upon receiving clearance to 8000, I disconnected the autoplt and automatic throttle and elected to manually fly the descent. As I pitched the nose over and closed the throttles to begin the descent, I felt the aircraft begin to yaw, it felt exactly like an engine failure as I had to apply rudder to hold the aircraft straight. I immediately informed the captain that something was wrong and it felt like an engine failure. We both looked at the engine instruments which all displayed normal readings and matched each other. Additionally they all indicated an idle condition. As we reached 8000' I pitched and manually pushed up the throttles and had to add further rudder input to hold heading. At this time I reached up and cycled the yaw dampener switch off and on thinking that the problem might be in the yaw dampener system. This had no corrective effect at all. My next action was to reach down to the back of the center console to trim out the rudder pressure I was holding. At this time I found the rudder trim tab position indicator pegged on the right side. I began to center the rudder trim which required me to hold it continuously as it motored back. At the same time I returned my attention to the instruments as I was hand flying the aircraft. At some point as the trim was about 2/3 of the way back toward center, I released the trim knob so as to use my left hand to adjust the throttles. As I glanced down to place my left hand back on the trim knob to finish centering the trim, I was amazed to see the trim running away to the right. It was motoring as if the knob was twisted to the right, but the rudder trim knob was centered. I told the captain that the rudder trim was running away at which time he turned the trim knob left, centered the trim position indicator, released it, and we had no further difficulties during the remainder of the approach. There was no jump seat rider or cockpit visitor just prior to the runaway, and when I disengaged the autoplt the aircraft was in level flight, power on and in trim. It is my belief that the runaway began at the time of the autoplt automatic throttle disconnect or shortly thereafter as the timing was just about right, ie, the yaw problem came on slowly and got worse. Upon arrival at ord the problem was written up in the maintenance log and we spent about 15 mins debriefing company mechanics. The aircraft was taken out of service. We were unable to duplicate the runaway on the ground.

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Original NASA ASRS Text

Title: FLT CREW OF ADVANCED MLG EXPERIENCED A RUNAWAY RUDDER TRIM COINCIDENTAL WITH AUTOPLT AND AUTO THROTTLE DISCONNECT WHILE DESCENDING.

Narrative: UPON RECEIVING CLRNC TO 8000, I DISCONNECTED THE AUTOPLT AND AUTO THROTTLE AND ELECTED TO MANUALLY FLY THE DSCNT. AS I PITCHED THE NOSE OVER AND CLOSED THE THROTTLES TO BEGIN THE DSCNT, I FELT THE ACFT BEGIN TO YAW, IT FELT EXACTLY LIKE AN ENGINE FAILURE AS I HAD TO APPLY RUDDER TO HOLD THE ACFT STRAIGHT. I IMMEDIATELY INFORMED THE CAPT THAT SOMETHING WAS WRONG AND IT FELT LIKE AN ENGINE FAILURE. WE BOTH LOOKED AT THE ENGINE INSTRUMENTS WHICH ALL DISPLAYED NORMAL READINGS AND MATCHED EACH OTHER. ADDITIONALLY THEY ALL INDICATED AN IDLE CONDITION. AS WE REACHED 8000' I PITCHED AND MANUALLY PUSHED UP THE THROTTLES AND HAD TO ADD FURTHER RUDDER INPUT TO HOLD HDG. AT THIS TIME I REACHED UP AND CYCLED THE YAW DAMPENER SWITCH OFF AND ON THINKING THAT THE PROBLEM MIGHT BE IN THE YAW DAMPENER SYSTEM. THIS HAD NO CORRECTIVE EFFECT AT ALL. MY NEXT ACTION WAS TO REACH DOWN TO THE BACK OF THE CENTER CONSOLE TO TRIM OUT THE RUDDER PRESSURE I WAS HOLDING. AT THIS TIME I FOUND THE RUDDER TRIM TAB POSITION INDICATOR PEGGED ON THE RIGHT SIDE. I BEGAN TO CENTER THE RUDDER TRIM WHICH REQUIRED ME TO HOLD IT CONTINUOUSLY AS IT MOTORED BACK. AT THE SAME TIME I RETURNED MY ATTN TO THE INSTRUMENTS AS I WAS HAND FLYING THE ACFT. AT SOME POINT AS THE TRIM WAS ABOUT 2/3 OF THE WAY BACK TOWARD CENTER, I RELEASED THE TRIM KNOB SO AS TO USE MY LEFT HAND TO ADJUST THE THROTTLES. AS I GLANCED DOWN TO PLACE MY LEFT HAND BACK ON THE TRIM KNOB TO FINISH CENTERING THE TRIM, I WAS AMAZED TO SEE THE TRIM RUNNING AWAY TO THE RIGHT. IT WAS MOTORING AS IF THE KNOB WAS TWISTED TO THE RIGHT, BUT THE RUDDER TRIM KNOB WAS CENTERED. I TOLD THE CAPT THAT THE RUDDER TRIM WAS RUNNING AWAY AT WHICH TIME HE TURNED THE TRIM KNOB LEFT, CENTERED THE TRIM POSITION INDICATOR, RELEASED IT, AND WE HAD NO FURTHER DIFFICULTIES DURING THE REMAINDER OF THE APCH. THERE WAS NO JUMP SEAT RIDER OR COCKPIT VISITOR JUST PRIOR TO THE RUNAWAY, AND WHEN I DISENGAGED THE AUTOPLT THE ACFT WAS IN LEVEL FLT, POWER ON AND IN TRIM. IT IS MY BELIEF THAT THE RUNAWAY BEGAN AT THE TIME OF THE AUTOPLT AUTO THROTTLE DISCONNECT OR SHORTLY THEREAFTER AS THE TIMING WAS JUST ABOUT RIGHT, IE, THE YAW PROBLEM CAME ON SLOWLY AND GOT WORSE. UPON ARR AT ORD THE PROBLEM WAS WRITTEN UP IN THE MAINT LOG AND WE SPENT ABOUT 15 MINS DEBRIEFING COMPANY MECHANICS. THE ACFT WAS TAKEN OUT OF SERVICE. WE WERE UNABLE TO DUPLICATE THE RUNAWAY ON THE GND.

Data retrieved from NASA's ASRS site as of August 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.