Narrative:

At about XA30 hours EDT, while flying from ind to bwi, the rudder trim indicator 'ran away' to a full left rudder trim indication, displacing the rudder neutral point and thereby displacing the rudder to approximately 10.5 degrees to the left of center at approximately 270 IAS at FL265, while climbing to FL370 at best rate of climb speed for the existing airplane confign. The cause (later determined by maintenance inspection) was a short in the rudder trim wiring within the center instrument panel, which caused power to the motor to run, fully displacing the rudder actuator neutral point, and the rudder. As the speed was decreased, the rudder displacement increased due to the effects of diminishing rudder 'blow down' effect. We were able to land the aircraft without injury to passenger or crew, and no damage to the aircraft. Callback conversation with reporter revealed the following information: the reporter stated the trim moved about 3 times as fast on the runaway as the trim moves with the trim knob. The trim moved a full 17 units in about 3 seconds, fast enough to spill the coffee cart. The belief is there were cannon plugs deteriorated which allowed micro voltages to short across the pins. Standard ohm meters will not pick up the micro voltages so the system would check out normal except with special microvoltage measuring devices. The reporter feels the foreign matter that had accumulated in the pedestal contributed to the short that caused the rudder trim neutral to position to the far left. He said the foot pressures got worse as he slowed down. The deploying half spoilers really helped and in fact he landed with the spoilers half deployed and 15 degrees flaps and with the assistance of the copilot on the rudder actually able to align the airplane with the runway. There was left crosswind which aggravated his problem. If there were anything he would recommend it would be not to turn off any flight controls. Do just as the book recommends unless of course the book recommendations are not saving the day. He could not think of anything he would have done differently. Supplemental information from acn 317183: captain was hand flying when the rudder trim moved completely left. Aircraft banked 15- 20 degrees left. Captain maintained control, turned off yaw damper. Aircraft would dutch roll as rudder was being fed in. Captain pulled out speed brake and lowered nose which helped the situation. We asked for priority handling to day. Our flight was from ind-bwi. We descended overhead day. When given to approach, we declared an emergency and asked to overfly runway 24R at 2000 ft, then come back for a landing. At that time both the captain and myself were holding in the right rudder. We both maintained the right rudder input for the landing, which was uneventful. We tracked straight down the runway and used the tiller to taxi to the gate. Aircraft was a B737-300. Callback conversation with reporter of acn 317183 revealed the following information: the reporter stated the rudder trim went to full travel in about 3 seconds. The normal time of travel she said would be at least 10 seconds. The flight attendants said the catering cart was tipped over and the coffee was spilled. There were no injuries. She had just taken a simulator check ride a few hours before this analyst talked with her and she said the recommended rudder trim to compensate for a V1 chop during or after second segment is 5 units. The rudder trim has 17 units travel capability. A short was the cause of the runaway trim. This analyst asked how much rudder pressure in pounds the pilot had to apply to offset the trim. Her response was the captain was holding the pressure and his leg was shaking when she assisted him on the rudder. And even so, the airplane was still in a crab. She said the manufacturer of the trim motor on this aircraft had had problems before and the trim motors with this manufacturer are being replaced. Also, an aural warning is going to be installed on the electric rudder trim.

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Original NASA ASRS Text

Title: BOEING B737-300 FLC EXPERIENCE RUNAWAY RUDDER TRIM.

Narrative: AT ABOUT XA30 HRS EDT, WHILE FLYING FROM IND TO BWI, THE RUDDER TRIM INDICATOR 'RAN AWAY' TO A FULL L RUDDER TRIM INDICATION, DISPLACING THE RUDDER NEUTRAL POINT AND THEREBY DISPLACING THE RUDDER TO APPROX 10.5 DEGS TO THE L OF CTR AT APPROX 270 IAS AT FL265, WHILE CLBING TO FL370 AT BEST RATE OF CLB SPD FOR THE EXISTING AIRPLANE CONFIGN. THE CAUSE (LATER DETERMINED BY MAINT INSPECTION) WAS A SHORT IN THE RUDDER TRIM WIRING WITHIN THE CTR INST PANEL, WHICH CAUSED PWR TO THE MOTOR TO RUN, FULLY DISPLACING THE RUDDER ACTUATOR NEUTRAL POINT, AND THE RUDDER. AS THE SPD WAS DECREASED, THE RUDDER DISPLACEMENT INCREASED DUE TO THE EFFECTS OF DIMINISHING RUDDER 'BLOW DOWN' EFFECT. WE WERE ABLE TO LAND THE ACFT WITHOUT INJURY TO PAX OR CREW, AND NO DAMAGE TO THE ACFT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE TRIM MOVED ABOUT 3 TIMES AS FAST ON THE RUNAWAY AS THE TRIM MOVES WITH THE TRIM KNOB. THE TRIM MOVED A FULL 17 UNITS IN ABOUT 3 SECONDS, FAST ENOUGH TO SPILL THE COFFEE CART. THE BELIEF IS THERE WERE CANNON PLUGS DETERIORATED WHICH ALLOWED MICRO VOLTAGES TO SHORT ACROSS THE PINS. STANDARD OHM METERS WILL NOT PICK UP THE MICRO VOLTAGES SO THE SYS WOULD CHK OUT NORMAL EXCEPT WITH SPECIAL MICROVOLTAGE MEASURING DEVICES. THE RPTR FEELS THE FOREIGN MATTER THAT HAD ACCUMULATED IN THE PEDESTAL CONTRIBUTED TO THE SHORT THAT CAUSED THE RUDDER TRIM NEUTRAL TO POS TO THE FAR L. HE SAID THE FOOT PRESSURES GOT WORSE AS HE SLOWED DOWN. THE DEPLOYING HALF SPOILERS REALLY HELPED AND IN FACT HE LANDED WITH THE SPOILERS HALF DEPLOYED AND 15 DEGS FLAPS AND WITH THE ASSISTANCE OF THE COPLT ON THE RUDDER ACTUALLY ABLE TO ALIGN THE AIRPLANE WITH THE RWY. THERE WAS L XWIND WHICH AGGRAVATED HIS PROB. IF THERE WERE ANYTHING HE WOULD RECOMMEND IT WOULD BE NOT TO TURN OFF ANY FLT CTLS. DO JUST AS THE BOOK RECOMMENDS UNLESS OF COURSE THE BOOK RECOMMENDATIONS ARE NOT SAVING THE DAY. HE COULD NOT THINK OF ANYTHING HE WOULD HAVE DONE DIFFERENTLY. SUPPLEMENTAL INFO FROM ACN 317183: CAPT WAS HAND FLYING WHEN THE RUDDER TRIM MOVED COMPLETELY L. ACFT BANKED 15- 20 DEGS L. CAPT MAINTAINED CTL, TURNED OFF YAW DAMPER. ACFT WOULD DUTCH ROLL AS RUDDER WAS BEING FED IN. CAPT PULLED OUT SPD BRAKE AND LOWERED NOSE WHICH HELPED THE SIT. WE ASKED FOR PRIORITY HANDLING TO DAY. OUR FLT WAS FROM IND-BWI. WE DSNDED OVERHEAD DAY. WHEN GIVEN TO APCH, WE DECLARED AN EMER AND ASKED TO OVERFLY RWY 24R AT 2000 FT, THEN COME BACK FOR A LNDG. AT THAT TIME BOTH THE CAPT AND MYSELF WERE HOLDING IN THE R RUDDER. WE BOTH MAINTAINED THE R RUDDER INPUT FOR THE LNDG, WHICH WAS UNEVENTFUL. WE TRACKED STRAIGHT DOWN THE RWY AND USED THE TILLER TO TAXI TO THE GATE. ACFT WAS A B737-300. CALLBACK CONVERSATION WITH RPTR OF ACN 317183 REVEALED THE FOLLOWING INFO: THE RPTR STATED THE RUDDER TRIM WENT TO FULL TRAVEL IN ABOUT 3 SECONDS. THE NORMAL TIME OF TRAVEL SHE SAID WOULD BE AT LEAST 10 SECONDS. THE FLT ATTENDANTS SAID THE CATERING CART WAS TIPPED OVER AND THE COFFEE WAS SPILLED. THERE WERE NO INJURIES. SHE HAD JUST TAKEN A SIMULATOR CHK RIDE A FEW HRS BEFORE THIS ANALYST TALKED WITH HER AND SHE SAID THE RECOMMENDED RUDDER TRIM TO COMPENSATE FOR A V1 CHOP DURING OR AFTER SECOND SEGMENT IS 5 UNITS. THE RUDDER TRIM HAS 17 UNITS TRAVEL CAPABILITY. A SHORT WAS THE CAUSE OF THE RUNAWAY TRIM. THIS ANALYST ASKED HOW MUCH RUDDER PRESSURE IN POUNDS THE PLT HAD TO APPLY TO OFFSET THE TRIM. HER RESPONSE WAS THE CAPT WAS HOLDING THE PRESSURE AND HIS LEG WAS SHAKING WHEN SHE ASSISTED HIM ON THE RUDDER. AND EVEN SO, THE AIRPLANE WAS STILL IN A CRAB. SHE SAID THE MANUFACTURER OF THE TRIM MOTOR ON THIS ACFT HAD HAD PROBS BEFORE AND THE TRIM MOTORS WITH THIS MANUFACTURER ARE BEING REPLACED. ALSO, AN AURAL WARNING IS GOING TO BE INSTALLED ON THE ELECTRIC RUDDER TRIM.

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.