Narrative:

Approximately 1 hour 15 mins out of lax at FL350 after going through light turbulence, our jump seat rider asked if we felt a yaw movement. Yes, it was noticeable. Captain then asked me if this was normal on the B737. I told him it was not. We decided to try autoplt 'B'. The pulsation every 3-5 seconds continued at cruise and on descent. Once the flaps were extended beyond 10 degrees, it then took 2-3 units of aileron trim to fly straight and level. We continued the visual approach and landed in lax with no further incident. Callback conversation with reporter revealed the following information: the reporter stated the yaw inputs were noticeable every 3-5 seconds and the first reaction was to try switching to another autoplt channel but no change occurred, so the autoplt was switched off. Reporter advised in descent with 10 degrees flaps selected had to add 3-5 units of aileron trim for straight and level flight. The reporter said the reports in the log were written so the aircraft had to be worked on on the overnight layover and maintenance reported replacing the yaw damper coupler to correct the rudder pulsations and adjusted 1 flap jackscrew for trim correction. Reporter states the aircraft test flew ok. The reporter related that maintenance advised the aircraft manufacturer of the rudder report. The reporter reports flying the aircraft the next day back to ZZZ and all system were ok, but on arrival at ZZZ the aircraft manufacturer advised replacing the rudder power unit. The reporter states this was disturbing because the unit should have been replaced on the overnight layover when the other items were corrected. Callback conversation with reporter acn 377167 revealed the following information: reporter stated the maintenance action to correct the reports were available but did know that 28-29 inputs were recorded on the flight data recorder.

Google
 

Original NASA ASRS Text

Title: A B737-400 IN CRUISE AND DSCNT EXPERIENCED RUDDER PULSATIONS EVERY 3-5 SECONDS AND WAS OBSERVED ON THE YAW DAMPER RUDDER INDICATOR.

Narrative: APPROX 1 HR 15 MINS OUT OF LAX AT FL350 AFTER GOING THROUGH LIGHT TURB, OUR JUMP SEAT RIDER ASKED IF WE FELT A YAW MOVEMENT. YES, IT WAS NOTICEABLE. CAPT THEN ASKED ME IF THIS WAS NORMAL ON THE B737. I TOLD HIM IT WAS NOT. WE DECIDED TO TRY AUTOPLT 'B'. THE PULSATION EVERY 3-5 SECONDS CONTINUED AT CRUISE AND ON DSCNT. ONCE THE FLAPS WERE EXTENDED BEYOND 10 DEGS, IT THEN TOOK 2-3 UNITS OF AILERON TRIM TO FLY STRAIGHT AND LEVEL. WE CONTINUED THE VISUAL APCH AND LANDED IN LAX WITH NO FURTHER INCIDENT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE YAW INPUTS WERE NOTICEABLE EVERY 3-5 SECONDS AND THE FIRST REACTION WAS TO TRY SWITCHING TO ANOTHER AUTOPLT CHANNEL BUT NO CHANGE OCCURRED, SO THE AUTOPLT WAS SWITCHED OFF. RPTR ADVISED IN DSCNT WITH 10 DEGS FLAPS SELECTED HAD TO ADD 3-5 UNITS OF AILERON TRIM FOR STRAIGHT AND LEVEL FLT. THE RPTR SAID THE RPTS IN THE LOG WERE WRITTEN SO THE ACFT HAD TO BE WORKED ON ON THE OVERNIGHT LAYOVER AND MAINT RPTED REPLACING THE YAW DAMPER COUPLER TO CORRECT THE RUDDER PULSATIONS AND ADJUSTED 1 FLAP JACKSCREW FOR TRIM CORRECTION. RPTR STATES THE ACFT TEST FLEW OK. THE RPTR RELATED THAT MAINT ADVISED THE ACFT MANUFACTURER OF THE RUDDER RPT. THE RPTR RPTS FLYING THE ACFT THE NEXT DAY BACK TO ZZZ AND ALL SYS WERE OK, BUT ON ARR AT ZZZ THE ACFT MANUFACTURER ADVISED REPLACING THE RUDDER PWR UNIT. THE RPTR STATES THIS WAS DISTURBING BECAUSE THE UNIT SHOULD HAVE BEEN REPLACED ON THE OVERNIGHT LAYOVER WHEN THE OTHER ITEMS WERE CORRECTED. CALLBACK CONVERSATION WITH RPTR ACN 377167 REVEALED THE FOLLOWING INFO: RPTR STATED THE MAINT ACTION TO CORRECT THE RPTS WERE AVAILABLE BUT DID KNOW THAT 28-29 INPUTS WERE RECORDED ON THE FLT DATA RECORDER.

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.