AN ACR B-737 HAD AN UNCOMMANDED YAW JUST PRIOR TO TOUCHDOWN.

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APPROX 1-2 FT PRIOR TO TOUCHDOWN; ACFT ABRUPTLY YAWED 4-6 DEGS R. YAW WAS MOMENTARY BUT VERY ABRUPT.NO CTL COLUMN OR RUDDER INPUT WAS GENERATED BY CREW. TOUCHDOWN OCCURRED WITHIN A SECOND AFTER YAW OCCURRED. DISCUSSION WITH TWR INDICATED NO WINDSHEAR OR GUSTS WERE PRESENT NOR WERE ANY ACFT IN THE VICINITY OF TOUCHDOWN ZONE CREATING JET BLAST HAZARD. PRECEDING ACFT WAS A KING AIR. LNDG ROLLOUT WAS UNEVENTFUL. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR IS A CAPT OF A UNITED STATES ACR. HE IS A RETIRED FIGHTER PLT FROM THE MIL RESERVE; HAVING FLOWN THE F4 AND THE F16 AND INSTRUCTED AEROBATICS IN THE T37. HE BELIEVES THAT HE WAS FLYING THE B737-400; WHILE HIS FO STATES THAT THEY WERE FLYING THE B737-300. THIS RPTR SAYS THAT THERE ARE NO DIFFERENCES IN THE FLT CTL SYS BTWN THE 2 ACFT. THIS RPTR BELIEVES THAT THE PROB WAS CAUSED BY THE YAW DAMPER SYS AS THERE WAS NO RUDDER PEDAL MOVEMENT. THE DFDR TRACE SHOWED THAT THERE WAS A RUDDER MOVEMENT FROM MINUS 1.33 DEGS TO PLUS 7.5 DEGS IN LESS THAN 1 SECOND. THE INPUT FELT LIKE A HARD SLAP AND THE REMOVAL WAS VERY GENTLE. THIS RPTR WOULD LIKE BOEING TO LOOK AT THE YAW DAMPER AGAIN. HE ALSO THINKS THAT UPSET TRAINING WOULD BE NICE. IMMEDIATELY ON ROLLOUT; THE RPTR CALLED PHX ATCT TO DETERMINE IF THERE WERE ANY GUSTS OR ACFT RUNNING UP AT THE END OF THE RWY. THE WIND HAD BEEN STEADY ALL MORNING AT LESS THAN 6 KTS. THERE WERE NO ACFT NEAR THE END OF THE RWY THAT WERE RUNNING THEIR ENGS. THE ACFT WAS OTS 2 DAYS AT PHX TO AFFECT REPAIRS. THE STANDBY RUDDER SYS FAILED A FIELD AND BENCH TEST AND AN ACTUATOR WAS REPLACED. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO ON 314352: THE RPTR SAT IN ON SOME OF THE HEARINGS REGARDING A CRASH OF A SIMILAR ACFT AND DEPLORES THE FACT THAT THERE WERE MORE LAWYERS THAN ENGINEERS PRESENT. THE STANDBY RUDDER SYS FAILED A TEST AND WAS REPAIRED. THE RPTR AND HIS CAPT WENT TO AN INFORMAL HEARING WITH THEIR CHIEF PLT. THE FAA AND NTSB WERE NOTIFIED BUT NOT PRESENT.