An air carrier B-737 had an uncommanded yaw just prior to touchdown.

Attributes
ACN	313458
Time
Date	199508
Day	Thu
Local Time Of Day	0601 To 1200
Place
Locale Reference	airport : phx
State Reference	AZ
Altitude	agl bound lower : 10 agl bound upper : 20
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	tower : phx
Operator	common carrier : air carrier
Make Model Name	B737-400
Operating Under FAR Part	Part 91
Navigation In Use	Other Other
Flight Phase	descent : approach landing other
Route In Use	approach : straight in
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 180 flight time total : 12500 flight time type : 8600
ASRS Report	313458
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : flight engineer pilot : atp
Experience	flight time last 90 days : 180 flight time total : 15000 flight time type : 2500
ASRS Report	314352
Events
Anomaly	aircraft equipment problem : less severe other anomaly other
Independent Detector	other flight crewa
Resolutory Action	aircraft : equipment problem dissipated
Consequence	faa : reviewed incident with flight crew Other
Supplementary
Primary Problem	Aircraft
Air Traffic Incident	other

Narrative:

Approximately 1-2 ft prior to touchdown, aircraft abruptly yawed 4-6 degrees right. Yaw was momentary but very abrupt.no control column or rudder input was generated by crew. Touchdown occurred within a second after yaw occurred. Discussion with tower indicated no windshear or gusts were present nor were any aircraft in the vicinity of touchdown zone creating jet blast hazard. Preceding aircraft was a king air. Landing rollout was uneventful. Callback conversation with reporter revealed the following information: the reporter is a captain of a united states air carrier. He is a retired fighter pilot from the military 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 first officer states that they were flying the B737-300. This reporter says that there are no differences in the flight control system between the 2 aircraft. This reporter believes that the problem was caused by the yaw damper system as there was no rudder pedal movement. The DFDR trace showed that there was a rudder movement from minus 1.33 degrees to plus 7.5 degrees in less than 1 second. The input felt like a hard slap and the removal was very gentle. This reporter would like boeing to look at the yaw damper again. He also thinks that upset training would be nice. Immediately on rollout, the reporter called phx ATCT to determine if there were any gusts or aircraft running up at the end of the runway. The wind had been steady all morning at less than 6 KTS. There were no aircraft near the end of the runway that were running their engines. The aircraft was OTS 2 days at phx to affect repairs. The standby rudder system failed a field and bench test and an actuator was replaced. Callback conversation with reporter revealed the following information on 314352: the reporter sat in on some of the hearings regarding a crash of a similar aircraft and deplores the fact that there were more lawyers than engineers present. The standby rudder system failed a test and was repaired. The reporter and his captain went to an informal hearing with their chief pilot. The FAA and NTSB were notified but not present.

Original NASA ASRS Text

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

Narrative: 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.

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.