Narrative:

Uncommanded roll/yaw. On jun/xa/99, I picked up aircraft (a B737-300) at portland, me, for the first leg of a 2 leg day to houston. The captain that brought the airplane in had no complaints nor were there any log write-ups. We had 1 1/2 hours ground hold at pwm at the gate due to traffic and WX at phl. First officer flew the leg from pwm to phl. Everything went normal and all appeared to be ok with the aircraft. I did notice and mentioned that my localizer did not pick up a signal until 3 or 4 mins after his did. On the ground at phl, instead of changing aircraft, we were told to keep this aircraft for the leg to houston. I flew the leg from phl to iah with the aircraft flying normal during takeoff, climb out and leveloff. It was a normal leg flown at the flight planned altitude of FL310. Descent was normal with no erratic performance. Traffic was light and we were cleared for the ILS to runway 26 at iah. There were no aircraft in front of us while we were on the approach and the winds were light to calm. This being my first time into iah. I purposely slowed down and started configuring the aircraft a little early. I called for gear down and flaps 15 degrees at the 11 DME point from the runway. I was on the GS hand flying the aircraft with the autothrottles engaged. Bugging down to 150 KTS with flaps 15 degrees, I called for flaps 25 degrees and 30 degrees and bugged down to bug speed +5 KTS. The autothrottles retarded the throttles to idle (at 8 DME from field). When the speed approached bug the throttles advanced to maintain the dialed in speed which was at 134 KTS. At this point the aircraft rolled right then left at 10 degrees, 2-3 times and there seemed to be some yaw first right then left. I disengaged the autothrottles and said to the first officer, 'I guess that was asymmetrical thrust (spool up).' what was so noticeable about the event was that when I tried to counteract the conditions the controls were very soft to almost non existent. I continued on the approach with the runway in sight and after say 10-20 seconds with speed and descent stabilized, the aircraft started the uncommanded rolling motion again, again the flight controls did not initially stop nor correct the action. Again the controls did not appear to be responding to inputs normally. It took exaggerated left aileron to stop the rolls to the right and vice versa, with the rudder feeling soft and mushy and not responding to inputs. Once again I regained control. (At the onset of the second series of rolls I called for yaw damper off and at the same time reached up and turned it off.) at this point I asked what were the winds from page 2 of the progress heading in the FMC. He said 'calm.' all appropriate checklists had been called for and completed I might add. For a short time -- say 5-10 seconds -- control inputs appeared to return to normal, and then from say 500-800 ft out they appeared to be gradually just fading out with the aircraft continuing the roll action with very little rudder response. In any normal aircraft I would have gone around at this point, but at this point I did not feel the airplane was airworthy. I decided I had to land even if it meant a hard landing due to lack of normal control inputs. I had marginal rudder and aileron control at touchdown. I used nose-up trim at flare and it gave me a normal landing. From the 500 ft point on down, I confirmed symmetrical thrust, normal sink and realized that I would have to accept drift at touchdown rather than have an exaggerated aileron input in and have the controls return to normal and hit an engine or wingtip. Rollout and reverse appeared to be normal. Taxi in was normal. Callback conversation with reporter revealed the following information: the crew was on the second leg of the day with the aircraft. On approach, with flaps 15 degrees and approximately 150 KTS, the aircraft began an uncommanded yaw and roll. At this time the rolling motion was about 10 degrees from side-to-side and becoming divergent. The crew disconnected the autoplt and autothrottles, no change. The captain then verified that the engines were not surging by observing no change in N1 or fuel flow. Next, the yaw damper was turned off, again no change. Hydraulic system 'a' and 'B' were verifiedas normal. All this time the captain was attempting to control the roll with control yoke input. Using the technique he was taught to control dutch rolls, he inputted sharp aileron and spoiler inputs. Control yolk angles of at least 45 degrees were used. At no time did the captain feel any unwanted movement in the rudder peddles. The crew did not attempt to turn on standby rudder because of the low aircraft altitude. The first officer became concerned that if allowed to continue for several more cycles, the aircraft would go on its back. The captain felt the lateral and rudder control were soft and did not provide normal response. The captain debriefed the air carrier maintenance department, and had an extensive discussion with air carrier flight test department. The aircraft was given an extensive flight test and ferried back to the air carrier main maintenance base. The only thing that was found was a loose flap fairing canoe on the left wing. The captain does not feel that this could have caused the problems he experienced.

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Original NASA ASRS Text

Title: B737 CREW HAD UNCOMMANDED CTL INPUT AND UNRESPONSIVE FLT CTLS ON FINAL APCH AT IAH.

Narrative: UNCOMMANDED ROLL/YAW. ON JUN/XA/99, I PICKED UP ACFT (A B737-300) AT PORTLAND, ME, FOR THE FIRST LEG OF A 2 LEG DAY TO HOUSTON. THE CAPT THAT BROUGHT THE AIRPLANE IN HAD NO COMPLAINTS NOR WERE THERE ANY LOG WRITE-UPS. WE HAD 1 1/2 HRS GND HOLD AT PWM AT THE GATE DUE TO TFC AND WX AT PHL. FO FLEW THE LEG FROM PWM TO PHL. EVERYTHING WENT NORMAL AND ALL APPEARED TO BE OK WITH THE ACFT. I DID NOTICE AND MENTIONED THAT MY LOC DID NOT PICK UP A SIGNAL UNTIL 3 OR 4 MINS AFTER HIS DID. ON THE GND AT PHL, INSTEAD OF CHANGING ACFT, WE WERE TOLD TO KEEP THIS ACFT FOR THE LEG TO HOUSTON. I FLEW THE LEG FROM PHL TO IAH WITH THE ACFT FLYING NORMAL DURING TKOF, CLBOUT AND LEVELOFF. IT WAS A NORMAL LEG FLOWN AT THE FLT PLANNED ALT OF FL310. DSCNT WAS NORMAL WITH NO ERRATIC PERFORMANCE. TFC WAS LIGHT AND WE WERE CLRED FOR THE ILS TO RWY 26 AT IAH. THERE WERE NO ACFT IN FRONT OF US WHILE WE WERE ON THE APCH AND THE WINDS WERE LIGHT TO CALM. THIS BEING MY FIRST TIME INTO IAH. I PURPOSELY SLOWED DOWN AND STARTED CONFIGURING THE ACFT A LITTLE EARLY. I CALLED FOR GEAR DOWN AND FLAPS 15 DEGS AT THE 11 DME POINT FROM THE RWY. I WAS ON THE GS HAND FLYING THE ACFT WITH THE AUTOTHROTTLES ENGAGED. BUGGING DOWN TO 150 KTS WITH FLAPS 15 DEGS, I CALLED FOR FLAPS 25 DEGS AND 30 DEGS AND BUGGED DOWN TO BUG SPD +5 KTS. THE AUTOTHROTTLES RETARDED THE THROTTLES TO IDLE (AT 8 DME FROM FIELD). WHEN THE SPD APCHED BUG THE THROTTLES ADVANCED TO MAINTAIN THE DIALED IN SPD WHICH WAS AT 134 KTS. AT THIS POINT THE ACFT ROLLED R THEN L AT 10 DEGS, 2-3 TIMES AND THERE SEEMED TO BE SOME YAW FIRST R THEN L. I DISENGAGED THE AUTOTHROTTLES AND SAID TO THE FO, 'I GUESS THAT WAS ASYMMETRICAL THRUST (SPOOL UP).' WHAT WAS SO NOTICEABLE ABOUT THE EVENT WAS THAT WHEN I TRIED TO COUNTERACT THE CONDITIONS THE CTLS WERE VERY SOFT TO ALMOST NON EXISTENT. I CONTINUED ON THE APCH WITH THE RWY IN SIGHT AND AFTER SAY 10-20 SECONDS WITH SPD AND DSCNT STABILIZED, THE ACFT STARTED THE UNCOMMANDED ROLLING MOTION AGAIN, AGAIN THE FLT CTLS DID NOT INITIALLY STOP NOR CORRECT THE ACTION. AGAIN THE CTLS DID NOT APPEAR TO BE RESPONDING TO INPUTS NORMALLY. IT TOOK EXAGGERATED L AILERON TO STOP THE ROLLS TO THE R AND VICE VERSA, WITH THE RUDDER FEELING SOFT AND MUSHY AND NOT RESPONDING TO INPUTS. ONCE AGAIN I REGAINED CTL. (AT THE ONSET OF THE SECOND SERIES OF ROLLS I CALLED FOR YAW DAMPER OFF AND AT THE SAME TIME REACHED UP AND TURNED IT OFF.) AT THIS POINT I ASKED WHAT WERE THE WINDS FROM PAGE 2 OF THE PROGRESS HDG IN THE FMC. HE SAID 'CALM.' ALL APPROPRIATE CHKLISTS HAD BEEN CALLED FOR AND COMPLETED I MIGHT ADD. FOR A SHORT TIME -- SAY 5-10 SECONDS -- CTL INPUTS APPEARED TO RETURN TO NORMAL, AND THEN FROM SAY 500-800 FT OUT THEY APPEARED TO BE GRADUALLY JUST FADING OUT WITH THE ACFT CONTINUING THE ROLL ACTION WITH VERY LITTLE RUDDER RESPONSE. IN ANY NORMAL ACFT I WOULD HAVE GONE AROUND AT THIS POINT, BUT AT THIS POINT I DID NOT FEEL THE AIRPLANE WAS AIRWORTHY. I DECIDED I HAD TO LAND EVEN IF IT MEANT A HARD LNDG DUE TO LACK OF NORMAL CTL INPUTS. I HAD MARGINAL RUDDER AND AILERON CTL AT TOUCHDOWN. I USED NOSE-UP TRIM AT FLARE AND IT GAVE ME A NORMAL LNDG. FROM THE 500 FT POINT ON DOWN, I CONFIRMED SYMMETRICAL THRUST, NORMAL SINK AND REALIZED THAT I WOULD HAVE TO ACCEPT DRIFT AT TOUCHDOWN RATHER THAN HAVE AN EXAGGERATED AILERON INPUT IN AND HAVE THE CTLS RETURN TO NORMAL AND HIT AN ENG OR WINGTIP. ROLLOUT AND REVERSE APPEARED TO BE NORMAL. TAXI IN WAS NORMAL. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE CREW WAS ON THE SECOND LEG OF THE DAY WITH THE ACFT. ON APCH, WITH FLAPS 15 DEGS AND APPROX 150 KTS, THE ACFT BEGAN AN UNCOMMANDED YAW AND ROLL. AT THIS TIME THE ROLLING MOTION WAS ABOUT 10 DEGS FROM SIDE-TO-SIDE AND BECOMING DIVERGENT. THE CREW DISCONNECTED THE AUTOPLT AND AUTOTHROTTLES, NO CHANGE. THE CAPT THEN VERIFIED THAT THE ENGS WERE NOT SURGING BY OBSERVING NO CHANGE IN N1 OR FUEL FLOW. NEXT, THE YAW DAMPER WAS TURNED OFF, AGAIN NO CHANGE. HYD SYS 'A' AND 'B' WERE VERIFIEDAS NORMAL. ALL THIS TIME THE CAPT WAS ATTEMPTING TO CTL THE ROLL WITH CTL YOKE INPUT. USING THE TECHNIQUE HE WAS TAUGHT TO CTL DUTCH ROLLS, HE INPUTTED SHARP AILERON AND SPOILER INPUTS. CTL YOLK ANGLES OF AT LEAST 45 DEGS WERE USED. AT NO TIME DID THE CAPT FEEL ANY UNWANTED MOVEMENT IN THE RUDDER PEDDLES. THE CREW DID NOT ATTEMPT TO TURN ON STANDBY RUDDER BECAUSE OF THE LOW ACFT ALT. THE FO BECAME CONCERNED THAT IF ALLOWED TO CONTINUE FOR SEVERAL MORE CYCLES, THE ACFT WOULD GO ON ITS BACK. THE CAPT FELT THE LATERAL AND RUDDER CTL WERE SOFT AND DID NOT PROVIDE NORMAL RESPONSE. THE CAPT DEBRIEFED THE ACR MAINT DEPT, AND HAD AN EXTENSIVE DISCUSSION WITH ACR FLT TEST DEPT. THE ACFT WAS GIVEN AN EXTENSIVE FLT TEST AND FERRIED BACK TO THE ACR MAIN MAINT BASE. THE ONLY THING THAT WAS FOUND WAS A LOOSE FLAP FAIRING CANOE ON THE L WING. THE CAPT DOES NOT FEEL THAT THIS COULD HAVE CAUSED THE PROBS HE EXPERIENCED.

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.