Narrative:

On climb out at bur, altitude above 10000 ft, I was flying the aircraft. I noticed that the flight characteristics of the aircraft were very sloppy. The aircraft seemed to swish around when making turns, like it was fishtailing. I mentioned it to first officer. There were no indications or warning lights showing anything wrong. We leveled off at FL250 due to reported moderate turbulence above FL270. Over dag VOR, we hit a pocket of moderate to severe turbulence for about 30 seconds. The aircraft reacted violently and very unstable. The autoplt seemed to be overctling the roll, causing the aircraft to roll and yaw more. After further observation, I determined that the yaw dampener was inoperative. The yaw dampener indicator on the forward upper instrument panel was centered and not moving. I contacted the flight attendants to make sure they were not injured. They were not injured and all the passenger were ok and in their seats. The seat belt sign had been on prior to the event. I made announcement to the passenger about the unstable air and not to be alarmed. After las VOR, we tried to climb to a higher altitude because FL330 was reported smooth. While climbing through FL300, we again encountered light to moderate turbulence. The aircraft became very unstable. It was fishtailing and was getting into a bad dutch roll situation. I had to take the aircraft off the autoplt and use dutch roll recovery techniques (learned in my past B727 training) to stabilize the aircraft. The autoplt could not handle the roll problem and made it worse. We notified ATC of the problem and requested an immediate descent back down to FL270, which they gave us, for the duration of the cruise portion of the flight. I put the aircraft back on the autoplt and we had no further problems during the cruise portion of the flight. The air was relatively smooth. We then followed up with the flight manual irregularity procedure 7-40. There was not much about the yaw dampener out condition other than we were restr to land with flaps 15 degrees and no greater than light turbulence. I spoke with maintenance regarding the problem and told him of the unstable flight characteristics and that we highly suspected that the yaw dampener was inoperative. We told him we had reset the circuit breakers and turned the yaw dampener switch off and on. The master caution and the switch and the light functioned normally. Maintenance said that he would notify den maintenance of the problem and be ready for us upon our arrival. I checked the WX at least 4 times to make sure that den did not have any WX problems. There were no special WX notices and basically good for our arrival. It was a little windy (140 degrees at 14 KTS), but there was no llwsa alerts in effect. So, we continued on to den. We realized that we were burning more fuel due to our lower flight altitude, but I had added my own 1000 pounds of comfort fuel for the flight since we were flight planned to land in den with 6.2 at the gate with no alternate, and only .5 for the descent. I was glad I took the extra fuel and wish I had taken more later. About 80 mi out den, we received the ATIS. The winds were 140 degrees at 16 KTS, with moderate turbulence reported from 16000-18000 ft and an llwsa was in effect. I told first officer that I would hand fly the aircraft below FL200 since the autoplt could not handle the yaw dampener problem in rough air. We descended down to 11000 ft with no problem. There was only light turbulence. First officer switched over to approach control and received the news that there was mod turbulence reported by all aircraft and moderate to severe turbulence on final approach to runway 16. Approach said that some aircraft had gone around. We told approach control that we had a flight control problem and we were restr to light turbulence, and furthermore, we were approaching a low fuel situation. We asked if any other runways were better. They said that they were only using the operations on runway 16 and runway 17L&right. We were being vectored for a visual ILS backed approach to runway 16. ATC asked us what our intentions were. The last 2-3 aircraft had gone around. The first officer and I discussed our situation and decided because of our low fuel situation (we were down to +/-5.5 on the fuel) that we should attempt an approach. The conditions could change. If it got bad, we would go around. On final, below 8000 ft, we encountered moderate or greater turbulence. The aircraft became very unstable, almost uncontrollable. The rapid roll from side-to-side was greater than 30 degrees with the nose and tail swishing around violently. I immediately applied go around thrust, hit the toga button and made a missed approach. We informed ATC of the situation. And again, we told him that we had a bad control problem and we were now in a low fuel situation. We had to land soon. We needed to get on the ground, and we didn't have enough fuel to go to another airport. ATC asked us if we were declaring an emergency. I said only if you don't get us in immediately. They immediately gave us vectors to bring us around for an approach to runway 26. They were changing runway operations to land on runway 26 and runway 25. We climbed up to 13000 ft and were turned to the east. We were cleared to 8000 ft and turned onto final approach about 10 mi out for a visual to runway 26. The 2 aircraft in front of us had landed with no problems and only light turbulence. The wind was approximately 330 degrees at 19 KTS. We briefed the approach and a flaps 15 degrees landing. Fortunately, there was only light turbulence on final and we landed without further incident. The usable fuel on board was approximately 4.2, discounting the .2 in the center tank which was unusable. I did not prepare the cabin for evacuate/evacuation. I did not feel that the situation warranted it at the time. Also, there was no need to alarm the passenger since a normal landing was expected. Upon gate arrival, the mechanic met the flight. The aircraft was written up 'this aircraft is unsafe to fly. Not suitable for safe flight in turbulence.' callback conversation with reporter revealed the following information: reporter advised maintenance did change the rudder control power unit. But in accordance with their check, the yaw damper was working ok. Reporter confirmed the aircraft was nearly uncontrollable. An emergency was not declared but the wind on runway 26 was more favorable for the landing. A company conference call was made to discuss the problem. Has not had any feedback reference the incident.

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Original NASA ASRS Text

Title: B737-300 OFF BUR-DEN. CAPT, PF, SUSPECTED YAW DAMPER INOP ACCOUNT YAWING THROUGH THE AUTOPLT. ENCOUNTERED TURB DAG AND NECESSARY TO DISCONNECT THE AUTOPLT TO MAINTAIN CTL. ON ARR DEN HAD TO USE 15 DEGS FLAPS AND ONLY LIGHT TURB TO MAKE THE APCH. ENCOUNTERED LLWS MAKING IT NECESSARY TO MAKE A GAR. ACCOUNT LOW FUEL, REQUESTED AND LANDED RWY 26 WITH LIGHT TURB. MAINT CLAIMS YAW DAMPER OK BUT CHANGED RUDDER PWR CTL UNIT AS A PRECAUTION.

Narrative: ON CLB OUT AT BUR, ALT ABOVE 10000 FT, I WAS FLYING THE ACFT. I NOTICED THAT THE FLT CHARACTERISTICS OF THE ACFT WERE VERY SLOPPY. THE ACFT SEEMED TO SWISH AROUND WHEN MAKING TURNS, LIKE IT WAS FISHTAILING. I MENTIONED IT TO FO. THERE WERE NO INDICATIONS OR WARNING LIGHTS SHOWING ANYTHING WRONG. WE LEVELED OFF AT FL250 DUE TO RPTED MODERATE TURB ABOVE FL270. OVER DAG VOR, WE HIT A POCKET OF MODERATE TO SEVERE TURB FOR ABOUT 30 SECONDS. THE ACFT REACTED VIOLENTLY AND VERY UNSTABLE. THE AUTOPLT SEEMED TO BE OVERCTLING THE ROLL, CAUSING THE ACFT TO ROLL AND YAW MORE. AFTER FURTHER OBSERVATION, I DETERMINED THAT THE YAW DAMPENER WAS INOP. THE YAW DAMPENER INDICATOR ON THE FORWARD UPPER INST PANEL WAS CTRED AND NOT MOVING. I CONTACTED THE FLT ATTENDANTS TO MAKE SURE THEY WERE NOT INJURED. THEY WERE NOT INJURED AND ALL THE PAX WERE OK AND IN THEIR SEATS. THE SEAT BELT SIGN HAD BEEN ON PRIOR TO THE EVENT. I MADE ANNOUNCEMENT TO THE PAX ABOUT THE UNSTABLE AIR AND NOT TO BE ALARMED. AFTER LAS VOR, WE TRIED TO CLB TO A HIGHER ALT BECAUSE FL330 WAS RPTED SMOOTH. WHILE CLBING THROUGH FL300, WE AGAIN ENCOUNTERED LIGHT TO MODERATE TURB. THE ACFT BECAME VERY UNSTABLE. IT WAS FISHTAILING AND WAS GETTING INTO A BAD DUTCH ROLL SIT. I HAD TO TAKE THE ACFT OFF THE AUTOPLT AND USE DUTCH ROLL RECOVERY TECHNIQUES (LEARNED IN MY PAST B727 TRAINING) TO STABILIZE THE ACFT. THE AUTOPLT COULD NOT HANDLE THE ROLL PROB AND MADE IT WORSE. WE NOTIFIED ATC OF THE PROB AND REQUESTED AN IMMEDIATE DSCNT BACK DOWN TO FL270, WHICH THEY GAVE US, FOR THE DURATION OF THE CRUISE PORTION OF THE FLT. I PUT THE ACFT BACK ON THE AUTOPLT AND WE HAD NO FURTHER PROBS DURING THE CRUISE PORTION OF THE FLT. THE AIR WAS RELATIVELY SMOOTH. WE THEN FOLLOWED UP WITH THE FLT MANUAL IRREGULARITY PROC 7-40. THERE WAS NOT MUCH ABOUT THE YAW DAMPENER OUT CONDITION OTHER THAN WE WERE RESTR TO LAND WITH FLAPS 15 DEGS AND NO GREATER THAN LIGHT TURB. I SPOKE WITH MAINT REGARDING THE PROB AND TOLD HIM OF THE UNSTABLE FLT CHARACTERISTICS AND THAT WE HIGHLY SUSPECTED THAT THE YAW DAMPENER WAS INOP. WE TOLD HIM WE HAD RESET THE CIRCUIT BREAKERS AND TURNED THE YAW DAMPENER SWITCH OFF AND ON. THE MASTER CAUTION AND THE SWITCH AND THE LIGHT FUNCTIONED NORMALLY. MAINT SAID THAT HE WOULD NOTIFY DEN MAINT OF THE PROB AND BE READY FOR US UPON OUR ARR. I CHKED THE WX AT LEAST 4 TIMES TO MAKE SURE THAT DEN DID NOT HAVE ANY WX PROBS. THERE WERE NO SPECIAL WX NOTICES AND BASICALLY GOOD FOR OUR ARR. IT WAS A LITTLE WINDY (140 DEGS AT 14 KTS), BUT THERE WAS NO LLWSA ALERTS IN EFFECT. SO, WE CONTINUED ON TO DEN. WE REALIZED THAT WE WERE BURNING MORE FUEL DUE TO OUR LOWER FLT ALT, BUT I HAD ADDED MY OWN 1000 LBS OF COMFORT FUEL FOR THE FLT SINCE WE WERE FLT PLANNED TO LAND IN DEN WITH 6.2 AT THE GATE WITH NO ALTERNATE, AND ONLY .5 FOR THE DSCNT. I WAS GLAD I TOOK THE EXTRA FUEL AND WISH I HAD TAKEN MORE LATER. ABOUT 80 MI OUT DEN, WE RECEIVED THE ATIS. THE WINDS WERE 140 DEGS AT 16 KTS, WITH MODERATE TURB RPTED FROM 16000-18000 FT AND AN LLWSA WAS IN EFFECT. I TOLD FO THAT I WOULD HAND FLY THE ACFT BELOW FL200 SINCE THE AUTOPLT COULD NOT HANDLE THE YAW DAMPENER PROB IN ROUGH AIR. WE DSNDED DOWN TO 11000 FT WITH NO PROB. THERE WAS ONLY LIGHT TURB. FO SWITCHED OVER TO APCH CTL AND RECEIVED THE NEWS THAT THERE WAS MOD TURB RPTED BY ALL ACFT AND MODERATE TO SEVERE TURB ON FINAL APCH TO RWY 16. APCH SAID THAT SOME ACFT HAD GONE AROUND. WE TOLD APCH CTL THAT WE HAD A FLT CTL PROB AND WE WERE RESTR TO LIGHT TURB, AND FURTHERMORE, WE WERE APCHING A LOW FUEL SIT. WE ASKED IF ANY OTHER RWYS WERE BETTER. THEY SAID THAT THEY WERE ONLY USING THE OPS ON RWY 16 AND RWY 17L&R. WE WERE BEING VECTORED FOR A VISUAL ILS BACKED APCH TO RWY 16. ATC ASKED US WHAT OUR INTENTIONS WERE. THE LAST 2-3 ACFT HAD GONE AROUND. THE FO AND I DISCUSSED OUR SIT AND DECIDED BECAUSE OF OUR LOW FUEL SIT (WE WERE DOWN TO +/-5.5 ON THE FUEL) THAT WE SHOULD ATTEMPT AN APCH. THE CONDITIONS COULD CHANGE. IF IT GOT BAD, WE WOULD GAR. ON FINAL, BELOW 8000 FT, WE ENCOUNTERED MODERATE OR GREATER TURB. THE ACFT BECAME VERY UNSTABLE, ALMOST UNCTLABLE. THE RAPID ROLL FROM SIDE-TO-SIDE WAS GREATER THAN 30 DEGS WITH THE NOSE AND TAIL SWISHING AROUND VIOLENTLY. I IMMEDIATELY APPLIED GAR THRUST, HIT THE TOGA BUTTON AND MADE A MISSED APCH. WE INFORMED ATC OF THE SIT. AND AGAIN, WE TOLD HIM THAT WE HAD A BAD CTL PROB AND WE WERE NOW IN A LOW FUEL SIT. WE HAD TO LAND SOON. WE NEEDED TO GET ON THE GND, AND WE DIDN'T HAVE ENOUGH FUEL TO GO TO ANOTHER ARPT. ATC ASKED US IF WE WERE DECLARING AN EMER. I SAID ONLY IF YOU DON'T GET US IN IMMEDIATELY. THEY IMMEDIATELY GAVE US VECTORS TO BRING US AROUND FOR AN APCH TO RWY 26. THEY WERE CHANGING RWY OPS TO LAND ON RWY 26 AND RWY 25. WE CLBED UP TO 13000 FT AND WERE TURNED TO THE E. WE WERE CLRED TO 8000 FT AND TURNED ONTO FINAL APCH ABOUT 10 MI OUT FOR A VISUAL TO RWY 26. THE 2 ACFT IN FRONT OF US HAD LANDED WITH NO PROBS AND ONLY LIGHT TURB. THE WIND WAS APPROX 330 DEGS AT 19 KTS. WE BRIEFED THE APCH AND A FLAPS 15 DEGS LNDG. FORTUNATELY, THERE WAS ONLY LIGHT TURB ON FINAL AND WE LANDED WITHOUT FURTHER INCIDENT. THE USABLE FUEL ON BOARD WAS APPROX 4.2, DISCOUNTING THE .2 IN THE CTR TANK WHICH WAS UNUSABLE. I DID NOT PREPARE THE CABIN FOR EVAC. I DID NOT FEEL THAT THE SIT WARRANTED IT AT THE TIME. ALSO, THERE WAS NO NEED TO ALARM THE PAX SINCE A NORMAL LNDG WAS EXPECTED. UPON GATE ARR, THE MECH MET THE FLT. THE ACFT WAS WRITTEN UP 'THIS ACFT IS UNSAFE TO FLY. NOT SUITABLE FOR SAFE FLT IN TURB.' CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR ADVISED MAINT DID CHANGE THE RUDDER CTL PWR UNIT. BUT IN ACCORDANCE WITH THEIR CHK, THE YAW DAMPER WAS WORKING OK. RPTR CONFIRMED THE ACFT WAS NEARLY UNCTLABLE. AN EMER WAS NOT DECLARED BUT THE WIND ON RWY 26 WAS MORE FAVORABLE FOR THE LNDG. A COMPANY CONFERENCE CALL WAS MADE TO DISCUSS THE PROB. HAS NOT HAD ANY FEEDBACK REF THE INCIDENT.

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.