Date: 201503
Local time of day: 1801-2400
Location: ZZZ.ARTCC, US
Flight conditions: Mixed
Light: Night
Aircraft: B737-300
Operator: Air Carrier
Flight plan: IFR
Flight phase: Cruise
Airspace: Class A ZZZ
ATC / advisory: Center ZZZ
Aircraft component: Autoflight Yaw Damper
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Not Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Situational Awareness
Second reporter function: Dispatcher
Anomaly: Aircraft Equipment Problem Less Severe; Flight Deck / Cabin / Aircraft Event Illness / Injury; Inflight Event / Encounter Loss Of Aircraft Control; Inflight Event / Encounter Weather / Turbulence
Detector: Person Flight Crew; Person Passenger; Person Flight Attendant
When detected: In-flight
Result: Flight Crew Took Evasive Action; Flight Crew Regained Aircraft Control
Contributing factors: Weather; Company Policy; Aircraft
Primary problem: Ambiguous

Narrative: Arriving at the aircraft we learned the yaw damper was inoperative and the aircraft was being operated under MEL 22-X-XX relief. I reviewed the requirements of the provisos of the MEL and discussed it with the First Officer. During the crew briefing with the Flight Attendants; I informed them of the MEL and inoperative yaw damper and advised they may feel a difference in the rear of the aircraft as a result. I continued that this was the first time I operated the B-737 with an inoperative yaw damper and was unsure of how significant a difference they would notice.We discussed the weather and the existence of a turbulence plot; but Dispatch had filed us with ample fuel and at an altitude below the tropopause. They were informed that if the ride did get bumpy; sit immediately and do not wait for us to tell them; but we would communicate with them about any adverse conditions we were informed about as far in advance as possible.During climb to FL300; the flight was normal and without incident. We encountered an occasion light bump and noticed the lack of yaw damper operation was immediately apparent. After reaching FL300; approximately two to three minutes later; the aircraft encountered moderate turbulence. The response from the aircraft was shocking. The tail began to yaw dramatically left; then right; then left. The resulting yaw induced a significant and severe 'Dutch roll'. The First Officer (PF) instinctively disconnected the autopilot; and attempted to control the aircraft. Seeing the severe yawing and accompanying Dutch roll; I instructed him to immediately re-engage the autopilot to prevent Pilot induced oscillations and possible loss of control.I made an immediate PA for the Flight Attendants (FA) to take their seats; contacted Center; advised that we were in moderate turbulence and operating with no yaw damper and the aircraft's reaction to the turbulence was unacceptable. ATC issued a descent to FL280. We leveled at FL280 and had another severe Dutch roll encounter. We sought and received another descent to FL260. Leveling at FL260; I instructed the First Officer to maintain a speed of 280 KIAS or less and be prepared for more turbulence. I then called the Flight Attendants on the service interphone. Both FA's (front and back) were on the interphone and were extremely shaken by the event. The B or C Flight Attendant on the line sounded almost panicked. She stated service items were tossed about; but they were able to get into a seat and were uninjured but Passengers were getting sick; and were terrified.I made a PA to the Passengers explaining that we had encountered clear air turbulence; to keep their seats with their seat belts securely fastened and that the Flight Attendants would be remaining in their seats as well.The aircraft handled much better at the lower altitude; but a subsequent descent to FL240 was required later to keep the aircraft manageable. I used ACARs to request voice contact with our Dispatcher. Voice contact was established and I informed our Dispatcher that it was our intention to continue but if we encountered ANY more severe yawing/Dutch roll we would be diverting immediately. The Dispatcher initially was confused and asked if we encountered severe turbulence. I explained that it was moderate turbulence; but a severe reaction by the aircraft to that turbulence. It was agreed that the Dispatcher would keep a listening watch on down line frequencies to monitor our progress. Eventually; the ride was smooth and the Flight Attendants were able to get a service out to the customers.Arriving at the gate; I was informed by the Flight Attendants that the flight was the worst they had ever experienced. They were still shaken by the event. As Passengers deplaned; numerous Passengers commented on their fear of the situation; but thanked us for getting them safely on the ground. Some of the Passengers had gotten airsick as well.In the future I will not accept an aircraft with an inoperative yaw damper if the possibility of adverse weather exists on the route of flight. I fully believed; as did the First Officer; that we were on the edge of departing controlled flight at least twice during the event. The apprehension of using manual rudder imputes without a yaw damper; possibly over controlling or experiencing PIO; leading to possible aircraft damage or upset is just far too real. I believe it is a real threat to safety operating an aircraft under this MEL when ANY adverse weather exists.


Second narrative: I received an ACARS for voice request from one of my airborne flights. We established contact and the Captain shared that the flight had to emergency descend to FL240 due to turbulence on climb out. He stated that they we're unable to physically type an ACARS message due to the turbulence they encountered and the reaction of the aircraft. The Yaw Damper was on MEL. He said the aircraft control was questionable. At the time of voice contact they had established more control of the aircraft- I asked if he wanted to divert he said no; however; if they encountered anymore turbulence we would divert to the nearest airport. I shared that rides down the coast along his route had no PIREP SIGMET plots. We established a plan as I monitored his progress to the frequency we would use or ACARS if they able to communicate that way.I checked in with flight 25 minutes later via ACARS and got PIREP. He made reference again to the lack of control of aircraft from before - in which I asked if SEV was the nature/ for inspection etc. and if the cabin crew was able to perform duty and well being of our passengers. He said the TURB was not SEVERE; but the reaction of the aircraft was with no Yaw Damper. Shared that the cabin crew was very scared and they stayed seated. We kept in communication via ACARS with ride reports along route; which all seemed to be handling much better - per Captain for the rest of flight. This flight landed without further incident.The leg prior the aircraft was flown on the reverse route at FL290 without issue. There were SIGMETS out at time of planning well to the West and not along planned route. This flight was late into my shift; and not one complaint or PIREP from any of my flights departing to the South. I put remark on the release stating that it was filed lower at FL300 due to MEL...and make sure tanker fuel was onboard so fuel would not be an issue. Around the same time of sending release; a SIGMET came out [the same area]. MOD FL270-FL330. I had no PIREPS or complaints from my flights departing; to cause any alarm to my flight planning nor did the flight crew check in for any flight brief or MEL concerns. This aircraft was routed for the MEL and Yaw Damper to be fixed.


Synopsis: B737-300 Captain reports being dispatched with an inoperative yaw damper and encounters moderate turbulence at FL300. The response from the aircraft was shocking with the tail yawing dramatically left; then right; then left producing a significant and severe 'Dutch roll'. Flight descends to a lower altitude and is able to continue to destination. The Dispatchers report is also included.