Structural damage found in Southwest 737 following 'Dutch roll' event

A preliminary report from the US National Transportation Safety Board (NTSB) on July 9 found structural damage on Southwest Airlines flight 746 on a 737 MAX 8 aircraft that went into a 'Dutch roll' on May 25. A Dutch roll is an imbalance in both lateral and directional stability, which ultimately leads to a side-to-side rocking through a combination of rolling and yawing. The event occurred at around 34,000 feet after some light turbulence. The reported described the event as a ""strange movement of the tail of the airplane back and forth, coupled with very slight rudder movement left and right."" The tail movement was described as ""noticeable, but not excessive"". The report added: ""The flight crew discussed the event and noted that they felt the rudder pedals move during the oscillations. They deduced that the oscillations were not caused by the turbulence, as the rudder pedals should not move with the yaw damper system, but rather by the airplane."" The aircraft - a domestic passenger flight from Phoenix Sky Harbour International Airport in Arizona - continued and arrived safely at the Metro Oakland International Airport with all 181 passengers and crew aboard uninjured. After arriving, maintenance crews inspected the aircraft, which was removed from service. ""The examination revealed damage to the vertical stabiliser trailing edge ribs above and below the standby rudder power control unit (PCU). The damage to the stabiliser ribs adversely affects the structural strength of the fitting and is considered substantial damage."" The report said the aircraft in question was parked overnight between May 16 and 17 at a ""remote spot"" at the Louis Armstrong New Orleans International Airport. Throughout the night, the area was impacted by a ""squall line of thunderstorms"" with heavy rain and wind shift. The report said a ""core check"" was performed on the aircraft on May 23 - including the test of the standby rudder PCU - with the check being passed and no issues noted. ""A review of flight data recorder (FDR) data found that this is the last time the standby rudder PCU was powered on,"" the report said. Boeing said it would ""continue to fully support the NTSB's investigation"". The OEM had notified all 737 operators in June of the importance of high wind event inspections, which is defined in the aircraft maintenance manual. Southwest notified the NTSB of the event and damage nearly two weeks later on June 7. As part of the investigation, Southwest conducted an inspection of its entire MAX fleet of 231 aircraft between June 17 and 20. No damage or anomalies were found, the report said. A Southwest spokesperson said: ""As noted in the NTSB’s preliminary report, Southwest took immediate steps to inspect our MAX 8 fleet and analyse all available flight data as part of our proactive safety management system with no additional findings. We continue to participate in the NTSB’s investigation."" The NTSB said the investigation is ongoing.