Southwest ‘dutch roll’ occurred after plane maintenance, NTSB says

A Southwest Airlines Co. plane that experienced an unusual rolling motion several times during a May flight had undergone regularly scheduled maintenance work two days earlier, US safety investigators said Tuesday.

After the May 25 event, a Southwest maintenance crew found structural damage to the plane’s rudder system. The National Transportation Safety Board said it’s still trying to determine when that damage occurred, according to a preliminary report by the agency.

Technicians also found damage to a backup system that helps control rudder movement. That part of the plane had passed tests following the May 23 maintenance work, the last time the system was turned on prior to the May 25 flight, the NTSB said.

Flight 746 experienced what is known as a “Dutch roll,” which involves the plane’s tail rocking from side to side and is named after the motion of a Dutch skating technique. Pilots at first thought the strange motion, which occurred at 34,000 feet on a Boeing Co. Max 737 8, might have been caused by turbulence but determined that wasn’t the case after noticing an unusual movement of the plane’s rudder pedals, according to the NTSB’s report. 

The plane departed from Phoenix and landed safety at its destination airport in Oakland. None of the 181 passengers and crew aboard the aircraft were injured. 

After landing, maintenance crews discovered damage in an area near the plane’s tail, which the NTSB said “adversely affects the structural strength of the fitting and is considered substantial damage.” 

After the incident, Southwest inspected its entire Max fleet of 231 aircraft between June 17 and June 20. No damage or anomalies like those found on Flight 746 were discovered, and there haven’t been any findings to date on new deliveries from Boeing, which are inspected, the NTSB said.  

The Federal Aviation Administration also launched an investigation of the incident.

Southwest declined to comment. The agency has said it’s working with both agencies on the probes.