Southwest Airlines’ Dutch roll incident occurred after maintenance, NTSB reports

A Southwest Airlines Co. plane that exhibited unusual rolling movements several times during a flight in May had undergone regular maintenance two days earlier, U.S. safety investigators said Tuesday.

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

Technicians also found that a backup system that controls rudder motion was damaged. That part of the plane had passed post-maintenance testing on May 23 when the system was turned on for the last time before the May 25 flight, the NTSB said.

Flight 746 experienced what is known as a “Dutch roll,” in which the tail of the plane rocks from side to side. The name comes from the Dutch rolling technique. Pilots initially thought the strange movement, which occurred at 34,000 feet on a Boeing Co. Max 737 8, might have been caused by turbulence, but realized that was not the case after noticing unusual movement of the plane’s rudder pedals, according to the NTSB report.

The plane took off from Phoenix and landed safely at its destination airport in Oakland. None of the 181 passengers and crew on board the plane were injured. After landing, maintenance crews discovered damage in an area near the tail of the plane that, according to the NTSB, “compromised the structural integrity of the armature and is considered significant damage.”

Following the incident, Southwest inspected its entire Max fleet of 231 aircraft between June 17 and 20. No damage or anomalies like those on Flight 746 were found, and no findings have been made to date on the new deliveries from Boeing that are being inspected, the NTSB said.

The Federal Aviation Administration (FAA) also launched an investigation into the incident.

Southwest declined to comment. The agency said it is cooperating with both agencies on the investigation.

Allyson Versprille and Mary Schlangenstein for Bloomberg