The US National Transportation Safety Board (NTSB) has launched an investigation into why a Southwest Airlines Boeing 737 Max 8 experienced what pilots called a “Dutch roll” during a 25 May flight.

Such a movement is unusual, and an inspection of the Southwest jet after the incident revealed structural damage, prompting the NTSB’s inquiry, the safety investigatory bureau said on 14 June.

A Dutch roll involves an aircraft yawing and rolling at the same time. The NTSB calls the movement an “oscillation event”.

November 2020

Source: Shutterstock

The event did not injure passengers or crew aboard. It involved Southwest flight 746 from Phoenix to Oakland and occurred at about 34,000ft.

“Following the event, [Southwest] performed maintenance on the airplane and discovered damage to structural components,” says the NTSB.

Southwest reported the in-flight upset to the NTSB on 7 June.

Other media outlets have reported that the jet’s back-up power control unit was damaged.

The NTSB has received data from the 737 Max 8’s digital flight-data recorder and will analyse the data at its laboratory in Washington DC. The jet’s cockpit voice recorder is of no use to investigators because it captures only 2h of audio and has since been overwritten.

The NTSB says it expects within 30 days to release a preliminary incident report.

Asked to comment, Boeing defers to Southwest, which itself defers to the NTSB and Federal Aviation Administration. The FAA did not immediately respond to a request for comment but other news outlets have reported that the agency is investigating the incident.