CNN  —  Federal authorities and Boeing are trying to figure out why a 737 Max 8 experienced a rare, unsafe back-and-forth roll during flight. The oscillating motion is known as a Dutch roll, and one characteristic described by the Federal Aviation Administration is the nose of an aircraft making a figure-eight. There were no injuries onboard Southwest Airlines flight 746 on May 25, according to the airline and a preliminary report by the FAA. The report said the crew “regained control,” and the plane safely landed. But the aircraft suffered “substantial” damage and the FAA classified the incident as an “accident.” The FAA report said an inspection “revealed damage to the standby PCU,” or power control unit, which controls the rudder. It is unclear if the damaged unit led to or was a result of the roll. The plane has not flown since landing in Oakland, California after the incident, except to move it to a Boeing facility in Washington state. Boeing did not immediately comment to CNN. Southwest told CNN it referred the incident to the FAA and National Transportation Safety Board and is participating in and supporting the investigation. The incident occurred almost three weeks ago and was added to a FAA database this week. There were 175 passengers and six crew onboard, according to the airline. CNN has reached out to the NTSB. It has not said whether it is investigating the incident. In February, the FAA required airlines flying some 737 Max 8 and similar aircraft to inspect the rudder assembly for loose or missing nut, washer and bolt. It said the flaw would prevent the pilots from controlling the rudder using foot pedals. Authorities have not said if this condition and the Dutch roll last month are related.