U.S. Investigating ‘Dutch Roll’ That Damaged a Boeing 737 During Flight

A rare midair sway-and-wobble that caused “substantial” damage to the tail section of a Southwest Airlines plane during a flight last month has become the focus of an investigation by the Federal Aviation Administration and the National Transportation Safety Board, the agencies said this week.

The plane, a Boeing 737 Max 8, was flying at an altitude of 34,000 feet from Phoenix to Oakland, Calif., around 8 a.m. Pacific time on May 25 when the flight crew members said they experienced what is known as a Dutch roll, the N.T.S.B. said in a statement on Friday.

A Dutch roll is “a coupled oscillation” that creates simultaneous side-to-side and rocking motions, producing a figure-8 effect. The phenomenon is believed to have been named by an aeronautical engineer who compared it to a traditional ice skating technique made popular in the Netherlands.

If unaddressed, the wobbling can become more exaggerated, creating a dangerous feedback loop.

“It’s a weird movement of the airplane, an oscillation that, if not dampened or stopped, could continue to get worse and worse,” said Jeff Guzzetti, a former accident investigator for the F.A.A. and N.T.S.B. “It’s a vicious cycle.”

Dutch rolls do happen from time to time, under the right circumstances. Most modern airplanes, including the Max, have equipment to dampen its effects, according to Mr. Guzzetti.

“There’s still a lot we don’t know about this,” he said. “Usually modern jets need computers to dampen out these potential ‘Dutch roll’ tendencies. It may just be an indication of the wrong circumstances at the wrong time with the wrong components out of commission.”