A Southwest Airlines plane performing a Dutch roll suffered structural damage, according to investigators

A Boeing 737 Max suffered structural damage during a Southwest Airlines flight last month when the plane entered a so-called “Dutch roll” position, US investigators said on Friday.

The incident occurred on May 25 as the plane was flying from Phoenix to Oakland, California, at 34,000 feet. However, Southwest did not notify the National Transportation Safety Board that the plane had rolled over or been damaged until June 7, the NTSB said.

“Following the incident, SWA performed maintenance on the aircraft and discovered damage to structural components,” the safety agency said.

The NTSB’s comment may indicate that the incident was more serious than previously known, but aviation experts say it is too early to say for sure.

A Southwest spokesman said the Dallas-based airline is participating in the investigation, but declined further comment.

A “Dutch roll” is a combination of yaw, i.e. the sideways sliding of the tail, and rocking of the aircraft, in which the wings roll up and down. The name comes from the fact that the rhythmic, swaying movement is reminiscent of a form of ice skating popular in the Netherlands.

“It’s just part of the aerodynamics,” says John Cox, a former pilot and now an aviation safety consultant. “At the back, you can feel the plane rocking in some way.”

Pilots train to recover from a Dutch roll, and most modern aircraft are equipped with a yaw damper that can correct the condition by adjusting the rudder. A preliminary report from the Federal Aviation Administration said that after the Southwest plane landed, damage was discovered to a unit that controls the rudder’s emergency power supply.

The damage was described as “significant.”

Cox said the structural damage likely occurred to the aircraft’s tail fin, where the propulsion units are housed. He was baffled that the spare unit was damaged, as it would not normally activate during a Dutch roll.

Cox said the two-way oscillation of a Dutch roll was a dangerous phenomenon on previous Boeing jets, but is no longer a problem on the 737 due to design changes.

“Boeing 737s are not prone to excessive Dutch roll. The design of the airplane is such that if you do absolutely nothing, it will dampen the Dutch roll on its own,” he said. “On older models of airplanes – 707s, 727s – it can develop to the point where you lose control of the airplane.”

The NTSB said it had downloaded data from the aircraft, a Boeing 737 Max 8, that would help investigators determine the duration and severity of the incident.

However, investigators do not know exactly what the pilots said: the voice recorder in the cockpit was overwritten after two hours.

The pilots regained control and landed in Oakland. No injuries were reported on the flight, which had 175 passengers and a crew of six on board.

The NTSB said it expects to release a preliminary report on the incident in about 30 days.