Southwest Airlines plane experiences rolling incident during severe storm

Investigators say a Southwest Airlines jet that exhibited an unusual “Dutch roll” motion during flight was parked outdoors during a severe storm and then underwent routine maintenance, after which the pilots noticed strange movements in the rudder pedals.

After the May 25 incident, Southwest mechanics found “significant” damage to the tail of the plane, where the rudder is located, but the National Transportation Safety Board said Tuesday it had not yet determined when the damage occurred.

The plane, a Boeing 737 Max, was grounded for more than a month but resumed operations last week, according to data from Flightradar24.com.

The Dutch roll is a swaying, rhythmic combination of yaw, or sideways sliding of the tail, and pitching of the wingtips. The Southwest jet experienced this motion at 34,000 feet and again after descending to 32,000 feet on the flight from Phoenix to Oakland, California.

This condition can be dangerous and modern aircraft have a yaw damper to stop the vibrations typical of the Dutch roll movement.

After the plane landed, Southwest mechanics discovered fractures in the metal bracket and ribs that hold an emergency power unit to the rudder system. Investigators examined the damaged parts last week in Ogden, Utah.

The NTSB said the plane was parked overnight at the New Orleans airport on May 16 during thunderstorms with wind gusts up to 85 mph, heavy rains and a tornado warning.

On May 23, the plane underwent a scheduled maintenance check, after which the pilots noticed that the rudder pedals moved when the yaw damper was activated. The pilots on the May 25 flight felt the pedals move during the Dutch roll and even after landing, the NTSB said.

Southwest inspected its 231 Max jets last month and found no further damage to the rudder actuators, according to the NTSB. No problems have been encountered in the new planes the company has received since then.

Dallas-based Southwest declined to comment.

It could take a year or more for the NTSB to determine a probable cause for the incident.