DALLAS (NEWSnet/AP) — Investigators say a Southwest Airlines jet that had an unusual “Dutch roll” in flight had been parked outdoors during a storm, then underwent routine maintenance.

After the May 25 incident, Southwest mechanics found substantial damage in the aircraft’s tail, where the rudder is located.

National Transportation Safety Board on Tuesday said it hasn’t determined when the damage occurred.

Dutch roll is a swaying, rhythmic combination of yaw, or the tail sliding sideways, and the wingtips rocking up and down. 

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

On May 23, the plane underwent scheduled maintenance, and afterward pilots noticed the rudder pedals moving when the yaw damper was engaged. John Cox, a former airline pilot and now a safety consultant, said the NTSB report indicates the plane most likely was damaged during the storm. Cox said there is “absolutely no way in the world” the Dutch roll caused such severe damage, nor does he think it was related to the maintenance work.

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