By DAVID KOENIG DALLAS (AP) — Investigators say a Southwest Airlines jet that experienced an unusual “Dutch roll” in flight had been parked outside during a strong storm and then underwent routine maintenance, after which pilots noticed odd movements of the rudder pedals. After the May 25 incident, Southwest mechanics found “substantial” damage in the aircraft’s tail, where the rudder is located, but the National Transportation Safety Board said Tuesday that it hasn’t determined when the damage occurred. The plane, a Boeing 737 Max, was grounded for more than a month but resumed flights last week, according to data from Flightradar24.com. Dutch roll is a swaying, rhythmic combination of yaw, or the tail sliding sideways, and the wingtips rocking up and down.