Southwest jet did 'Dutch roll' mid-air after being parked outside during severe storm, investigators say

A Southwest Airlines jet did a "Dutch roll" in midair after being parked outside during a severe storm and receiving standard maintenance, an investigation found.

Southwest mechanics found "substantial" damage in the aircraft's tail, where the rudder is located, after the incident on May 25. The National Transportation Safety Board said Tuesday that it hasn't determined when the damage occurred.

The aircraft, a Boeing 737 Max , was temporarily grounded for over a month but has since returned to the skies, as per Flightradar24.com data.

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Dutch roll is a swaying, rhythmic combination of yaw, or the tail sliding sideways, and rocking wingtips. The Southwest jet experienced the movement at 34,000 feet and again after descending to 32,000 feet while flying from Phoenix to Oakland, California .

The movements pose a threat to safety but modern aircraft are equipped with a "yaw damper" to counteract the Dutch roll's movements.

Upon landing, Southwest mechanics discovered cracks in the metal components that connect a power control unit to the rudder system. Last week, investigators scrutinized the damaged parts in Ogden, Utah.

The NTSB reported that the airplane had been left overnight at New Orleans airport on May 16 amidst thunderstorms that brought winds up to 84 mph, intense rainfall, and a tornado watch.

On May 23, the aircraft underwent routine maintenance, and afterward, pilots noticed the rudder pedals moving when the yaw damper was engaged. The NTSB reported that pilots on the May 25 flight felt the pedals moving during the Dutch roll and even after landing.

Southwest conducted an inspection of its 231 Max jets last month and found no other instances of damage around the rudder power units and no issues in new planes it has received since, according to the NTSB.

Dallas-based Southwest declined to comment. The NTSB may take a year or more to determine a probable cause for the incident.