US NTSB investigating 'Dutch roll' by Southwest Boeing 737 MAX

FILE PHOTO: FILE PHOTO: Boeing's logo

By David Shepardson

WASHINGTON (Reuters) - The U.S. National Transportation Safety Board said on Friday it is investigating why a Boeing 737 MAX operated by Southwest Airlines rolled during a flight last month.

The NTSB said the plane experienced what the crew said was a "Dutch roll" at 34,000 feet while en route from Phoenix, Arizona to Oakland, California on May 25. Such lateral asymmetric movements are named after a Dutch ice skating technique and can pose serious safety risks.

The board said pilots regained control of the plane, landed it safely and no one among the 175 passengers and six crew were injured during the incident. In a subsequent inspection, Southwest found damage to structural components, the NTSB said.

The Federal Aviation Administration (FAA), which said damage was seen in a standby power-control unit, said it is also investigating and working "closely with the NTSB and Boeing to investigate this event".

Boeing declined to comment on the Dutch Roll incident, referring questions to Southwest, which said it is participating in the investigation.

Separately, the FAA confirmed it is investigating a Southwest 737 MAX 8 passenger flight in April that came about 400 feet of the ocean off the coast of Hawaii after bad weather conditions prompted pilots to bypass a landing attempt at Lihue airport in Kauai.

During the go-around, the first officer "inadvertently pushed forward on the control column while following thrust lever movement commanded by the autothrottle," according to a June 7 memo seen by Reuters and the plane began to descend rapidly hitting a maximum descent rate of about 4,400 feet per minute.

The memo said "safety data confirmed the crew received a “DON’T SINK" oral warning followed by a "PULL UP" oral warning, but the first officer later said the clew did not hear the warnings.

The pilots in a post-debrief said seeing the severity of the flight "through the animations was a significant, emotional event," the Southwest memo said, adding the crew participated in comprehensive corrective actions and the airline is reviewing data and trends related to its procedures, training, standards, and performance.

Southwest said in a statement "the event was addressed appropriately as we always strive for continuous improvement." Bloomberg News first reported the Hawaii flight.

(Reporting by David Shepardson; editing by Diane Craft, Alexander Smith and David Gregorio)