WASHINGTON >> The National Transportation Safety Board said today it is seeking to determine when structural damage occurred to the rudder system on a Southwest Airlines Boeing 737 MAX that rolled in May.

The roll is one of a series of recent serious safety incidents involving Southwest flights that are under review by U.S. agencies.

WHY THIS IS IMPORTANT

The NTSB said last month the plane experienced what the crew said was a “Dutch roll” at 34,000 feet (10,360 metres) while en route from Phoenix, Arizona to Oakland, California. Such lateral asymmetric movements are named after a Dutch ice-skating technique and can pose serious safety risks.

Pilots regained control, landed safely and no one among the 181 people on board was injured.

KEY CONTEXT

Today, the NTSB said a review of data showed the unusual behavior of the rudder system began on the first flight after May 23 scheduled maintenance.

After the flight, a Southwest inspection revealed damage to the vertical stabilizer trailing edge ribs above and below the standby rudder power control unit, which is considered substantial damage.

Southwest inspected its entire MAX fleet of 231 airplanes from June 17 to 20 and found no damage or anomalies, the NTSB said.

OTHER INCIDENTS UNDER INVESTIGATION

The FAA is investigating a number of recent Southwest flights, including a flight last month that departed from a temporarily closed runway in Portland, Maine, as well as a flight that descended to a low altitude of around 500 feet about nine miles (14 km) from the Oklahoma City airport.

The FAA is also probing a Southwest 737 flight in April that came within about 400 feet of the ocean off Hawaii after bad weather conditions prompted pilots to bypass a landing attempt at Lihue airport.