WASHINGTON, July 9 (Reuters) - The National

Transportation Safety Board said on Tuesday 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 ( LUV ) 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

On Tuesday, 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 ( LUV ) 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 ( LUV ) 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 ( LUV )

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 ( LUV ) 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.