a Boeing 737 Max damage sustained to parts of the aircraft’s structure after it entered a “Dutch sandwich” during a Southwest Airlines flight last month, U.S. investigators said Friday.
The incident occurred as the plane flew from Phoenix to Oakland, California, at an altitude of 35,000 feet on May 25. Southwest not informed National Transportation Safety Board about the role or damage to the jet until June 7, the NTSB said.
“Following the event, SWA performed maintenance on the aircraft and discovered damage to structural components,” the safety board said.
The NTSB’s commentary could suggest the incident was more serious than previously known, but aviation experts said it was too early to know for sure.
A Southwest spokesperson said the Dallas-based airline is participating in the investigation. He declined further comment.
A Dutch roll is a combination of yawing, or the tail sliding back and forth, and rocking the plane so that the wings roll up and down. The name comes from the way the rhythmic, rocking motion resembles a form of ice skating that was popular The Netherlands.
“It’s just part of aerodynamics,” says John Cox, a former pilot and now aviation safety consultant. “What you feel in the back is that the plane is rolling a little.”
Pilots train to recover from a Dutch roll, and most modern aircraft include a device called a yaw damper that can correct the condition by adjusting the rudder. a preliminary report By the Federal Aviation Administration said that after the Southwest plane landed, damage was discovered to a unit that controls backup power to the rudder.
The damage was described as ‘significant’.
Cox said the structural damage likely occurred in the plane’s tailfin, where the power units are housed. He was baffled that the backup unit would be damaged, as it would not normally activate during a Dutch roll.
Cox said the two-way oscillation of a Dutch roll was a dangerous phenomenon on previous Boeing jets, but not on 737s because of design changes.
Boeing “737s are not sensitive to excessive Dutch role. The design of the aircraft is (such that) if you do nothing at all, the aircraft will naturally dampen the Dutch rollout,” he said. “On older aircraft – 707s, 727s – this can go so far that you lose control of the aircraft.”
The NTSB said it downloaded data from the plane Boeing 737 Max8which allows investigators to determine the duration and severity of the incident.
Researchers won’t know exactly what the pilots However, they said: the cockpit voice recorder was overwritten after two hours.
The pilots regained control and landed in Oakland. There were no reports of injuries on the flight, which carried 175 passengers and a crew of six.
The NTSB said it expected to issue a preliminary report on the incident within about 30 days.