A Boeing 737 Max suffered damage to parts of the plane’s structure after it crashed a “Dutch roll” during a Southwest Airlines flight last month, U.S. investigators said Friday.
The incident happened while the jet was sailing at 34,000 feet from Phoenix to Oakland, California, on May 25, but South-west did not notify the National Transportation Safety Board about the rollover or damage to the plane by June 7, the NTSB said.
“Following the event, SWA performed maintenance on the airplane and discovered damage to structural components,” the safety board said.
The NTSB comment could suggest the incident was more serious than previously known, but aviation experts said it was too early to know for sure.
A Dutch roll is a combination of yaw, or the tail sliding from side to side, and the plane rocking in a way that causes the wings to roll up and down. The name comes from the way the rhythmic, swaying movement resembles a form of ice skating that was popular in The Netherlands.
“It’s just part of aerodynamics,” said John Cox, a former airline pilot and now an aviation safety consultant. “What you feel back there is that the plane is wallowing.”
Pilots train to recover from a Dutch roll, and most modern airplanes include a device called a yaw damper that can correct the condition by adjusting the rudder. A preliminary report for the Federal Aviation Administration said that after the Southwest plane landed, damage was discovered to a unit that controls the rudder’s backup power.
The damage was described as “substantial”.
Cox said the structural damage likely occurred in the plane’s tail, where the power units are housed. He was perplexed that the backup unit would be damaged because normally it would not activate during a Dutch test.
Cox said the bidirectional oscillation of a Dutch bearing was a dangerous phenomenon in previous Boeing jets but not in the 737 due to design changes.
Boeing “737s are not prone to excessive Dutch rolls. The design of the plane is (such that) if you do absolutely nothing, the plane will dampen the Dutch launch naturally,” he said. “On older model planes – 707, 727 – this could escalate to the point where you would lose control of the plane.”
The NTSB said it downloaded data from the plane, a Boeing 737 Max 8which will help investigators determine the duration and severity of the incident.
Investigators won’t know exactly what the pilots we were saying, however: The cockpit voice recorder was replaced 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 expects to issue a preliminary report on the incident in about 30 days.
This story originally appeared on Time.com read the full story
