CNN
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Federal authorities and Boeing are trying to understand why a 737 Max 8 experienced a rare and dangerous back-and-forth during flight.
The oscillating motion is known as a Dutch roll, and one characteristic described by the Federal Aviation Administration is an airplane’s nose doing a figure eight.
There were no injuries aboard Southwest Airlines Flight 746 on May 25, according to the airline and a preliminary FAA report. The report said the crew “regained control” and the plane landed safely.
But the plane suffered “substantial” damage and the FAA called the incident an “accident.” The FAA report said an inspection “revealed damage to the backup PCU,” or power control unit, which controls the rudder.
It is unclear whether the damaged unit was driven or was a result of the jet.
The plane has not flown since landing in Oakland, California, after the incident, except to move it to a Boeing facility in Washington state. Boeing did not immediately comment to CNN.
Southwest told CNN it had referred the incident to the FAA and the National Transportation Safety Board and was participating in and supporting the investigation.
The incident occurred nearly three weeks ago and was added to an FAA database this week. There were 175 passengers and six crew members on board, according to the airline.
The NTSB confirmed that it has opened an investigation into this incident. Investigators downloaded data from the flight data recorder, which “will help them determine the duration and severity of the event.”
The voice recordings from the voice data recorder – the other of the black boxes – were overwritten.
In February, the FAA asked airlines flying 737 Max 8s and similar planes to inspect the rudder assembly for loose or missing nuts, washers and bolts. He said the defect would prevent pilots from controlling the rudder using the pedals. Authorities did not say whether this condition and last month’s Dutch role were linked.
Most passengers have never felt an airplane make this motion – and most airline pilots have never experienced it in actual flight.
“It’s very murky,” Kathleen Bangs, an aviation safety analyst and former airline pilot, told CNN. “It’s a very uncomfortable movement and you can feel the tail swinging.”
When moving forward in flight, planes can rotate in three axes: nose up and down, called pitch; wings descending or raised, called roll; and the tail moves left or right, which is called yaw.
Airliners turn using a generally seamless combination of roll and yaw coordinated by the plane’s computers. These large planes are also equipped with yaw dampers that make small adjustments throughout the flight.
In the Dutch roll, the airplane rolls and yaws excessively. Passengers would feel the plane moving to one side and back to the other – moving back and forth, Bangs said.
She said airline pilots train for scenarios where their yaw dampers fail. They could take an airplane simulator to high altitude and disable the yaw damper.
“Then you press the rudder really hard to try to initiate (the roll) in the simulator,” Bangs said.
To exit a Dutch roll, pilots can slow the plane and descend into thicker air. Modern airliners are designed to be inherently stable in the air, she said, so the plane can return to level flight with minimal additional effort.
But the forces can be powerful. In 1959, four of the eight occupants of a Boeing 707 test and training flight were killed just outside Washington, D.C., after extremely steep Dutch rolls.
“The plane immediately yawed and rolled violently to the right,” read a report from the Civil Aeronautics Council, which investigated the incident. “Several gyrations followed and after regaining control of the aircraft, it was determined that three of the four engines had separated from the aircraft and it was on fire.”
