“Dutch roll” incident on Boeing aircraft is being investigated

Boeing has been under scrutiny since a door stopper burst during an Alaska Airlines flight in January. This week, the manufacturer disclosed a quality issue with 787 Dreamliner jets. The FAA is also investigating how titanium with falsified documentation was used in some Boeing and Airbus planes.

On May 25, the Southwest plane was flying at an altitude of 34,000 feet around 8 a.m. when the “Dutch roll” incident occurred, the NTSB said in a statement.

A “Dutch roll” occurs when an aircraft rolls, or pitches from side to side, and the tail moves from left to right, a motion called yaw. The name refers to a skating technique common in the Netherlands. Eric Wydra, director of the University of Oklahoma’s School of Aviation, said in an email that the motion is not dangerous in most cases but “can be very uncomfortable for passengers.”

“The nose can move from left to right while the airplane simultaneously pitches from side to side,” says a post on Boeing’s website about Dutch rolls. The action is triggered by wind or pilot input, Boeing says, “and causes a series of oscillations that continue until the movement stops on its own or the pilot corrects it.”

Pilots are trained to recognize and correct a Dutch roll movement, Wydra said. However, planes are also equipped with so-called yaw dampers to automatically compensate for the movement.

In the case of the Southwest flight, the FAA’s initial report said the flight crew “regained control.” During maintenance after the incident, the airline discovered damage to some structural components, according to the NTSB. The FAA’s initial report said the “standby PCU,” or power control unit, was damaged.

John Cox, a retired pilot and instructor at the University of Southern California, said in an interview that the information available so far about the incident left him with “more questions than answers.”

He said a Dutch roll is a “natural aerodynamic condition” that should resolve itself on a 737. On some other airplanes from earlier years, such a movement could have caused the pilot to lose control of the plane, he said. But he is not aware of any case in which a Dutch roll on a 737 has ever caused damage.

“A Dutch roll incident is unusual,” said Cox, CEO of consulting firm Safety Operating Systems. “One that causes damage is unthinkable.”

Investigators are currently examining whether the damaged power control unit contributed to the unexpected movement or whether the movement caused the damage, the FAA said.

Southwest notified the NTSB of the incident and damage on June 7. The safety agency said it had obtained data from the plane’s flight data recorder that will help investigators determine “the duration and severity of the incident.” The cockpit voice recorder, which is limited to two hours of audio, was overwritten.

Flight tracking data shows that the plane did not leave Oakland until June 6 and continued on to Everett, Washington, where it has been since then. The FAA released information about the incident on Wednesday.

A preliminary report from the NTSB is expected shortly – within 30 days of the May 25 event.

Southwest said it was “participating in and cooperating with the investigation” and referred questions to the FAA and NTSB. Boeing declined to answer questions and referred questions to Southwest.

The incident only gained widespread attention on Wednesday when the Aviation Herald, a website that reports on incidents and industry news, published an article about it.