- A Boeing 737 Max has been out of service since May 25.
- The Southwest Airlines jet encountered a rare stability problem called a Dutch roll.
- A post-flight inspection found damage to the power control unit, which controls the rudder.
A Boeing 737 Max has been out of service for 20 days as officials investigate an in-flight incident.
The Southwest Airlines jet was flying from Phoenix to Oakland on May 25 when it ran into a rare but potentially serious problem.
An incident report filed Wednesday by the Federal Aviation Administration says the "aircraft experienced a Dutch roll."
A Dutch roll describes a plane rocking from side to side and changing in yaw — the direction the nose is facing — simultaneously resembling a horizontal figure-eight pattern.
It can be particularly nauseating for passengers, and Dutch roll has been cited in accidents after pilots were unable to regain control of the aircraft. Pilots are trained in how to counter it, and modern airliners are built with a system called a yaw damper to try to prevent it.
The video below shows an aircraft (not the Southwest 737 Max) experiencing a Dutch roll:
Luckily, in the Southwest incident, pilots regained control of the 737 Max. The FAA's report added that the post-flight inspection revealed damage to the standby PCU (power control unit), which controls the rudder. No injuries were reported.
Data from FlightAware shows the jet, which is less than two years old, remained in Oakland for 12 days following the incident. It was then flown to Everett, Washington, where Southwest has a maintenance facility.
The FAA is investigating the cause of the incident.
It's a less-than-ideal situation for Boeing, with the planemaker experiencing a crisis following January's Alaska Airlines blowout involving a 737 Max. The Max has faced increased scrutiny ever since it was involved in two crashes in 2018 and 2019, when a combined 346 people died.
Boeing and Southwest Airlines did not immediately respond to a request for comment from Business Insider outside US working hours.