A Southwest Boeing 737 Max experienced a rare but serious 'Dutch roll,' and has now been out of service for almost 3 weeks

• 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 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, California, on May 25 when it ran into a rare but serious problem.

An incident report filed Wednesday by the Federal Aviation Administration said 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.

It can be particularly nauseating for passengers, and Dutch rolls have 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 postflight inspection revealed damage to the standby power-control unit, which controls the rudder. No injuries were reported.

Data from FlightAware shows the jet, which is less than 2 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, in which a combined 346 people died.

When reached for comment, Boeing referred Business Insider to Southwest Airlines. The airline said it's supporting the investigation.