Explained: What Exactly Is a Dutch Roll?

DALLAS — The Federal Aviation Administration (FAA) and the National Transportation Safety Board (NTSB) are investigating an incident involving a Boeing 737-8, which experienced a 'Dutch roll' at approximately 32,000 feet.

The irregular aircraft motion took place on May 25 during a Southwest Airlines (WN) flight from Phoenix to Oakland. Despite the aircraft sustaining significant damage to the standby power control unit (PCU), the pilots managed to regain control and land safely, according to an initial report from the FAA.

On Thursday, the FAA released a statement saying it was working closely with the NTSB and Boeing to determine the cause of the incident, noting that no other airlines had reported similar issues.

What Is a Dutch Roll?

A Dutch roll is a type of aircraft motion that involves a combination of rolling and yawing oscillations. This phenomenon occurs when an aircraft rolls in one direction while simultaneously yawing in the opposite direction.

Another description is the combination of a yawing motion when the tail slides and the plane rocks from wingtip to wingtip. The name "Dutch roll" is derived from its resemblance to the motion of ice skaters performing a particular skating maneuver. Characteristics of a Dutch Roll include:

1. Oscillatory Motion: The aircraft oscillates between rolling and yawing, creating a noticeable side-to-side and up-and-down movement.
2. Coupled Motions: Roll and yaw motions are interconnected, meaning as the aircraft rolls to one side, it also yaws in the opposite direction, and vice versa.
3. Damping: Stability augmentation systems or specific design features in the aircraft can help dampen these oscillations, reducing their intensity and duration.

Another metaphor that uses the term comes from the world of cinema. The "Dutch angle" is a camera technique that tilts the camera to one side so the horizon line is not parallel to the bottom of the frame. The angle is deliberately askew to create a sense of uneasiness or tension between the characters.

Causes, Control of a Dutch Roll

According to Flight Stability and Automatic Control by Dr. Robert C. Nelson and Aircraft Dynamics: from Modeling to Simulation by Marcello R. Napolitano, the most salient causes of Dutch Rolls include:

• Aerodynamic Design: The design of the aircraft's wings and tail can influence its susceptibility to Dutch roll. Aircraft with swept wings are more prone to this phenomenon.
• Lateral Stability: An imbalance between lateral stability and directional stability can lead to Dutch roll. If an aircraft has high lateral stability (resistance to rolling) but lower directional stability (resistance to yawing), it may be more prone to Dutch roll.
• Pilot Input: Incorrect or delayed pilot inputs can exacerbate a Dutch roll.

Modern aircraft are often equipped with a yaw damper, an automatic control system that helps prevent or correct Dutch roll by applying small, automatic rudder corrections. Additionally pilots are trained to recognize and manage Dutch roll by using coordinated control inputs, typically involving the rudder and ailerons.

On May 25, the WN pilots were able to recognize the irregular aircraft movement and stabilize the equipment. As for why the Dutch roll incident happened in the first place, the NTSB expects to have a preliminary report in a month's time.