The dangers of Dutch rolls in aviation

Security Incidents in the aviation industry are not uncommon, but the complexity of each aircraft can make the nature of some incidents difficult to understand. The science of flight requires extensive research and investigation. As aircraft manufacturers across the industry have embraced innovation over the years, aviation has become safer than ever. Still, it is likely that some incidents will occur, whether to the aircraft or the operator.

Fortunately, the number of fatal incidents seems to have decreasedbut that doesn’t mean pilots, air traffic controllers, and other key people in the industry should let their guard down. As the world has seen in recent years, even small manufacturing errors can lead to catastrophic disasters. One particular incident that occurred earlier this year, while not fatal, may have been an incident some have never heard of before. A Southwest Airlines flight experienced a so-called “Dutch roll” in May, which reportedly caused damage to the aircraft..

What is a Dutch Roll?

In addition to the bad reputation of the Boeing 737 MAX, National Transportation Safety Board (NTSB) investigated WN746, which flew from Phoenix Sky Harbor International Airport (PHX) to San Francisco Bay Oakland International Airport (SFO) on May 25. The Dutch roll occurred at approximately 8:00 a.m. while the aircraft was flying at its cruising altitude of 34,000 feet. There were 175 passengers and six crew members on board, and no injuries were reported.

Photo: Angel DiBilio | Shutterstock

Although the incident occurred in late May, the first reports did not emerge until weeks later. It was also discovered that the aircraft in question had been grounded during a storm before deploying WN746.

The term “Dutch Roll” may sound like a delicious treat, but in aviation, according to the NTSB, it is anything but.

“A Dutch roll is a coupled oscillation of the yaw and roll axes of an aircraft that is inherent in the flight dynamics of all conventional aircraft.”

According to Daniel Kwasi Adjekum, an assistant professor in the aviation department at the University of North Dakota, who spoke to Travel + Leisure, “It is the series of antiphase turns in which the aircraft rolls and yaws in one direction [twists] in the other.”

Although rare, Dutch rolls can also cause crashes. On August 12, 1985, a Japan Airlines (JAL) Boeing 747 with 509 passengers and 15 crew on board experienced a structural failure and in-flight decompression. As a result, the aircraft’s rear pressure bulkhead ruptured, releasing compressed air in the cabin and breaking off the auxiliary power unit (APU) and parts of the tail cone. This resulted in the aircraft’s vertical stabilizer tearing off and the failure of all four hydraulic systems, rendering the flight control surfaces inoperable.

The plane flew with “strong Dutch roll and phugoid movements” for more than 30 minutes before it crashed into Mount Osutaka, killing all aboard.

Three main features

Although the Dutch roll movements can occur naturally, they are not common in commercial aircraft because, unlike fighter jets or other aerobatic aircraft, the planes have yaw dampers responsible for safely correcting unexpected movements. According to Airways Magazine, the term Dutch roll got its name because it closely resembles the movement of ice skaters when performing a certain ice skating maneuver. It has three main characteristics:

Oscillating movements are noticeable sideways and up and down movements that occur as the aircraft oscillates between rolling and yawing.
When roll and yaw movements are combined, they are called coupled movements and mean that the aircraft rolls to one side while yawing in the opposite direction.
Damping is achieved by stability enhancement systems or special design features that reduce the intensity and duration of vibrations

SF_Southwest Airlines Boeing 737 MAX 9 taking off from SEA_JAK

Photo: Joe Kunzler | Easy flying

The pilots of WN746 realized that the 737MAX8 made erratic movements and stabilized the plane. According to the Associated Press (AP), investigators found that the plane was parked outside during a strong storm before flying from PHX to OAK. Southwest reportedly performed routine maintenance on the plane, but pilots noticed that the control blades were moving erratically.

After the plane landed, Southwest discovered there were fractures in the metal bracket and ribs that housed the rudder system’s emergency power unit. The damaged parts were sent to investigators in Ogden, Utah, for examination. The plane remained grounded for over a month, but was returned to service in July.

The probable causes

According to Airways Magazine, quoting Flight Stability and Automatic Control by Dr. Robert C. Nelson and Aircraft Dynamics: from Modeling to Simulation by Marcello R. Napolitano, a Dutch roll is caused by three factors:

The aerodynamic design of an aircraft, such as its wings and tail, can make it vulnerable to the Dutch roll, especially aircraft with swept wings
When there is an imbalance between the directional stability and lateral stability of an aircraft – Dutch rolls are more prone to aircraft with high lateral stability, which is resistance to rolling, but lower directional stability, which is resistance to yaw
Pilot errors can also lead to a “Dutch roll,” such as incorrect or delayed inputs into the aircraft’s flight system.

Pilots in the cockpit of a Boeing 737 while flying.

Photo: Atosan | Shutterstock

Travel + Leisure also quoted Boeing as saying that wind can also cause “Dutch rolls.”

Older aircraft were built without yaw dampers, which means Pilots must intervene to save the aircraft from the phenomenon. They can manually control the rudder of an aircraft and counteract the Dutch roll. According to Pilot Mall, one of the first Dutch roll incidents occurred in 1955 on a 707 prototype. Pilot Tex Johnston regained control of the aircraft during a demonstration flight.

Pilots are trained to handle a Dutch roll, but it can have adverse effects on passengers. According to Travel + Leisure, it can be uncomfortable for travelers or even lead to nausea or airsickness.

“My only advice to passengers is to always wear seat belts to ensure their safety in such cases,” said Adjekum.