Southwest 737 hit by strong ground winds before ‘Dutch roll’ incident: NTSB | News

The Southwest Airlines Boeing 737 Max 8 that experienced a “Dutch roll” on May 25 had previously been grounded during a severe storm. Its stabilizer and rudder systems were later found to have sustained significant damage.

Following the incident, Southwest inspected all 231 737 Max aircraft in its fleet for such damage, according to a preliminary incident report from the National Transportation Safety Board (NTSB) dated July 8. Boeing also reminded operators to conduct inspections in “high winds.”

The new report does not provide any information on the cause of the incident, which caught the attention of safety experts because it was a problem with the flight controls. However, the report does provide new details on what investigators call a “rudder control system anomaly.”

The anomaly occurred during a May 25 Southwest flight from Phoenix to Oakland operated by a 737 Max 8 with registration N8825Q. Pilots reported that the jet exhibited a “Dutch roll” shortly after takeoff, which is simultaneous yaw and roll. The plane landed safely in Oakland and Southwest took it out of service.

The NTSB now says that nine days before the incident, on the night of May 16-17, the plane was parked at Louis Armstrong New Orleans International Airport when gusts with lightning and winds of 73 knots (135 km/h) swept through the area. The report also describes the 737 Max’s “gust attenuation system,” which is designed to prevent damage caused by strong winds that slam the rudder against its stops. The NTSB makes no explicit connection between the storm and the damage to the plane and says the investigation is ongoing.

When asked for comment, Boeing said the company reminded 737 operators in June to conduct high-wind inspections in accordance with the aircraft maintenance manual.

“We continue to fully support the NTSB’s investigation,” Boeing added. The manufacturer’s chief engineer, Howard McKenzie told lawmakers in June that the incident had nothing to do with Boeing’s production or the design of the 737 Max.

This claim was confirmed by the NTSB in a scalding letter of 27 June to Boeing CEO Dave Calhoun, who spoke about how the plane maker had disclosed details of ongoing safety investigations. At the time, the NTSB said it had not yet determined whether design or manufacturing played a role in the “Dutch Roll” incident.

Southwest mechanics examined the airplane after the May 25 flight. They found damage to the rudder components, including a control rod bearing, a bushing and a bracket that holds the backup power control unit (PCU). The main and backup PCUs move the rudder.

“The vertical stabilizer trailing edge rib above the standby PCU was also breached…The vertical stabilizer trailing edge rib below the standby PCU was dented/deformed,” the NTSB said. “The damage to the vertical stabilizer ribs compromises the structural integrity of the mount and is considered significant damage.”

In response, Southwest inspected its entire 737 Max fleet between June 17 and 20, looking for “damage to the main rudder PCU and standby PCU hardware, as well as structural attachment points,” the report said. The airline found no other problems.

Neither Southwest, Boeing nor the Federal Aviation Administration (FAA) responded to requests for comment.

It is unclear whether the incident prompted other airlines to inspect their jets for damage. U.S. 737 Max operators American Airlines and United Airlines did not respond to questions.

Another airline, Alaska Airlines, says: “We have evaluated our data and found no issues. We are not taking any further action at this time.”

The NTSB report shows that the Southwest jet experienced rudder problems before the May 25 flight.

When the captain reviewed the jet’s logbook that morning, he found an entry about a “yaw damper discrepancy” that had been corrected by “resetting some of the yaw damper stall management computer codes.”

Then, during the taxi for takeoff, “the captain noticed a momentary stiffness in the rudder pedals,” the NTSB said. After takeoff, during the climb in light waves, the jet experienced “a slight Dutch roll… The roll was stable, more noticeable in frequency, with only slight yaw,” the captain told investigators. “The captain felt a faint movement of the rudder pedals in time with the oscillations.”

The first officer describes the disturbance as a “strange movement of the aircraft’s tail back and forth, combined with a very slight rudder movement to the left and right… The tail movement was noticeable, but not excessive.”

The vibrations occurred “several times during cruise,” the report says. “The yaw damper light did not illuminate and there were no major warnings.”

The NTSB notes that on the evening of May 23 – seven days after the storm hit New Orleans and two days before the accident flight – Southwest mechanics performed a “core check” on the jet, which included an evaluation of the rudder standby hydraulic actuation system and the standby PCU.

Although no problems were identified, the NTSB states that data from the jet’s flight recorder showed that “abnormal behavior of the rudder system” occurred “on the first flight” after maintenance.

“Prior to maintenance, the yaw damper commands did not match the rudder pedal movements. However, after scheduled maintenance was performed on the airplane, rudder pedal movements were observed when the yaw damper was activated,” the NTSB said.