The Southwest Airlines Boeing 737 Max 8 that experienced “Dutch roll” on 25 May had previously sat on the ground through a severe wind storm and was later discovered to have substantial damage to its stabiliser and rudder system.

Following the incident, Southwest has inspected all 231 737 Max in its fleet for such damage, according to an 8 July preliminary incident report from the National Transportation Safety Board (NTSB). Boeing has also reminded operators about “high-wind” inspections.

The new report does not say what caused the incident, which caught attention of safety experts because it involved a flight-control problem. But the report does provide new details about what investigators call a “rudder control system anomaly”.

The anomaly occured during a 25 May Southwest flight from Phoenix to Oakland operated by a 737 Max 8 with registration N8825Q. The pilots reported that shortly after taking off the jet experienced “Dutch roll”, or yawing and rolling at the same time. The aircraft landed safety in Oakland and Southwest pulled it from service.

Southwest 737 Max 8

Source: Southwest Airlines

The NTSB now says that nine days before the event, on the night of 16-17 May, the aircraft was parked at Louis Armstrong New Orleans International airport as squalls with lightning and 73kt (135km/h) winds passed the area. The report also describes the 737 Max’s “gust-damping” system, which is intended to prevent damage caused by high winds slamming the rudder against its stops. The NTSB does not make an explicit link between the storm and the aircraft’s damage and says the investigation is ongoing.

Asked to comment, Boeing says that in June it had reminded 737 operators of performing high-wind event inspections in accordance with the aircraft maintenance manual. 

“We continue to fully support the NTSB’s investigation,” adds Boeing. The manufacturer’s chief engineer Howard McKenzie told lawmakers in June that the incident was unrelated to Boeing’s production or to the 737 Max’s design. 

That assertion was disputed by the NTSB in a scalding 27 June letter to Boeing chief executive Dave Calhoun regarding how the airframer had disclosed details of ongoing safety investigations. At the time, the NTSB said it had not determined whether design or manufacturing played a role in the Dutch roll event. 

Southwest mechanics inspected the aircraft after the 25 May flight. They found damage to rudder components, including to a control rod bearing, a bushing and a bracket holding the standby power-control unit (PCU). The main and standby PCUs move the rudder.

“The vertical stabiliser trailing-edge rib above the standby PCU was also fractured through… The vertical stabiliser trailing-edge rib below the standby PCU was dented/deformed,” says the NTSB. “The damage to the stabiliser ribs adversely affects the structural strength of the fitting and is considered substantial damage.”

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

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

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

Images of Southwest 737 Max 8's damaged rudder control components

Source: National Transportation Safety Board

These images show damage to the Southwest 737 Max 8’s rudder control components.

Another, Alaska Airlines, says, “We have evaluated our data and found no issues related to this one. There are no further actions we are currently taking.”

The NTSB’s report reveals that the Southwest jet had rudder issues before the 25 May flight.

That morning, when reviewing the jet’s logbook, the captain found an entry for a “yaw damper discrepancy” that had been addressed by “resetting a few stall management yaw damper computer codes”.

Then, while taxiing for take-off, “the captain noticed momentary stiffness in the rudder pedals”, says the NTSB. After take-off, while climbing in light chop, the jet experienced “a small amount of Dutch roll… The roll was stable, more noticeable in frequency, with only a slight amount of yaw”, the captain told investigators. “The captain felt faint rudder pedal movement in phase with the oscillations.”

The first officer describes the upset as a “strange movement of the tail of the airplane back and forth, coupled with very slight rudder movement left and right… The tail movement was noticeable, but not excessive”.

The oscillations occurred “a few more times during the cruise portion of the flight”, says the report. “The yaw damper light did not illuminate and there were no master caution warnings.”

The NTSB notes that on the evening of 23 May – seven days after the New Orleans storm and two days before the incident flight – Southwest mechanics completed an “A Core Check” on the jet that included evaluating its standby hydraulic actuation system and rudder standby PCU.

Though no problems were found, the NTSB says data from the jet’s flight recorder reveals that “anomalous behavior of the rudder system began on the first flight” following that maintenance.

“Before the maintenance, yaw damper commands did not correspond to rudder pedal movements. However, after scheduled maintenance was performed on the airplane, rudder pedal movements were noted when the yaw damper was engaged,” the NTSB says.