WN746 - Accident: Southwest B38M enroute on May 25th 2024

A Southwest Airlines Boeing 737-8 MAX, registration N8825Q performing flight WN-746 from Phoenix,AZ to Oakland,CA (USA) with 175 passengers and 6 crew, was enroute at FL340 when the aircraft experienced Dutch Roll. The crew was able to regain control, descended the aircraft to FL320 and landed the aircraft on Oakland's runway 30 about 55 minutes later. The aircraft sustained substantial structural damage.

The FAA reported: "AIRCRAFT EXPERIENCED A DUTCH ROLL, REGAINED CONTROL AND POST FLIGHT INSPECTION REVEALED DAMAGE TO THE STANDBY PCU, OAKLAND, CA." and stated the aircraft sustained substantial damage, the occurrence was rated an accident.

The aircraft remained on the ground in Oakland until Jun 6th 2024, then positioned to Everett,WA (USA), ATS facilities, and is still on the ground in Everett 6 days later.

Dutch Roll is a coupled out of phase movement of the aircraft as result of weakened directional stability (provided by the vertical tail and rudder), in which the aircraft oscillates around its vertical as well as longitudinal axis (coupled yaw and roll).

The PCU is the power control unit, an actuator controlling the (vertical) rudder.

On Jun 13th 2024 The Aviation Herald learned that two ribs, that the stand by PCU is being mounted to, were damaged as well as the mounts of the stand by actuator. A temporary repair was done in Oakland replacing the damaged PCU, the aircraft was then ferried to Everett to replace the damaged ribs.

On Jun 14th 2024 the NTSB reported:

The NTSB has opened an investigation into an inflight oscillation event on a Southwest Airlines B-737-MAX8.

The event, which was described by the flight crew as a “Dutch roll,” occurred on May 25 at about 8 am PT at an altitude of 34,000 feet as Southwest flight 746 was enroute from Phoenix, Arizona (PHX), to Oakland, California (OAK).

A Dutch roll is a coupled oscillation in an airplane’s yaw and roll axes, inherent to the flight dynamics of all conventional airplanes.

None of the 175 passengers or crew of six aboard the flight were injured in the event.

Following the event, SWA performed maintenance on the airplane and discovered damage to structural components. SWA notified the NTSB of the event and damage on June 7.

The NTSB’s Vehicle Recorder Laboratory in Washington has received data downloaded from the airplane’s digital flight data recorder. Data from the recorder will aid investigators in determining the length and severity of the event.

The cockpit voice recorder, which is currently limited to two hours of audio, was overwritten and unavailable to investigators.

A preliminary report is expected within 30 days of the date of the event.

On JUl 9th 2024 the NTSB released their preliminary report stating for the history of flight:

According to the flight crew, the captain was the pilot flying and the first officer (FO) was the pilot monitoring. The captain said that while reviewing the logbook before the flight, he noted a previously recorded yaw damper discrepancy described as “the yaw damper over-correcting in flight”. He recalled that the corrective action consisted of resetting a few stall management yaw damper computer codes.

The preflight, pushback, and initial taxi segments of the flight were uneventful. However, when cleared for takeoff and while turning onto the runway, after transitioning to rudder pedal steering, the captain noticed momentary stiffness in the rudder pedals.

Following departure, occasional light turbulence was encountered during an uneventful climb to the cruise altitude of 34,000 feet. The captain said that shortly after reaching their cruise altitude, and while flying through some light chop, the aircraft began to experience “a small amount of Dutch roll”. He stated that “the roll was stable, more noticeable in frequency, with only a slight amount of yaw.” The oscillations lasted a few seconds and the autopilot remained engaged throughout the event. The FO characterized the event 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, and I remember my first thought being this is some odd light chop”.

The flight crew discussed the event and noted that they felt the rudder pedals move during the oscillations. They deduced that the oscillations were not caused by the turbulence, as the rudder pedals should not move with the yaw damper system, but rather by the airplane.

After coordinating with Air Traffic Control (ATC), they descended to 32,000 where they encountered similar flight conditions. The same type of aircraft movements was experienced a few more times during the cruise portion of the flight and the captain felt faint rudder pedal movement in phase with the oscillations. The yaw damper light did not illuminate and there were no master caution warnings for the duration of the flight.

SWA maintenance control was notified by the flight crew of a yaw damper issue via the aircraft communication addressing and reporting system (ACARS) and the flight continued to OAK and landed without incident. However, after clearing the runway, the captain felt the same oscillations and rudder pedal stiffness observed earlier in the flight.

After arriving at the gate, the aircraft was met by SWA maintenance, and the airpoance was removed from service. Post-incident troubleshooting and inspection of the airplane was accomplished by SWA maintenance. The examination revealed damage to the vertical stabilizer trailing edge ribs above and below the standby rudder power control unit (PCU). The damage to the stabilizer ribs adversely affects the structural strength of the fitting and is considered substantial damage. As part of the investigation, a timeline of events was created.

With respect to the flight data the NTSB annotated:

A review of the data also showed that the anomalous behavior of the rudder system began on the first flight after a scheduled maintenance on May 23, 2024. 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. All occurrences of these oscillations occurred with the yaw damper engaged; when the yaw damper was disengaged in-flight, or when the airplane was dispatched with the yaw damper on the minimum equipment list (MEL), the anomalous behavior was not observed.

The NTSB also stated: "Part of the investigation will be determining when the structural damage to the rudder system occurred."

On Nov 6th 2026 the NTSB released their final report concluding the probable causes of the accident were:

Damage to the standby rudder power control unit connection to the vertical fin due to an extreme high wind event while the airplane was parked, which remained undetected and resulted in anomalous operation of the standby rudder power control unit. Contributing to the nondetection of the damage was 1) the improper accomplishment of the standby hydraulic system operational test by Southwest Airlines maintenance personnel; 2) the lack of a defined wind speed threshold, in the Boeing 737 aircraft maintenance manual, to perform the extreme high wind conditional inspection for parked and moored aircraft; and 3) the absence of an adequate process at Southwest Airlines to notify maintenance control when parked aircraft experience an extreme high wind event that may require the relevant conditional inspection.

The NTSB analysed:

Southwest Airlines (SWA) flight 746 experienced a rudder control system anomaly during flight between Phoenix Sky Harbor International Airport (PHX), Phoenix, Arizona and Metro Oakland International Airport (OAK), Oakland, California. The captain stated, shortly after reaching their cruise altitude and while flying through light turbulence, the airplane began to experience “a small amount of Dutch roll.” The oscillations lasted a few seconds and the autopilot remained engaged throughout the event.

The first officer (FO) characterized the event as a “strange movement of the tail of the airplane back and forth, coupled with very slight rudder movement left and right.” The flight crew discussed the event and noted that they felt the rudder pedals move during the oscillations. The flight crew deduced that the oscillations were not caused by turbulence but rather by the airplane. SWA flight 746 continued to OAK and landed without incident. Postlanding inspection found damage to include the vertical stabilizer trailing edge ribs, both above and below the standby rudder power control unit (PCU).

On the evening of May 16, 2024, the accident airplane was parked overnight at Louis Armstrong New Orleans International Airport (MSY), New Orleans, Louisiana. Overnight, thunderstorms at MSY generated high winds with gusts up to 73 kts. The Boeing 737 rudder system does not incorporate a gust-lock function. It is likely that these high winds resulted in movement of the rudder that damaged the vertical stabilizer ribs to which the standby rudder PCU was connected.

At the time of the accident, the Boeing 737 aircraft maintenance manual conditional inspection for parked or moored airplanes that experienced an extreme high wind event lacked a defined wind threshold for requiring the conditional inspection to be performed. Additionally, SWA did not have a process in place to notify maintenance control for when their parked aircraft may have experienced an extreme high wind event. Therefore, the conditional inspection for extreme high wind events, which could have detected structural damage to the vertical stabilizer, was not accomplished after the thunderstorms. The accident airplane resumed flying the next morning.

Between May 17 to 23, 2024, the accident airplane flew a total of 38 flights with no reported anomalies of the rudder control system. Therefore, the damaged vertical stabilizer ribs remained undetected for these flights. The airplane underwent scheduled maintenance inspections on the evening of May 23, 2024. These inspections included an operational test of the standby hydraulic system.

The damage to the vertical stabilizer trailing edge ribs, to which the standby rudder PCU was attached, that existed at the time of this operational test would have revealed itself as a rudder that would not return to its neutral position. However, contrary to the required procedure, SWA maintenance personnel did not visually confirm the rudder position during this operational test.

Nonetheless, SWA maintenance signed off that the accident airplane passed the operational test of the standby hydraulic system, and the airplane was released for flight. As a result, the damage to the vertical stabilizer remained undetected. Investigators could not determine if the existing vertical stabilizer rib damage was further damaged during this maintenance activity.

On May 24, 2024, during the accident airplane’s fourth flight of the day, the damaged vertical stabilizer ribs exhibited the first operational symptoms in which that flight crew believed the yaw damper had malfunctioned. The accident airplane subsequently flew another flight on May 24, 2024, with the yaw damper disabled. Flight 746 was the first flight the next day.