Southwest B38M enroute on May 25th 2024, Dutch Roll

Last Update: July 9, 2024 / 13:12:40 GMT/Zulu time

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Incident Facts

Date of incident
May 25, 2024

Classification
Accident

Flight number
WN-746

Aircraft Registration
N8825Q

Aircraft Type
Boeing 737-800MAX

ICAO Type Designator
B38M

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."
Aircraft Registration Data
Registration mark
N8825Q
Country of Registration
United States
Date of Registration
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Manufacturer
BOEING
Aircraft Model / Type
737-8
Number of Seats
ICAO Aircraft Type
B38M
Year of Manufacture
Serial Number
Aircraft Address / Mode S Code (HEX)
Engine Count
Engine Manufacturer
Engine Model
KkbihhgmkigAg Subscribe to unlock
Engine Type
Pounds of Thrust
Main Owner
EnihjghqAkgdl nqlpgjdiAbfAAmpfibjcgghhlh Alkimd phd decAjjfdnpqmnbibeblbbicjc pjjjgiqA Subscribe to unlock
Incident Facts

Date of incident
May 25, 2024

Classification
Accident

Flight number
WN-746

Aircraft Registration
N8825Q

Aircraft Type
Boeing 737-800MAX

ICAO Type Designator
B38M

This article is published under license from Avherald.com. © of text by Avherald.com.
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