Incident Overview
On May 25, 2024, at about 0744 Pacific daylight time, Southwest Airlines flight 746, a Boeing 737-8 (registration N8825Q), experienced a rudder control system anomaly while operating as a scheduled domestic passenger flight from Phoenix Sky Harbor International Airport (PHX) to Metro Oakland International Airport (OAK). The flight continued to Oakland and landed without incident. None of the 181 passengers and crew were injured.
Flight Crew Observations
The captain, who was the pilot flying, noted a previously recorded yaw damper discrepancy in the logbook before departure, described as "the yaw damper over-correcting in flight." The corrective action had involved resetting stall management yaw damper computer codes.
Preflight, pushback, and initial taxi were uneventful. However, while turning onto the runway for takeoff, the captain noticed momentary stiffness in the rudder pedals after transitioning to rudder pedal steering.
After departure, the climb to 34,000 feet was uneventful except for occasional light turbulence. Shortly after reaching cruise altitude, while in light chop, the aircraft experienced a "small amount of Dutch roll," as described by the captain. The oscillations lasted a few seconds, with the autopilot engaged throughout. The first officer 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 rudder pedal movement during the oscillations. They deduced the oscillations were not caused by turbulence, as the rudder pedals should not move with the yaw damper system.
After coordinating with Air Traffic Control, they descended to 32,000 feet, where similar conditions were encountered. The same movements occurred several more times during cruise, with the captain feeling faint rudder pedal movement in phase with the oscillations. No yaw damper light or master caution warnings illuminated.
The crew notified Southwest Airlines maintenance control via ACARS about a yaw damper issue. The flight continued to Oakland and landed without incident. After clearing the runway, the captain again felt oscillations and rudder pedal stiffness.
Post-Flight Inspection and Damage
Upon arrival at the gate, maintenance met the aircraft, which was removed from service. Troubleshooting and inspection revealed damage to the vertical stabilizer trailing edge ribs above and below the standby rudder power control unit (PCU). The damage adversely affects the structural strength of the fitting and is considered substantial.
During replacement of the main rudder PCU, a damaged bearing was found in the forward end of the upper input control rod. A new control rod was installed. Further rigging/adjustment identified additional structural damage near the standby rudder PCU. The standby PCU forward support bracket and anti-rotation bushing were damaged. The vertical stabilizer trailing edge rib above the standby PCU was fractured through and adjacent to two of three attachment holes. The rib below the standby PCU was dented and deformed.
Previous Maintenance and Weather Events
On May 16, 2024, the aircraft had been parked at a remote spot at Louis Armstrong New Orleans International Airport (MSY) during a squall line with gusting winds up to 73 knots. The NWS recorded a peak gust of 73 knots at 2336 CDT on May 16. Thunderstorms lasted from 2212 CDT May 16 to 0234 CDT May 17.
On May 23, 2024, the aircraft underwent a scheduled "A Core Check" at William P Hobby Airport (HOU). An operational test of the standby hydraulic actuation system included moving the rudder pedals to stops at a high rate for 5 cycles. The test passed with no issues noted. Flight data recorder data showed this was the last time the standby rudder PCU was powered on.
Rudder System Description
The 737-8 rudder is controlled via a closed-loop system from the pilots' rudder pedals through cables, an aft rudder quadrant, and a torque tube in the vertical fin. Two input rods from the torque tube feed the main rudder PCU; one feeds the standby PCU. Both PCUs are in the rear of the vertical stabilizer, mounted on support brackets attached to internal rib flanges. The system has no gust lock; gust damping is provided by bypass valves in the main PCU. When hydraulic pressure is below 250 psi, the valves are spring-biased to bypass mode, allowing actuator movement with reduced resistance. The standby PCU lacks gust damping.
Investigation and Data Analysis
The National Transportation Safety Board (NTSB) was notified on June 7, 2024, and began an incident investigation. Participating parties include the FAA, Southwest Airlines, the Southwest Airlines Pilots Association, Boeing, and Parker Hannifin.
On June 26-27, 2024, the systems group examined the removed main and standby PCUs at Parker Hannifin's facility. No anomalies were found; both passed acceptance test procedures. The damaged structural components were examined by the NTSB structures group starting July 1, 2024.
Flight data recorder data corroborated the flight crew's statements. Data showed periodic oscillations at 34,000 feet, lasting about 20 seconds, with rudder deflection, pedal position, and yaw damper command oscillations at about 0.9 Hz and ±0.75 degrees of rudder movement, resulting in ±0.03 G lateral acceleration. No significant heading or bank angle changes occurred.
The anomalous behavior began on the first flight after the May 23 maintenance. Before maintenance, yaw damper commands did not correspond to pedal movements; after maintenance, pedal movements occurred when the yaw damper was engaged. All oscillations occurred with the yaw damper engaged; when disengaged or dispatched on MEL, no anomalies were observed.
Safety Actions
Southwest Airlines conducted an inspection campaign on its entire MAX fleet (231 aircraft) between June 17 and June 20. Detailed visual inspections of main and standby PCU hardware and structural attach points found no damage or anomalies. New Boeing deliveries are inspected at make-ready with no findings to date.
Southwest developed an algorithm to search Quick Access Recorder data for rudder oscillations. No aircraft exhibited the same behavior as N8825Q. Three aircraft had small oscillations without control surface movement and were inspected with no findings. Monitoring via the algorithm continues.
Part of the investigation will determine when the structural damage occurred. The investigation continues.