What happened
On May 25, 2024, a Boeing 737-8, registration N8825Q, operated by Southwest Airlines was flying from Phoenix Sky Harbor International Airport to Metro Oakland International Airport when the flight crew encountered a rudder control system anomaly. The flight involved 181 passengers and crew, all of whom were not injured.
During the initial taxi, the captain noted stiffness in the rudder pedals after transitioning to rudder pedal steering. Once the aircraft reached a cruise altitude of 34,000 feet, the crew experienced periodic oscillations described as a small amount of Dutch roll. The crew noted that the rudder pedals moved in phase with these oscillations. While the autopilot remained engaged and the yaw damper light did not illuminate, the crew observed the tail moving back and forth with slight rudder movement.
After coordinating with Air Traffic Control, the crew descended to 32,000 feet, where similar movements occurred several more times. The crew notified maintenance control via ACARS regarding the yaw damper issue. Upon landing at Oakland, the captain reported that the rudder pedal stiffness and oscillations recurred after clearing the runway.
The investigation
Maintenance inspections following the event revealed significant structural damage to the aircraft. Technicians found damage to the vertical stabilizer trailing edge ribs located above and below the standby rudder power control unit (PCU). The damage included a fractured rib above the standby PCU and a dented/deformed rib below it, which was considered substantial damage. Additionally, a damaged bearing was discovered in the forward end of the upper input control rod.
Flight data recorder (FDR) analysis confirmed the flight crew's reports, showing periodic oscillations in rudder deflection, rudder pedal position, and yaw damper command lasting approximately 20 seconds with a frequency of 0.9 Hz. The data also indicated that the anomalous behavior occurred only when the yaw damper was engaged; the oscillations were not observed when the yaw damper was disengaged.
Investigators also reviewed the aircraft's history, noting that on May 16, 2024, the aircraft was at New Orleans International Airport during a period of severe thunderstorms with wind gusts up to 73 knots. A review of maintenance records showed that on May 23, 2024, the aircraft underwent a scheduled maintenance check in Houston, Texas, which included an operational test of the standby hydraulic actuation system and the standby rudder PCU. The investigation is ongoing to determine when the structural damage to the rudder system occurred.