NTSB Eyes Latent Storm Damage In Southwest ‘Dutch Roll’ Probe

A Southwest Airlines Boeing 737-8 that exhibited anomalous rudder movements may have been damaged during heavy winds while parked overnight on the ground days before the inflight occurrence, a preliminary NTSB report suggests.

Investigators have pieced together an aircraft timeline that ends with the May 25 incident and includes an overnight stay during a storm with heavy, shifting wind and one routine overnight maintenance check, all within nine days. The storm winds reportedly exceeded certification limits for gust-damping systems.

Investigators are looking at the period leading up to the inflight incident to determine when the rudder system damage occurred.

While the report includes no analysis, it suggests that the inflight problems did not cause the damage but were in all likelihood a symptom of a pre-existing issue.

Southwest Flight 746, an 18-month-old 737-8 with registration N8825Q, was cruising at 34,000 ft. (FL340) on a routine flight from Phoenix to Oakland, California, when the pilots detected “a small amount of Dutch roll,” or repetitive rolling and yawing inherent in most aircraft designs, as it encountered light turbulence, the report said, quoting the captain.

The first officer (FO) said the back-and-forth tail movement “was noticeable, but not excessive,” the report said.

“The flight crew discussed the event and noted that they felt the rudder pedals move during the oscillations,” the report said. “They deduced that the oscillations were not caused by the turbulence, as the rudder pedals should not move with the yaw damper system” that automatically moves the rudder to offset Dutch roll, it added.

The oscillations lasted “a few seconds,” and the crew asked for and received a lower cruise altitude. The aircraft encountered similar flight conditions, prompting more oscillations.

The flight crew advised Southwest maintenance control of its issue via the aircraft communication addressing and reporting system. The crew continued to Oakland without incident.

Southwest then removed the aircraft from service and inspected it.
“The examination revealed damage to the vertical stabilizer trailing edge ribs above and below the standby rudder power control unit (PCU),” the NTSB said. The nature of the damage “is considered substantial” because it weakened affected fittings, the board added.

Southwest mechanics replaced the main PCU and, during functional testing of the new part, “identified additional structural damage” to the standby rudder PCU support bracket and ribs.
Southwest notified NTSB of its findings in early June, prompting the board to open its investigation.

Among the areas of interest was the aircraft’s overnight stay at Louis Armstrong New Orleans International Airport May 16-17. A line of storms passed through the area that included heavy rain, shifting winds, and a peak gust of 73 kt (84 mph), weather data examined by NTSB found. FAA Part 25 certification requirements for flight control systems include the ability to withstand 65 kt ground gusts “from any direction.”

The 737 rudder system does not have a gust-lock function. The main rudder PCU bypass valves provide gust-damping, which protects the system against damage from strong winds.

The bypass valves are automatically activated when hydraulic pressure falls below a certain level, such as when the aircraft is parked and shut down. The standby PCU does not have similar functionality, NTSB said.

Investigators are also reviewing a scheduled overnight check the aircraft underwent May 23 in Houston. Among the tasks was testing the standby rudder actuation system, including the standby PCU. The system passed with no issues noted, NTSB found. Flight data recorder data showed this was the last time the standby system was powered on, NTSB said.

The report does not make any direct links from the inflight incident and the storm or routine overnight work in Houston.

Investigators, working with PCU supplier Hannifin, tested both units removed from the incident aircraft. No issues were found.

Southwest inspected all 231 of its 737 MAX-family aircraft from June 17-20 for PCU system damage. “No damage or anomalies were found,” NTSB said.