A Southwest Airlines Boeing 737 Max jet last week flew an unusual flight condition known as a “Dutch roll” during a May 25 flight from Phoenix to Oakland. Preliminary findings were released by the NTSB, detailing an intricate series of events leading up to and following the occurrence of the incident.
This exposed the involved aircraft to the New Orleans airport on May 16 to the harsh weather conditions that involved thunderstorms, wind gusts of up to 84 mph, heavy rainfalls in the area, and a tornado watch. After being exposed to such extreme weather conditions, this plane was taken for routine maintenance on May 23.
Soon after the maintenance, pilots began to report that when the yaw damper was on they felt strange movements in the rudder pedals. The yaw damper is a system designed to prevent oscillations characteristic of a Dutch roll—a combination of sideways tail movement and rocking wingtips.
During the May 25 flight, the Dutch roll phenomenon was encountered at altitudes of 34,000 feet and again at 32,000 feet. The pilots reported that the rudder pedals continued to move during the incident and even on the ground after landing.
After the flight, Southwest mechanics inspected the aircraft and found “substantial” damage to the tail section where the rudder is attached. Specifically, they found cracks in a metal bracket and ribs that hold a standby power control unit in place against the rudder system. Investigators took those very damaged parts to a lab in Ogden, Utah, for analysis.
The NTSB said it had not yet determined when the damage occurred but noted Southwest Airlines inspected its fleet of 231 Max jets last month and did not find any similar damage or issues with new aircraft deliveries.
The incident caused the plane to be grounded for more than a month. According to data from Flightradar24.com, the plane returned to service last week.
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One aviation safety consultant, who was still a flying pilot at the time, John Cox, hypothesized that the acute damage likely was the result of the worst storm in New Orleans, with near hurricane-force winds that would have slammed the rudder back and forth as the aircraft sat on the ground. Cox specifically pointed out he does not believe the Dutch roll caused such damage, nor did he think it had anything to do with the maintenance job. He sees it as an isolated incident and nothing that could show faults intrinsic to the 737 Max or even to the 737 model in general.
This NTSB investigation is still open, and more may come to light as they continue to probe the incident and its causes.