A Southwest Boeing 737 MAX 8 flew 44 times before maintenance crews discovered tail damage that investigators traced to a thunderstorm more than a week earlier. In a final report released Thursday, the National Transportation Safety Board said the damage went undetected through a scheduled maintenance check and caused a standby rudder control unit to operate abnormally.
The damage came to light after Southwest flight 746 experienced unexpected rudder movements on May 25, 2024, while flying from Phoenix to Oakland, California. All 175 passengers and six crew members were uninjured. The aircraft, registered N8825Q, sustained substantial damage.
The NTSB traced the damage to the night of May 16, when the jet was parked in New Orleans, and thunderstorms brought gusts of up to 73 knots, or about 84 mph. Investigators said the wind likely moved the rudder enough to damage ribs inside the vertical stabilizer where the standby rudder control unit was mounted.
At the time, Boeing’s 737 maintenance manual called for a special inspection after an extreme wind event but gave no wind speed at which the check was required. The manual listed signs such as an aircraft moving over its wheel chocks or leaving tire skid marks.
Southwest also lacked a process to notify maintenance control when a parked aircraft had been exposed to extreme winds. The jet flew the next morning without the inspection.
Scheduled Maintenance
Over the next week, the aircraft completed 38 flights with no reported rudder or yaw damper problems. It then underwent scheduled maintenance in Houston on May 23, including a test of the standby hydraulic system. The procedure required a mechanic at the tail to verify that the rudder moved in response to pedal inputs.
Hangar video showed the rudder moving to the right but failing to move to the left. Investigators said no one was at the tail to verify the movement. Southwest’s maintenance records showed the test was signed off as passed, and the aircraft returned to service. The NTSB could not determine whether the test worsened the existing damage.
On the fourth flight the next day, a crew disconnected the yaw damper after reporting that it was overcorrecting in turbulence. The yaw damper uses the rudder to counter side-to-side motion.
The jet flew one more leg with the system listed as inoperative. It then passed a functional test, and the yaw damper was returned to service before Flight 746.
On the Oakland flight, the captain noticed momentary stiffness in the rudder pedals while turning onto the runway. Shortly after reaching 34,000 feet, he felt what he described as “a small amount of Dutch roll.” Both pilots felt the pedals move, and the oscillations recurred after the crew descended to 32,000 feet.
Flight data showed one episode of rudder and pedal oscillations lasting about 20 seconds, with no significant change in heading or bank angle. Investigators found that the abnormal movement appeared in the data from the first flight after the Houston maintenance check, before crews reported it. They observed it only when the yaw damper was engaged.
Southwest removed the aircraft from service after it landed in Oakland. The airline later inspected the rudder control unit mounting areas across its 737 MAX fleet and found no similar damage. It also added individual mechanic sign-offs to the hydraulic test and set up a process to alert maintenance control after extreme winds. Boeing revised its maintenance manual to require the special inspection at a wind threshold of 65 knots, or 75 mph.
