NTSB: Southwest 737 MAX flew 44 times with undetected storm damage

The US National Transportation Safety Board (NTSB) has said a Southwest Airlines Boeing 737 MAX 8 flew 44 times after thunderstorm winds damaged its tail, before the problem was found. According to AirlineGeeks, which reported on the final report released on Thursday, the damage passed through a scheduled maintenance check and led to abnormal rudder movements on Flight 746 from Phoenix to Oakland on 25 May 2024. All 175 passengers and six crew were uninjured.

What the NTSB found

The aircraft, registered N8825Q, was parked in New Orleans on the night of 16 May 2024 when thunderstorms brought gusts of up to 73 knots (about 84 mph). Investigators concluded the wind likely moved the rudder enough to damage ribs inside the vertical stabiliser, where the standby rudder control unit was mounted. The aircraft sustained substantial damage.

AirlineGeeks reports that Boeing's 737 maintenance manual then required a special inspection after an extreme wind event but gave no wind speed at which it applied, listing signs such as the aircraft moving over its wheel chocks or leaving tyre skid marks. Southwest also had no process to tell maintenance control when a parked aircraft had been exposed to extreme winds. The jet flew the next morning without an inspection.

The missed maintenance check

The aircraft completed 38 flights with no reported rudder or yaw damper problems, then underwent scheduled maintenance in Houston on 23 May. A test of the standby hydraulic system required a mechanic at the tail to confirm the rudder moved in response to pedal inputs. Hangar video showed the rudder moving right but not left, and investigators said nobody was at the tail to verify it. The test was nonetheless signed off as passed. The NTSB could not determine whether the test worsened the existing damage.

On the fourth flight the following day, a crew disconnected the yaw damper after reporting that it overcorrected in turbulence. After one more leg, it passed a functional test and was returned to service. Investigators found the abnormal movement appeared in flight data from the first flight after the Houston check, and only when the yaw damper was engaged.

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 descending to 32,000 feet. Data showed one episode lasting about 20 seconds, with no significant change in heading or bank angle. Southwest removed the aircraft from service after landing.

Changes made

• Southwest inspected the rudder control unit mounting areas across its 737 MAX fleet and found no similar damage.
• It added individual mechanic sign-offs to the hydraulic test.
• It set up a process to alert maintenance control after extreme winds.
• Boeing revised its manual to require the special inspection at a wind threshold of 65 knots (75 mph).

What this means for travellers

The reported fleet inspection found no similar damage on other Southwest 737 MAX aircraft, and the incident caused no injuries. The procedural changes at Southwest and Boeing are intended to close the gaps identified: unclear inspection triggers, no wind-exposure alerts and an unverified test. The source does not describe any current flight disruption or further regulatory action.

Sources: AirlineGeeks.