NTSB: Storm-Cracked Rudder Ribs Flew 43 Sectors Before Dutch Roll

A Boeing 737 MAX 8 flew 43 flights with cracked vertical stabiliser ribs before its crew felt a Dutch roll at FL340, the US National Transportation Safety Board (NTSB) concluded in a final report released on 5 November 2026. The damage came from a thunderstorm in New Orleans, survived a scheduled maintenance check in Houston, and surfaced only because the pilots of Southwest flight 746 treated an unfamiliar oscillation as a fault rather than turbulence.

What happened on flight 746?

On 25 May 2024, the aircraft, registered N8825Q, departed Phoenix Sky Harbor for Oakland with 175 passengers and six crew. Before departure the captain noted a logbook entry describing the yaw damper (a system that suppresses side-to-side swinging of the tail) overcorrecting; engineers had cleared it by resetting fault codes. Turning onto the runway, he felt brief stiffness in the rudder pedals.

After reaching FL340 in light turbulence, the captain observed what he described as "a small amount of Dutch roll" — a coupled rolling and yawing motion. The first officer saw the tail moving back and forth with slight rudder deflections. Both pilots felt the pedals moving, so they concluded the aircraft, not the air, was the cause.

The autopilot stayed engaged. After coordinating with ATC the crew descended to FL320, but the oscillations returned several times. No yaw damper light or master caution appeared. The crew alerted Southwest maintenance control via ACARS (the aircraft's text-link with ground systems) and landed safely in Oakland. The captain felt the same motion and pedal stiffness after touchdown, and Southwest withdrew the jet at the gate.

Flight data validated the crew's account: oscillations lasting about 20 seconds at roughly 0.9 Hz, with about 0.75 degrees of rudder travel either side — every episode with the yaw damper engaged.

What did engineers find?

Troubleshooting uncovered structural damage inside the vertical stabiliser:

• a damaged bearing on the main rudder power control unit (PCU, the hydraulic actuator that moves the rudder) input rod;
• damage to the standby PCU forward support bracket;
• cracked trailing-edge ribs above and below the standby PCU, with the upper rib fractured through, next to two of the three bracket mounting holes.

The NTSB found fracture signatures consistent with overload failure.

How did storm damage go undetected?

The trail led to 16 May 2024, when the aircraft parked overnight at remote stand R3W at Louis Armstrong New Orleans International Airport. A squall line crossed the field under a tornado watch, and a station about 5,000 feet from the jet recorded gusts of 73 knots.

The 737's rudder has no gust lock, and only the main PCU provides gust damping; the standby unit has none. The NTSB concluded wind likely moved the rudder and cracked the ribs carrying the standby PCU.

Boeing's maintenance manual contained a conditional inspection for parked aircraft exposed to extreme winds, but it referenced only tornadoes and microbursts and set no wind speed threshold. Southwest also had no process to alert maintenance control when storms hit parked aircraft, so nobody ordered the inspection. The jet flew the next morning.

Between 17 and 23 May it completed 38 uneventful flights. On 23 May it underwent an A Core Check at Houston Hobby, including an operational test of the standby hydraulic system that requires an observer at the tail watching the rudder move to each stop. Southwest's records showed a clean pass. Hangar security footage showed the rudder travelled right but not left, with nobody at the tail. The damage, the NTSB said, would have appeared as a rudder that would not return to neutral. Abnormal rudder behaviour began on the first flight after the check.

On 24 May the aircraft flew five more sectors. On the fourth, flight 5816, the crew manually disconnected the yaw damper; engineers in San Jose deferred it under the minimum equipment list. After a functional test passed, the jet returned to service — making flight 746 its 44th since the storm.

What does this change for operators?

The NTSB's probable cause was storm damage to the standby PCU's connection to the fin while parked, undetected and leading to abnormal PCU operation. Three factors contributed: an incorrectly performed standby hydraulic test, Boeing's manual lacking a wind speed threshold, and Southwest's inadequate alerting process after extreme winds.

Both companies have acted. Southwest inspected rudder PCU mountings across its entire 737 MAX fleet, found nothing abnormal, introduced a dedicated work card with individual mechanic sign-offs, and now alerts maintenance control when parked aircraft face extreme winds. Boeing revised the 737 maintenance manual so the extreme high wind inspection now triggers at 65 knots.

The Oakland flight ended without injury, but the report shows how latent damage can pass through multiple safety layers. The crew's decision to treat the motion as a mechanical fault — not turbulence — gave engineers the chance to find the cracked ribs before the failure progressed.