The NTSB has now released its Preliminary Report into an inflight ‘Dutch roll’ incident involving a Southwest Airlines Boeing 737 MAX 8 operating flight WN746 from Phoenix, AZ to Oakland, CA 0n 25 May 2024.
Whilst en route at FL340 (34,000 feet), the aircraft experienced a ‘Dutch Roll’ instability event. The crew regained control and descended to FL320, before continuing to the destination.
Southwest Airlines WN746 Phoenix-Oakland
On May 25th, 2024, a Southwest Airlines flight WN746, a Boeing 737 MAX 8 flying from Phoenix to Oakland, encountered a period of instability while cruising at FL340.
The incident involved a phenomenon known as Dutch roll. This is characterized by an unstable motion affecting both the aircraft’s direction (yaw) and bank (roll).
The flight crew regained control, descended to FL320 and continued to the destination airport. They landed safely with no injuries to the 175 passengers and 6 crew members on board.
Understanding Dutch Roll
Dutch roll is a coupled oscillation in both the yaw and roll axes of movement. This can occur when the airplane’s directional stability, typically maintained by the vertical stabilizer and rudder, is weakened.
While not inherently dangerous, Dutch roll can be uncomfortable for passengers and must be corrected by the pilot or the airplane’s automated systems.
Event Timeline
The NTSB’s preliminary report outlines the sequence of events:
- Pre-flight: During a pre-flight check, the captain noticed a previous log entry regarding a yaw damper issue where the system was overcorrecting. Records indicated corrective action had been taken before this flight.
- Taxi and Takeoff: While taxiing and during takeoff, the captain experienced temporary stiffness in the rudder pedals.
- Cruise: After reaching cruising altitude, the aircraft encountered light turbulence followed by a short period of Dutch roll. The autopilot remained engaged throughout, and both pilots described the movement as minimal. The yaw damper light, which would indicate a system malfunction, did not illuminate.
Flight Crew Response and Actions
Following the Dutch roll event, the crew discussed the situation and determined the turbulence wasn’t the cause.
Since the yaw damper is designed to counteract such disturbances and prevent rudder movement, they suspected an issue with the system itself.
They descended to 32,000 feet to assess the situation further. Here, they experienced similar brief periods of Dutch roll, with the captain again feeling faint rudder pedal movement.
The crew contacted Air Traffic Control and continued to Oakland for a safe landing. Upon arrival, the aircraft was met by maintenance personnel and removed from service for inspection.
The inspection revealed substantial damage to the vertical stabilizer, a critical component for maintaining directional stability.
This discovery prompted the National Transportation Safety Board (NTSB) to launch a full investigation into the incident.
Flight Data Analysis
The NTSB report includes insights gleaned from analyzing flight data. Notably, the data revealed that the anomalous behavior of the rudder system began on the first flight after scheduled maintenance performed on May 23rd.
Before this maintenance, yaw damper commands didn’t affect rudder pedals. However, after the maintenance, rudder pedal movements were observed when the yaw damper was engaged.
Interestingly, all occurrences of Dutch roll coincided with yaw damper engagement. When the yaw damper was disabled during the flight or when the airplane was dispatched with the yaw damper listed as inoperative (Minimum Equipment List – MEL), the anomalous behavior was not observed.
NTSB Focus: Determining the Cause of Damage
A key focus of the NTSB investigation is determining when the structural damage to the rudder system occurred.
Did the damage exist prior to the May 23rd maintenance and contribute to the yaw damper issue? Or did the maintenance activity itself cause the damage?
The investigation will also delve deeper into the cause of the yaw damper malfunction and its potential role in the Dutch roll event.
Understanding these factors will be crucial in preventing similar incidents in the future and ensuring the continued safety of passengers and crew.
This ongoing investigation highlights the importance of thorough maintenance procedures and robust safety protocols in aviation.
The Southwest Airlines 737 MAX 8 incident serves as a reminder that even minor technical issues can potentially lead to flight instability.
The NTSB’s findings will be instrumental in ensuring that all parties involved, from airlines and maintenance crews to aircraft manufacturers, take the necessary steps to prevent such occurrences.
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