The National Transportation Safety Board (NTSB) has published a preliminary report into an incident involving “stuck” rudders on a United Airlines Boeing 737 MAX 8.
The Boeing 737-8, registered N47280, landed at Newark Liberty International Airport (EWR) on February 6, 2024, when the captain reported that during the landing rollout, the rudder pedals failed to move in response to the “normal” application.
The pilot said that while trying to maintain the runway centerline the pedals remained in their neutral position resulting in the nosewheel steering tiller being used until the aircraft slowed to a safe taxi speed.
The first officer also reported the same problem when he was asked by the captain to test his rudder pedals.
In a statement given to the NTSB the captain said that the pedals began working as normal shortly after.
A review of preliminary flight data recorder (FDR) data showed that around 30 seconds after touchdown, a “significant pedal force input was observed along with corresponding rudder surface movement.”
On February 9, 2024, following the 737-8 being removed from service, United conducted a test flight in which the same rudder system malfunction was identified, and the NTSB was subsequently notified of the issue.
“Post-incident troubleshooting and inspection of the rudder control system found no obvious malfunctions with the system or any of its components whose failure would have resulted in the restricted movement observed during flight 1539 and the test flight,” the NTSB said in its preliminary report.
The decision was then made to remove the aft rudder input torque tube and associated upper and lower bearings and the rudder rollout guidance servo for further examination by the NTSB systems group.
“Following the removal of the rudder system components, UAL conducted a second test flight on the airplane and found the rudder control system operated normally,” the NTSB explained in its report.
On February 28, 2024, the SVO-730 rollout guidance servo was tested at the Collins Aerospace facility in Cedar Rapids.
“The testing was conducted to evaluate the effects that temperature “cold soaking” of the servo might have on the torque required to move the servo’s output crank arm,” according to the NTSB.
At room temperature the inspectors found no issue with the rollout guidance servo, but after the unit was cold soaked for one hour, testing found that the torque to move the servo’s output crank arm was “significantly beyond the specified design limits”.
“Because the servo output crank arm is mechanically connected to the rudder input torque tube, the restricted movement of the servo’s output crank arm would prevent the rudder pedals from moving as observed during flight 1539 and the test flight. Further examination of the SVO-730 rollout guidance servo will be conducted as the investigation continues.,” the NTSB concluded in its preliminary report.
N47280 is a four-year-old aircraft and was delivered to United Airlines on February 20, 2023.
