Incident Overview
On February 6, 2024, at approximately 15:55 eastern standard time, United Airlines (UAL) flight 1539, a Boeing 737-8 (registration N47280), experienced a rudder pedal malfunction during landing rollout at Newark Liberty International Airport (EWR), Newark, New Jersey. The flight, operating as a scheduled international passenger service under 14 CFR Part 121 from Lynden Pindling International Airport (NAS), Nassau, Bahamas, to EWR, landed without further incident. All occupants—2 flight crewmembers, 4 cabin crewmembers, and 155 passengers—deplaned at the gate without injuries.
Flight Crew Report
In a post-incident statement, the captain reported that during the landing rollout, the rudder pedals did not move in response to normal foot pressure while attempting to maintain the runway centerline. The pedals remained stuck in their neutral position. The captain used the nosewheel steering tiller to keep the airplane near the centerline while slowing to a safe taxi speed, then exited the runway onto a high-speed turn-off. On the turn-off, the captain asked the first officer to check his rudder pedals, who reported the same issue. Shortly thereafter, the rudder pedals began operating normally. After parking, the flight crew notified UAL maintenance of the malfunction. The airplane was removed from service for troubleshooting.
Test Flights and Maintenance
On February 9, 2024, UAL conducted a test flight on the incident aircraft at EWR, duplicating the rudder system malfunction reported during the incident. The test flight was discontinued and the aircraft returned to EWR uneventfully. Post-incident troubleshooting and inspection of the rudder control system found no obvious malfunctions with the system or its components whose failure would have resulted in restricted movement. As a precaution, the aft rudder input torque tube, associated upper and lower bearings, and the rudder rollout guidance servo were removed for further examination. Following removal of these components, UAL conducted a second test flight on the aircraft, finding the rudder control system operated normally.
Investigation
The National Transportation Safety Board (NTSB) was notified after the test flight and initiated an incident investigation. Qualified parties were invited to participate, including the Federal Aviation Administration (FAA), United Airlines, The Boeing Company, and Collins Aerospace. NTSB group chairs in systems and flight data recorders were assigned. FDR data from the incident flight corroborated the pilot statements: during landing and rollout, the rudder surface remained near neutral despite increasing pedal force inputs. About 30 seconds after touchdown, a significant pedal force input was observed with corresponding rudder surface movement; thereafter, the rudder pedals and surface moved as commanded for the remainder of taxi.
System Description and Component Examination
Pilot control of the Boeing 737-8 rudder is transmitted through a closed-loop system from the pilots’ rudder pedals to an aft rudder quadrant, pedal force transducer, and aft rudder input torque tube in the vertical fin. Rotation of the torque tube provides command inputs to two main and standby rudder power control units (PCUs) to move the rudder surface. UAL received the incident aircraft from Boeing on February 20, 2023, configured with a rudder SVO-730 rollout guidance servo that was disabled per UAL’s delivery requirements to reconfigure the autoflight system from CAT IIIB to CAT IIIA capability. Although disabled, the servo remained mechanically connected to the upper portion of the aft rudder input torque tube by its output crank arm and pushrod.
On February 28, 2024, the NTSB Systems group examined and tested the SVO-730 rollout guidance servo at the Collins Aerospace facility in Cedar Rapids, Iowa. Testing evaluated the effects of temperature cold soaking on the torque required to move the servo’s output crank arm. At room temperature, torque was within design specifications. After a 1-hour cold soak, the torque to move the output crank arm was significantly beyond design limits. Because the servo output crank arm is mechanically connected to the rudder input torque tube, restricted movement would prevent rudder pedal movement as observed. Further examination of the servo is ongoing.