DCA24LA094

United Airlines flight 1539, a Boeing 737-8, experienced a rudder pedal anomaly while landing at Newark Liberty International Airport (EWR), Newark, New Jersey. In a postincident statement, the captain reported that, during the landing rollout, the rudder pedals were “stuck” in their neutral position and did not move in response to the “normal” application of foot pressure to maintain alignment with the runway centerline. Postincident troubleshooting by United Airlines, including test flights, reproduced the pedal control anomaly and identified the rudder rollout guidance actuator (RRGA), part of the rudder control system, as the source of this anomaly. Examination of the incident RRGA confirmed evidence of an accumulation of moisture inside the servo. Specifically, staining patterns within the lower gearbox and motor bowl indicated that accumulated water was sufficient to submerge portions of the lower geartrain and associated bearings. At room temperature, the servo’s crank arm rotated freely. However, when the RRGA was exposed to cold temperatures (“cold soaked”) in a climate chamber, the crank arm became difficult to move by hand, and the measured torque exceeded the test requirement. After the unit returned to room temperature, allowing any frozen moisture to melt, the crank arm again rotated freely. Evidence of water intrusion was observed originating at the pressure port inlet and extending across the upper gearbox area. It is likely that sufficient moisture entered the RRGA gearbox and subsequently froze when the airplane was operated in cold temperatures. As a result, the frozen moisture restricted movement of the internal geartrain, output crank arm, rudder torque tube, and the flight deck rudder pedals.

Safety Actions As a result of the findings from this investigation, on September 26, 2024, the NTSB issued Urgent Safety Recommendations A-24-27 through A-24-30. Following the incident, United Airlines requested technical guidance from Boeing to permanently remove the SVO-730 from N47280. United Airlines also determined that Boeing delivered an additional eight 737 airplanes with SVO-730s that were electrically disabled but mechanically connected to the rudder control system. Boeing subsequently issued Service Bulletin 737-27-1458 with instructions for operators not utilizing CATIIIB Autoland capability to physically remove the RRGA. United subsequently accomplished the service bulletin on all affected airplanes in its fleet. In September 2024, Collins Aerospace issued a service bulletin titled Auto Flight – SVO-730 – Bearing Installation Orientation that stated a modification procedure was applicable to a portion of the SVO-730 units. The modifications were for two sealed ball bearings located in the upper gearbox. The service bulletin provided instructions to operators on how to return the units to Collins Aerospace for disassembly, inspection and, as needed, replacement of the affected SVO-730s. In November 2025, Collins Aerospace informed investigators that they would introduce a series of SVO-730 changes to prevent moisture ingress. The improvements included changes to the pressure port screw, gearbox washer, and the output shaft seal. Boeing released two Flight Crew Operations Manual Bulletins (OMB) on October 9, 2024 covering the specific failure mode of the jammed RRGA on the B737- 700/800/900 and B737-8/9. The OMBs provide expanded guidance on how to accomplish the Jammed or Restricted Flight Controls Non-Normal Checklist (NNC). They both recommended: If a jammed or restricted rudder condition is encountered prior to touchdown, the flight crew should accomplish the Go-Around and Missed Approach Procedure, climb to a safe altitude, and accomplish the Jammed or Restricted Flight Controls NNC. The Jammed or Restricted Flight Controls NNC must only be performed airborne, at a safe altitude. If restricted flight controls are encountered after touchdown and rollout is continued, Boeing recommends applying differential braking to maintain runway centerline. Avoid using nosewheel steering above 100 KIAS unless necessary for airplane control as a potential for overcontrol exists. Boeing 737 Rudder Control System Description Rudder control of the 737-8 airplane is accomplished using a single panel rudder located on the trailing edge of the vertical stabilizer. Rudder position is controlled manually by the pilot or copilot via the rudder pedals, electrically through rudder trim, and automatically through the system’s yaw damper. Rudder movement is provided by two main hydraulic systems in normal operation and an additional standby hydraulic system in the event of a main system failure (see figure 1). Figure 1. Rudder control system with rudderrollout guidance actuator. (Image © Boeing. Reprinted with permission.) Normal control of the rudder by the flight crew is transmitted in a closed-loop system from the pilots’ rudder pedals in the cockpit through a single cable system, to the rudder aft control cable quadrant, to the rudder pedal force transducer to the aft rudder input torque tube in the vertical fin. Rotation of the torque tube provides the command inputs to the two main and standby rudder power control units (PCU’s) to move the rudder. The rudder control system incorporates a feel and centering unit mounted at the bottom of the rudder torque tube. The unit is connected to the aft rudder quadrant through the rudder pedal force transducer and attaches to the bottom of the torque tube via the lower input crank. The feel and centering unit provides an artificial feel to the rudder pedals, centers the input to the rudder PCU and provides the rudder trim inputs to the aft control components. The 737-8 autopilot does not have rudder control authority. However, for high precision Category IIIB (CATIIIB) approaches using autoland, the airplane can be equipped with an optional customer-selectable system that incorporates a Collins Aerospace SVO 730 RRGA. When installed, the SVO-730 is located in the upper portion of the vertical stabilizer and provides input to the rudder via the unit’s output shaft and connecting link to the top of the rudder input torque tube (see figure 2). Figure 2. Location of SVO-730 Rudder Rollout Guidance Servo in vertical stabilizer, with connection to upper end of rudder torque tube. (Image © Boeing. Reprinted with permission.) SVO-730 Rudder Rollout Guidance Servo Description The Collins Aerospace SVO-730 RRGA is a self-contained actuator that provides yaw control/rudder inputs during CATIIIB instrument approaches (see figure 3). The RRGA is only electrically active when both autopilots are engaged for a CATIIIB approach. The servo receives commands calculated by the flight control computers via electrical signals and moves the rudder to keep the airplane aligned with the runway centerline throughout the approach and landing rollout. The actuator is mechanically connected to the rudder control system and is back-driven by mechanical inputs from the flight deck and by electrical inputs from the yaw damper and rudder trim system. Figure 3. SVO-730 Rudder Rollout Guidance Actuator. (Photograph courtesy of United Airlines) The SVO-730 is located in the upper portion of the vertical stabilizer and is connected directly to the upper end of the rudder input torque tube by a linkage. The unit consists of two separate, but similar motors and gearbox servos mounted on top of a reduction gearbox, output shaft and lever arm. The Motor 2 structure and gearbox are located further from the vertical stabilizer than Motor 1 (see figure 4). Figure 4. Overhead view of the SVO-730 as installed, noting position of the Motors. (Photo courtesy of United Airlines) Each of the twomotor/servos contain an electromagnetic, toothed clutch plate that is used to engage and disengage the respective motor from the main gearbox and output shaft. The SVO-730 data plate is mounted on the exterior of the gearbox housing. The housing contains the servo pressure port, which provides access to the interior of the gearbox through an opening at the top of the housing that enters the gearbox ceiling. The port is normally sealed with a set screw and is opened only during production testing to monitor internal leakage, see figure 5. Figure 5. The SVO-730 modification plate with red arrow pointing to pressure port screw. The main gearbox housing includes the SVO-730’s gearing from the two servo motors to the main output shaft and incorporates a bearing that provides a moisture seal. The SVO-730’s output crank arm incorporates shear pins designed to fail at a prescribed rudder pedal force. According to Boeing, the force required to shear the pins is 220 ± 30 pounds at the rudder pedals. This threshold was established to provide a margin above the maximum force required for autopilot override through the pedals, allowing the flight crew to override the autopilot without causing inadvertent shear-pin failures. When the shear pins fail, the SVO-730 actuator is mechanically disconnected from the rudder control system, restoring normal rudder operation through the pedals and enabling continued flight crew control of the rudder (see figure 6). A jam in the rudder aft quadrant does not affect yaw damper operation. The yaw damper remains functional and is capable of providing up to ±3° of rudder movement. Figure 6. The underside of an SVO-730 with shear pin rivets (red arrows). The white arrow points to the servo output crank arm. (Photo courtesy of United Airlines) N47280 Incident Airplane The incident airplane was originally manufactured for delivery to another airline, which established the aircraft’s initial production specifications. These specifications included the optional CATIIIB autoland capability, enabled through installation of a RRGA. Before delivery, the airplane was acquired by United Airlines and was subsequently delivered to United on February 20, 2023. United Airlines does not require CAT-IIIB capability for its Boeing 737 fleet. Therefore, United Airlines elected not to incorporate the optional CATIIIB autoland capability on the incident airplane. As a result, the airplane was delivered with a Collins Aerospace SVO 730 RRGA that was electrically disabled. Before delivery, Boeing deactivated the CATIIIB autoland function on the incident airplane by incorporating Boeing Service Bulletin 737-22-1392, “Autoflight – Autopilot – Flight Control Computer Program Pin Change from Category IIIB to IIIA” (Revision 1, dated November 10, 2022). The service bulletin provided instructions to modify the flight control computer pin programming, which removed the software required for CATIIIB autoland approaches. However, the service bulletin didnot remove the RRGA actuator. The deactivated actuator remained installed and mechanically connected to the upper portion of the aft rudder input torque tube through its output crank arm and pushrod. Examination of SVO-730 The incident RRGA was removed and sent to the Collins Aerospace facility in Cedar Rapids, Iowa for detailed examination (see figure 7). Figure 7. The SVO-730, as installed in N47280 prior to removal. The investigation determined that, after exposure to cold conditions (“cold-soak”) specified in Collins Aerospace production test requirements, the incident SVO-730 output crank arm could seize, causing the servo to fail the crank arm friction production test. Postincident examination found that the torque required to rotate the output crank arm at room temperature was 2.8 inch-pounds, which was within design limits. However, after the actuator was cold-soaked for one hour, the crank arm could not be rotated at that torque. Incremental increases in applied torque showed that 520 inch-pounds were required to initiate rotation and about 100 inch-pounds was needed to sustain rotation, both of which exceeded specified design limits. Disassembly of the actuator revealed evidence of moisture in the gearbox assembly (see figure 8), and corrosion on the gearbox bearings. Evidence of moisture intrusion was noted from the pressure port inlet. Figure 8. Incident actuator bearing with area of moisture. (Photo courtesy of Collins Aerospace) Examination of additional SVO-730 units corroborated the findings of moisture ingress and demonstrated that exposure to cold temperatures could result in failure of applicable production test requirements. One unit, removed from service by another operator and provided to Collins Aerospace, exhibited evidence of moisture ingress and failed the cold-soak test because of the elevated torque required to rotate the output crank arm. Conversely, none of the units that passed cold-soak testing showed evidence of moisture in the gearbox area. On February 6, 2024, about 1555 eastern standard time (EST), United Airlines (UAL) flight 1539, a Boeing 737-8, N47280, experienced a rudder pedal jam while landing at EWR, Newark, New Jersey. None of the 161 occupants onboard the airplane were injured and the airplane was not damaged. The flight was operated under the provisions of Title 14 Code of Federal Regulations Part 121 as a scheduled international passenger flight from Lynden Pindling International Airport, Nassau, Bahamas, to EWR. In a postincident statement, the captain reported that, during the landing rollout, the rudder pedals remained stuck in their neutral position and did not respond to the “normal” foot pressure applied to maintain runway centerline alignment. Boeing reported that rudder pedal force ranges from 15 to 75 pounds (the latter at full pedal travel), with about 45 pounds representing average operational use. Flight data recorder (FDR) derived data showed that, before touchdown, the flight crewapplied about 32 pounds of force to the rudder pedals with no corresponding change in rudder position or airplane heading. Immediately after touchdown, with the airplane traveling about 120 knots, the crew increased pedal force to approximately 75 pounds in an attempt to free the jammed rudder system; however, the rudder position and heading remained unchanged. As the airplane continued to decelerate, the crew applied about 42 pounds of force to the rudder pedals, but the jam persisted. The captain then transitioned to nosewheel steering to maintain directional control as the airplane slowed to taxi speed. After entering the assigned taxiway, the captain asked the first officer (FO) to check the rudder pedals on his side of the flight deck, and the first officer reported experiencing the same condition. About 31 seconds after touchdown, when the airplane's groundspeed had decreased to less than 20 knots, the flight crew applied about 59 pounds of force to the rudder pedals. Subsequently, the rudder pedals and rudder surface began responding normally and continued to operate as commanded. The airplane taxied to the gate without further incident. On February 9, 2024, a maintenance test flight was conducted to evaluate the rudder system malfunction reported during the incident flight. The rudder jam was successfully reproduced and recorded by the flight data recorder (FDR). The airplane climbed to between 32,000 and 33,000 ft and remained at cruise altitude for about 33 minutes before descending to 13,000 ft for a flight control check. During the check, successive left and right rudder pedal inputs initially produced little corresponding rudder movement. A subsequent right rudder pedal input of about 93 pounds restored normal rudder pedal and rudder surface response, which continued for the remainder of the flight. Following the flight test, Boeing notified United Airlines of the presence of a Collins Aeropsace SVO 730 rudder rollout guidance actuator (RRGA) and its potential contribution to the jammed rudder pedals. United Airlines subsequently removed the RRGA from the incident airplane to eliminate it as a potential contributor to the rudder pedal restriction anomaly. Following the component removal, a second test flight was conducted and confirmed that no further rudder pedal restrictions were observed. Consequently, and in consideration of United Airlines’ troubleshooting actions and test flight results, the investigation focused on the airplane’s rudder control system, with particular emphasis on the RRGA. An excerpt from the Boeing B737 Flight Crew Training Manual used by United Airlines dated June 30, 2023, stated in part: The flight control system consists of control surfaces mechanically linked to flight deck controls. Although rare, jamming of the flight control system has occurred on commercial airplanes. A jammed flight control can result from ice accumulation due to water leaks onto cables or components, dirt accumulation, componentfailure such as cable break or worn parts, improper lubrication, or foreign objects. A flight control jam may be difficult to recognize, especially in a properly trimmed airplane. If any jammed flight control condition exists, refer to the Jammed or Restricted Flight Controls NNC [non-normal checklist]. The Jammed or Restricted Flight Controls non-normal checklist stated in part: Overpower the jammed or restricted system. Use maximum force, including a combined effort of both pilots, if needed. A maximum two-pilot effort on the controls will not cause a cable or system failure. The failure could be due to freezing water and conditions allow a descent: Consider a descent to a warmer temperature and attempt to overpower the jammed or restricted system again. Simulator Study On October 10, 2024, the NTSB conducted a simulator study at Boeing in Seattle, Washington, to evaluate pilot and aircraft response to a simulated rudder jammed by a frozen RRGA. Testing showed that a jammed rudder could be cleared by applying sufficient pedal force, after which normal rudder control was restored. Pilot recognition and response, including use of the QRH Jammed or Restricted Flight Controls checklist, were qualitatively assessed. Response times were not measured because all participants were aware of the simulated failure. The study evaluated controllability across multiple jam-release force levels. Three B737 rated pilots served as pilot flying, supported by a Boeing experimental test pilot acting as pilot monitoring. The engineering simulator, which is used to develop Level D training simulator models, allowed rudder jams to be programmed to release at specified pedal forces. Ground tests evaluated pedal forces, pedal movement, and pilot perception of rudder breakout forces of 0, 80, 150, and 230 pounds. All pilots considered the 80-pound force minor and successfully cleared jams up to 230 pounds without assistance. In-flight testing assessed aircraft response before and after clearing rudder jams at representative holding altitudes and airspeeds. All jam conditions up to 230 pounds were cleared without difficulty. No pilot required assistance. The 80-pound jam produced little noticeable aircraft response before release. For higher-force jams, release resulted in approximately 20° of roll and 0.2 g of lateral acceleration. Pilots reported that higher breakout forces produced less post-release aircraft response because they anticipated the release and reduced pedal pressure accordingly. The Jammed or Restricted Flight Controls checklist was evaluated and found to be clear, executable, and adequate, with no reported issues. Landing tests were conducted with a 10-knot crosswind under no-jam, 150-pound, and 230-pound jam conditions. The jam was cleared before touchdown, at touchdown, or during rollout. One pilot detected the jam at 300 to 400 ft AGL, another at 100 to 150 ft AGL, and the pilot not current in the B737 did not recognize the jam until touchdown orshortly thereafter. Following jam release, a slight right drift occurred in all tests. The airplane remained controllable throughout, and differential braking aided directional control. Crosswind Approach and Go-Around Additional tests were conducted with a 25-knot crosswind and higher approach speeds. Consistent with Boeing guidance, pilots initiated a go-around upon recognizing a rudder jam. Two pilots detected the jam above 300 ft AGL and executed immediate go-arounds. The pilot who was not current in the B737 delayed rudder application, failed to detect the jam during one run, and landed after clearing the jam at about 30 ft AGL. In a repeat test, the pilot recognized the condition and initiated a go-around at 50 to 100 ft AGL. No controllability issues were reported. One pilot noted that delayed rudder application during a crosswind approach could reduce the time available to recognize and respond to a rudder jam before touchdown.