Boeing 737-8 N47280 incident near Newark, New Jersey

Probable cause

Moisture intrusion through the pressure port inlet of the rudder rollout guidance actuator that accumulated and froze during flight at altitude, resulting in jammed rudder pedal movement.

NTSB analysis

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.

Text from the NTSB. Open the NTSB docket for DCA24LA094 for the full record.