The National Transportation Safety Board (NTSB) issued "urgent safety recommendations" for the 737 to Boeing and the Federal Aviation Administration (FAA), relating to the potential for some 737s' rudder control system to jam.
The notice was in response to an incident on February 6, 2024, in which the rudder pedals on a United Airlines 737 MAX 8 became stuck in a neutral position during the landing rollout at Newark Liberty International Airport.
The captain had managed to control the jet's direction after touching down by using the nosewheel steering tiller. No injuries or damage to the aircraft was reported.
The NTSB said it tested the rollout guidance actuator - a rudder control component - from the incident airplane at Collins Aerospace, where the component was manufactured.
"When the incident actuator and an identical unit from another airplane were tested in a cold environment," NTSB said in a statement, "the actuators' function was significantly compromised."
It found moisture in both actuators, which failed the testing of the components.
"Collins Aerospace subsequently determined that a sealed bearing was incorrectly assembled during production of the actuators, leaving the unsealed side more susceptible to moisture that can freeze and limit rudder system movement," the NTSB continued.
Pilots are instructed to overpower the system using "maximum force" when confronted with a jammed or restricted rudder system in the 737 flight manual.
The NTSB "expressed concern" that the application of maximum force could result in loss of control of the aircraft as well as it running off the runway.
The safety board recommended Boeing to "determine appropriate flight crew responses besides applying maximum pedal force" for such situations. It also recommended that Boeing notify flight crews operating 737s with affected actuators that the rudder control system can jam due to accumulated moisture that has frozen.
In addition, it recommended that the FAA determine if actuators with incorrectly assembled bearings should be removed from affected 737 aircraft and, if so, to direct US to do so and notify foreign aviation regulators of its recommendations.
The notice comes as some 33,000 Boeing factory workers are on strike at the Portland and Seattle sites. It was reported that the 737 production has been halted temporarily.