The US National Transportation Safety Board (NTSB) has issued “urgent recommendations” to address an actuator problem that could cause rudder control systems on some Boeing 737NG and 737 Max to jam.
The safety investigatory agency on 26 September said it has recommended that the Federal Aviation Administration determine whether the flight-safety issue warrants ordering airlines to remove affected actuators from the 737s of US airlines.
The FAA says United Airlines is the only US carrier with jets that had the faulty components, adding, “It is the FAA’s understanding that the units are no longer in service”.
However, American Airlines says some of its 737s had the bad actuators. “We replaced the affected actuators on the nine aircraft that had them earlier this year,” American says.
Neither United nor Boeing immediately responded to requests for comment. Delta Air Lines and Southwest Airlines, which also operate 737s, did not immediately respond to requests for information.
The FAA “has been monitoring this situation closely,” it adds. “Tomorrow, we will convene a corrective action review board based upon the NTSB’s interim recommendations and determine next steps.”
The NTSB discovered “the potential for a jammed or restricted rudder control system on some 737 airplanes” after examining a rudder control component taken from a United 737 Max 8 that suffered a rudder system failure on 6 February.
That jet’s rudder pedals “were stuck in their neutral position during the landing rollout at Newark”, the NTSB says. “The captain used the nose wheel steering tiller to control the airplane’s direction after touching down.”
NTSB investigators have since tested the jet’s “rollout guidance actuator”, made by Collins Aerospace. The component controls the rudder’s movement during “Category III” precision instrument landings. “Although it is not used in non-precision landings, the actuator remains mechanically engaged with the rudder system,” the NTSB says.
“When the incident actuator and an identical unit from another airplane were tested in a cold environment, the actuators’ function was significantly compromised,” it adds. “Investigators found evidence of moisture in both actuators, which failed testing.”
Collins did not respond to a request for comment.
But the NTSB says the Charlotte-based supplier “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”.
Collins delivered 353 actuators with that defect to Boeing since February 2017, the NTSB adds.
Boeing’s 737 flight manual calls for pilots to “overpower” jammed rudders using “maximum force, including a combined effort by both pilots”.
The NTSB, however, fears that manoeuvre could cause a rudder movement sufficient to leave pilots unable to control the jet.
It has therefore asked Boeing to “determine appropriate flight crew responses besides applying maximum pedal force”, and to ensure pilots flying affected jets are aware that the rudder systems “can jam due to moisture that has accumulated inside the actuators and frozen”.
