The US National Transportation Safety Board (NTSB) is investigating after a Boeing 737 MAX 8 plane experienced "stuck" rudder pedals last month.
The NTSB said, in a preliminary report on the 6 February incident, that the aircraft landed and taxied to the gate at Newark Airport without incident.
There were no injuries to the 161 passengers and crew on the United Airlines flight.
The NTSB reported that the captain said that, after landing, the rudder pedals did not move in response to "normal" application of foot pressure while attempting to maintain the runway centreline.
Boeing said it worked with United Airlines "to diagnose the rudder response issue observed during two 737-8 flights in early February. With coordination with United, the issue was successfully resolved with the replacement of three parts and the airplane returned to service last month".
The plane-make said this is the only report of the issue in the 737 MAX fleet, but is aware of two similar occurrences in 2019 that were resolved through component replacements.
United Airlines said the parts at issue are on just nine of its aircraft originally built for other airlines.
"We'll continue to work with Boeing, the NTSB and the FAA on next steps for these aircraft," United said.
Boeing has been under scrutiny in recent months after a 5 January mid-air blowout on another model, a Boeing 737 MAX 9.
Last month, the Federal Aviation Administration (FAA) in the US formally mandated inspections of Boeing 737 MAX planes for loose bolts in the rudder control systems after a recommendation from the plane-maker in December.
According to the NTSB report, the captain said that in the MAX 8 plane incident last month, the pedals remained "stuck" in their neutral position.
They used the nosewheel steering tiller to keep the plane near the runway centreline while slowing to a safe speed before exiting the runway onto a high-speed turn-off.
The NTSB said the captain asked the first officer to check his rudder pedals and the first officer reported the same problem.
The captain said that, shortly afterwards, the rudder pedals began to operate normally.
Three days after the incident, United Airlines conducted a test flight and was able to duplicate the reported rudder system malfunction identified during the incident on the same plane.
Post-incident inspection found no obvious malfunctions, the NTSB said.
After removal of the rudder system components, the board added, United Airlines conducted a second flight test and found the rudder controls operated normally.
