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The US National Transportation Safety Board (NTSB) on Thursday issued "urgent safety recommendations" regarding the rudder systems on certain Boeing 737 aircraft, highlighting a risk of jamming.

It came after a February 6 incident involving a Boeing 737 MAX 8 operated by United Airlines, whose rudder pedals were "stuck" in the neutral position while on the tarmac after landing at Newark airport in New Jersey.

None of the 155 passengers and six crew members were hurt, the NTSB said, with the captain using the nose landing gear controls to steer the plane.

Boeing has come under increasing pressure following a number of safety incidents involving its aircraft. It did not immediately respond to an AFP request for comment.

The cause of the February incident was found to be the rollout guidance actuator, one of the rudder control components, with tests revealing it was susceptible to moisture which could "freeze and limit rudder system movement," the NTSB said.

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