An Alaska Airlines Boeing 737 Max 9 aircraft.Kevin Carter/Getty ImagesThe National Transportation Safety Board has more concerns about Boeing's planes.It said at least 40 airlines outside the US may be operating aircraft with risky rudder parts.This is the latest blow to Boeing, which has been plagued by safety probes.The National Transportation Safety Board (NTSB) has flagged fresh concerns with Boeing's 737 planes, saying that at least 40 airlines outside the US may be operating aircraft with faulty components.The NTSB issued a press release on Thursday, saying it had issued urgent safety recommendations to Boeing and the Federal Aviation Administration (FAA).The release said that the NTSB was investigating a February 6 incident with a United Airlines Boeing 737 Max 8 plane, which experienced "stuck" rudder pedals while landing at Newark Liberty International Airport in New Jersey.The Board said that its investigators tested a rollout guidance actuator — one of the rudder control components from the affected airplane — at the component's manufacturer, Collins Aerospace.When tested with another identical unit from a separate airplane, both actuators failed.