NTSB chair Jennifer Homendy wrote a critical letter to the head of the FAA.Kevin Dietsch/Getty ImagesThe NTSB chair accused the FAA of "downplaying" issues with some Boeing 737 Max jets.Jennifer Homendy said more than 40 airlines outside the US could be operating jets with risky rudder parts.It stems from an investigation into a February incident involving stuck rudder pedals.Safety bosses took aim at aviation regulators over concerns about some Boeing 737 Max jets.Jennifer Homendy, chair of the National Transportation Safety Board, accused the Federal Aviation Administration of "downplaying the urgency" of potentially defective rudder systems.She warned that at least 40 airlines outside the US may be operating Boeing 737s with risky rudder parts.The comments follow the ongoing investigation into a February incident where a United Airlines 737 Max experienced "stuck" rudder pedals during landing.The NTSB issued urgent safety recommendations to Boeing and the FAA last week.