The nation’s top accident investigator said Thursday that a surge in close calls between planes at U.S. airports this year is a "clear warning sign” that the aviation system is under stress. “While these events are incredibly rare, our safety system is showing clear signs of strain that we cannot ignore," Jennifer Homendy, chair of the National Transportation Safety Board, told a Senate panel on Thursday. Homendy warned that air traffic and staffing shortages have surged since the pandemic.