United Airlines resumes operations after brief nationwide mainline ground stop

Core Points - United Airlines issued a nationwide ground stop for all mainline flights due to an emergency alarm at its Chicago operations center [1][3] - The ground stop was temporary, and operations have since returned to normal with employees back at the primary operations center [1] - No flights were diverted during the ground stop, and United Express, the regional line, remained unaffected [3] - United Airlines operates the largest fleet among American airlines, with a total of 1,023 aircraft [3]