A United Airlines Boeing 737 Max 9 aircraft lands at San Francisco International Airport on March 13, 2019 in Burlingame, California.Justin Sullivan | Getty ImagesUnited Airlines said Monday that it has found loose bolts on door plugs of several Boeing 737 Max 9 planes during inspections spurred when a panel of that type blew out during an Alaska Airlines flight at 16,000 feet last week.The Federal Aviation Administration on Saturday grounded dozens of 737 Max 9s after the panel blew out midflig...