DENVER—The Federal Aviation Administration on Tuesday ordered airlines in the United States to ground planes with the type of engine that blew apart after takeoff from Denver this past weekend until they can be inspected for stress cracks. The FAA’s order applies to U.S. operators of airplanes equipped with certain Pratt & Whitney PW4000 engines, which are used solely on Boeing 777s. Before the planes can fly again, operators must conduct a thermal acoustic image inspection of the large titanium fan blades at the front of each engine. The technology can detect cracks on the interior surfaces of the hollow blades that can’t be seen by the naked eye, according to the order. The FAA directive is a blow to United Airlines, which had 24 of the planes in service and is the only U.S. airline with the engine in its fleet. “On Sunday, we voluntarily removed 24 Boeing 777 …