EXPLAINER: Why a plane’s engine exploded over Denver

The investigation into the engine explosion on a United Airlines flight over the weekend is homing in on why engine fan blades broke off. A United Boeing 777 had to make an emergency landing in Denver after one of its engines spewed chunks of wreckage over suburban neighborhoods. U.S. officials say two fan blades in the Pratt & Whitney engine broke off. Dozens of planes in the U.S., Japan and South Korea with the same type of engine have been grounded. Safety experts say the investigation wi