US aviation investigators were on the ground in Oregon on Sunday trying to figure out what caused a door panel to blow out of a brand new Boeing passenger jet just minutes after takeoff, forcing pilots to make an emergency landing with a hole “the size of a refrigerator” in the side of the plane.The American jet maker was facing fresh scrutiny as regulators temporarily grounded the Boeing 737 Max 9 aircraft after a section of a plugged exit door on an Alaska Airlines flight detached 16,000ft (4,877 meters) ...