(Reuters) - Southwest Airlines grounded 81 aircraft from its Boeing 737 fleet for inspection after a gaping hole in the fuselage forced one of its planes to make emergency landing in Arizona on Friday, the company said on Saturday. Southwest (LUV.N) and Boeing (BA.N) engineers will inspect the aircraft to try to determine the cause, Southwest said in a statement. Passengers heard a loud noise and suddenly saw a hole about mid-cabin. Southwest Airlines is working with the National Transportation Safety Board and the Federal Aviation Administration on the ground in Arizona to determine the cause of a sudden drop...