The aircraft carrier used to train the Navy's sailors and pilots _ the USS Lexington _ will be decomissioned in mid-1991, the Navy announced Wednesday. The USS Forrestal, which is homeported in Mayport, Fla., has been named to replace the Lexington as the service's training carrier. The Lexington was the site of a major accident last fall, when a jet pilot making his first landing crashed into the deck, killing himself and four others. In a statement issued at the Pentagon, the Navy said it had opted to retire the 47-year-old carrier rather than perform an overhaul to extend its service life. Since 1962, the Lexington has been used to qualify thousands of naval aviators, and it has recorded more than 490,000 aircraft landings on its flight deck. The Lexington was launched in 1942. During World War II, its crew destroyed more than 1,000 enemy planes and sank 300,000 tons of shipping, earning it the nickname ``The Blue Ghost'' and the reputation that it couldn't be sunk.