PHILADELPHIA — Southwest has sent a letter of apology, a $5,000 check and a $1,000 travel voucher to passengers who were on a flight that made an emergency landing in Philadelphia after an engine exploded. “Please accept our deepest apologies,” Chairman Gary Kelly wrote in a letter dated April 18, a day after the accident. An engine on a Southwest jet exploded Tuesday while the plane was flying from New York to Dallas, and debris hit it, causing extensive damage.