NEW YORK • Southwest Airlines Co. reported an 18 percent increase in quarterly net profit on Thursday, as strong demand and changes to the U.S. corporate tax code boosted its bottom line. Southwest, the carrier at the center of a deadly mid-flight engine explosion last week, posted a first-quarter profit of $438 million, excluding special items, up from $372 million a year earlier. Southwest, the fourth largest U.S. carrier by passenger traffic, posted earnings of 75 cents per share, topping the Wall Street consensus projection of 74 cents, according to Thomson Reuters I/B/E/S. Southwest reported a modest increase in unit costs of 0.1 percent year over year. The carrier had forecast a 1 percent to 2 percent increase in unit costs, excluding fuel and some items. Shares fell 3.6 percent in premarket trading. The Dallas-based airline has been under intense scrutiny in the days since an engine on one of its Boeing 737 jets blew apart mid-flight, killing a passenger and raising concerns about the safety of similar engines. “It remains a somber time for the Southwest Family following the Flight 1380 accident,” Chief Executive Officer Gary Kelly said in a statement. “We continue to cooperate with the National Transportation Safety Board’s thorough investigation to understand the cause of the accident.” The incident marked the first fatality on a U.S. commercial passenger airline since 2009, and the first such passenger death in Southwest’s 51-year history.