United Airlines’ use of larger aircraft on routes previously flown by Boeing Co’s grounded 737 MAX jets is costing the carrier money in the short-term, the company’s President Scott Kirby said in a letter to employees on Tuesday. Chicago-based United, which owns 14 MAX jets and has dozens more on order, has been using its larger 777 or 787 aircraft to cover routes formerly flown by the suspended MAX planes, without being able to fill the extra last-minute seats.