WASHINGTON — U.S. Air Force Secretary Frank Kendall is having second thoughts about the Pentagon’s decision to upgrade the F-35 fighter jet’s current engines instead of developing a next-generation replacement. In a discussion at the McAleese & Associates conference in Washington, Kendall said the decision to upgrade the fighters’ current Pratt & Whitney-made F135 engines instead of developing a new adaptive engine “was the right decision, given the constraints that we have, but [a choice] that