Transportation Secretary Samuel K. Skinner said he will seek strong measures to combat air terrorism but nothing that will put U.S. carriers at a disadvantage in world travel markets. Skinner said at a news conference that he will attend a meeting of the Council of the International Civil Aviation Organization in Montreal next week to look for ways to prevent attacks on civilian aviation. Skinner also announced that Federal Aviation Administrator T. Allan McArtor, who was named to his post in 1987, is resigning. He said that McArtor turned down his offer to be considered to remain at the FAA. Responding to criticism of the FAA for not warning the public about threats against Pan Am Flight 103 before a plane on the flight exploded over Scotland killing 270 people, Skinner said air travel cannot be stopped every time there is a threat.