If youre flying from mainland USA to Alaska, do you need to carry?
No, for the same reason that Canadians flying from Toronto to Western Canada (the flight path usually crosses part of northern Michigan) do not have to carry passports. You are still a domestic passenger. If there were an emergency landing, however, you would be on a short leash. A Toronto-Winnipeg flight made an emergency landing in Grand Forks a few years ago, and no one was allowed off the plane (due to a lack of U.S. CBP personnel available to inspect them). The same thing happened with a United Airlines flight from New York to Hong Kong that landed in Labrador. These travelers obviously had passports, but again, border inspection constraints meant that no one was allowed off the plane. If you are flying to Alaska, you won't need a passport because if your aircraft makes an emergency landing in Canada, you won't be getting off unless the plane is on fire.