Persons residing in Alaska who are and have been employed there by the United States for at least two years, and their families, may be transported on vessels or airplanes operated by the Department of Defense, if—

((1)) the Secretary of Defense considers that accommodations are available;

((2)) the transportation is without expense to the United States;

((3)) the transportation is limited to one round trip between Alaska and the United States during any two-year period, except in an emergency such as sickness or death; and

((4)) in case of travel by air, the transportation cannot be reasonably handled by a United States commercial air carrier.