10 U.S.C. § 2648: Persons and supplies: sea, land, and air transportation

Whenever the Secretary of Defense considers that space is available, the following persons and supplies may be transported on vessels, vehicles, or aircraft operated by the Department of Defense:(1) Members of Congress. (2) Other officers of the United States traveling on official business. (3) Secretaries and supplies of the Armed Services Department of the Young Men’s Christian Association. (4) Officers and employees of the Commonwealth of Puerto Rico on official business. (5) The families of members of the armed forces, officers and employees of the Department of Defense or the Coast Guard, and persons described in paragraphs (1), (2), and (4). However, a person described in paragraph (4) or (5) may be so transported only if the transportation is without expense to the United States.