Cal. Rev. & Tax. Code § 6051.6

There are exempted from the taxes imposed by Section 6051.5 the gross receipts derived from the sale of tangible personal property, other than fuel or petroleum products, to operators of aircraft to be used or consumed principally outside the county in which the sale is made and directly and exclusively in the use of the aircraft as common carriers of persons or property under the authority of the laws of this state, the United States, or any foreign government.