; or (D) any other fee or surcharge determined appropriate by the Federal Communications Commission. 4. Seating young children adjacent to an accompanying adult passenger (a) In general Subchapter I of chapter 417 of title 49, United States Code is amended by adding at the end the following: 41727. Seating young children adjacent to an accompanying adult on aircraft. (a) Applicability The requirements of this section apply for the purposes of each scheduled flight segment to or from any airport in the United States. (b) General requirement (1) Assigned seats Subject to subsection (f), an air carrier or foreign air carrier that assigns seats, or allows individuals to select seats, in advance of the date of departure of a flight shall seat each young child adjacent to an accompanying adult, provided that adjacent seat assignments are available at any time after the ticket is issued for the young child and before the first passenger boards the flight. (2) Unassigned seats Subject to subsection (f), an air carrier or foreign air carrier that does not assign seats or allow individuals to select seats in advance of the date of departure shall board customers in a manner that ensures each young child is seated adjacent to an accompanying adult. (3) Prohibition on fees (A) In general An air carrier or foreign air carrier may not charge a fee or impose an additional cost to seat a young child adjacent to an accompanying adult within the same class of service. (B) Class of service requirement Every class of service an air carrier or foreign air carrier offers must allow for seating of a young child adjacent to an accompanying adult. (C) Prohibition on availability limits An air carrier or foreign air carrier may not limit the availability of adjacent seats in a manner that results in an undue burden on the ability of a young child to receive an advance seat assignment adjacent to an accompanying adult. (c) Additional requirements for advance seating (1) In general Subject to subsection (f), an air carrier or foreign air