49 U.S.C. § 47133 — Restriction on use of revenues

(a) Prohibition. Local taxes on aviation fuel (except ones already in effect on December 30, 1987) or revenue from a federally assisted airport can only be spent on the capital or operating costs of: the airport itself; the local airport system; or another local facility, owned or run by that same airport owner or operator, that is directly and substantially tied to moving passengers or property by air.

(b) Exceptions.
(1) Prior laws and agreements. Subsection (a) doesn't apply if a law passed by September 2, 1982, or a covenant or assurance in a debt obligation issued by that date, already lets the owner's or operator's revenue — including local aviation fuel taxes — support not just the airport but also that owner's or operator's general debts or other facilities.
(2) Sale of private airport to public sponsor. For a privately owned airport, subsection (a) doesn't apply to sale proceeds when a public sponsor buys the airport, if: the Secretary approves the sale; this subchapter funds part of the public sponsor's purchase of airport land; and the private owner repays the Secretary an amount equal to the unpaid part of any airport improvement grant (spread over a 20-year period), plus an amount equal to the federal share of the current market value of any land bought with such a grant on or after October 1, 1996.
(3) Treatment of repayments. Those repayments count as recovered money from a prior year's obligation.

(c) Rule of Construction. Nothing in this section stops a state from using its own aviation fuel tax to support a state aviation program, or from using airport revenue — on or off the airport — for a noise mitigation purpose.