(a) Offer. Upon approving a project for airport development, airport master planning, or airport system planning, the Administrator issues a written offer that sets forth the terms, limitations, and requirements of the proposed agreement.

(b) Acceptance. The acceptance of an offer or an amendment to a grant agreement must be in writing. The sponsor's or planning agency's attorney must certify that the acceptance complies with all applicable law, and constitutes a legal and binding obligation of the sponsor or planning agency.

(c) Amendment: Airport development grants. The maximum obligation of the United States under a grant agreement for an airport development project may be increased by an amendment if—

(1) Except as otherwise provided by the Uniform Relocation Assistance and Real Property Acquisition Policies Act of 1970, the maximum obligation of the United States is not increased by more than 10 percent;

(2) Funds are available for the increase;

(3) The sponsor shows that the increase is justified; and

(4) The change does not prejudice the interest of the United States.

(d) Reduction of U.S. Share: Airport development grants. When project work for which costs have been incurred is deleted from a grant agreement, the Administrator reduces the maximum obligation of the United States proportionately, based on the cost or value of the deleted work as shown on the project application.

(e) Amendment: Airport planning. A grant agreement for airport planning may be changed if—

(1) The change does not increase the maximum obligation of the United States under the grant agreement; and

(2) The change does not prejudice the interest of the United States.