(a) Each sponsor shall establish and maintain, for each individual project, an adequate accounting record to allow appropriate personnel of the FAA to determine all funds received (including funds of the sponsor and funds received from the United States or other sources), and to determine the allowability of all incurred costs of the project. The sponsor shall segregate and group project costs so that it can furnish, on due notice, cost information in the following cost classifications:

(1) Purchase price or value of land.

(2) Incidental costs of land acquisition.

(3) Costs of contract construction.

(4) Costs of force account construction.

(5) Engineering costs of plans and designs.

(6) Engineering costs of supervision and inspection.

(7) Other administrative costs.

(b) The sponsor shall obtain and retain in its files for a period of three years after the date of the final grant payment, documentary evidence such as invoices, cost estimates, and payrolls supporting each item of project costs.

(c) The sponsor shall retain, for a period of three years after the date of the final grant payment, evidence of all payments for items of project costs including vouchers, cancelled checks or warrants, and receipts for cash payments.

(d) The sponsor shall allow the Administrator and the Comptroller General of the United States, or an authorized representative of either of them, access to any of its books, documents, papers, and records that are pertinent to grants received under the Federal-aid Airport Program for the purposes of accounting and audit. Appropriate FAA personnel may make progress audits at any time during the project, upon notice to the sponsor. If work is suspended on the project for an appreciable period of time, an audit will be made before any semi-final payment is made. In each case an audit is made before the final payment.