(a) Subject to the final determination of allowable project costs as provided in §151.63 partial grant payments for project costs may be made to a sponsor upon application. Unless previously agreed otherwise, a sponsor may apply for partial payments on a monthly basis. The payments may be paid, upon application, on the basis of the costs of airport development that is accomplished or on the basis of the estimated cost of airport development expected to be accomplished.

(b) Except as otherwise provided, partial grant payments are made in amounts large enough to bring the aggregate amount of all partial payments to the estimated United States share of the project costs of the airport development accomplished under the project as of the date of the sponsor's latest application for payment. In addition, if the sponsor applies, a partial grant payment is made as an advance payment in an amount large enough to bring the aggregate amount of all partial payments to the estimated United States share of the estimated project costs of the airport development expected to be accomplished within 30 days after the date of the sponsor's application for advance payment. However, no partial payment may be made in an amount that would bring the aggregate amount of all partial payments for the project to more than 90 percent of the estimated United States share of the total estimated cost of all airport development included in the project, but not including contingency items, or 90 percent of the maximum obligation of the United States as stated in the grant agreement, whichever amount is the lower. In determining the amount of a partial grant payment, those project costs that the Administrator considers to be of questionable allowability are deducted both from the amount of proj- ect costs incurred and from the amount of the estimated total project cost.