Interim Federal Aviation Administration Authorization Act

Interim Federal Aviation Administration Authorization Act - Amends Federal aviation law to extend the Airport Improvement Program (AIP) for two months. Provides that not more than $1.3 billion of funds available under the Grants-in-Aid for Airports Program may be obligated before the enactment of a law extending contract authorization for such program beyond May 31, 1999.

Authorizes appropriations for FY 1999 for: (1) the Federal Aviation Administration (FAA) Facilities and Equipment Program; and (2) FAA operations.

Removes the maximum cap on AIP funds credited to the discretionary fund.

Terminates the Military Airport Program under the Federal Aviation Reauthorization Act of 1996.

Makes specified changes to the formula for crediting airport improvement fund amounts to the AIP discretionary fund and the small airport fund.

Authorizes the Secretary of Transportation to approve the application of the Metropolitan Washington Airports Authority (pending at the Department of Transportation on March 17, 1999) for the expenditure of up to $30 million that otherwise would have been made available to it for passenger facility fee-airport development project grants.