If an extension is not approved, most immediately affected would be FAA programs paid for with airline ticket taxes. Airlines would have to stop collecting federal ticket taxes, although they would continue to collect fees.
Air traffic controllers, deemed essential safety personnel, would stay on the job. But airport construction grants and FAA's implementation of a new air traffic control system based on GPS technology would halt.
Transportation Secretary Ray LaHood and FAA Administrator Randy Babbitt said about 4,000 of the FAA's 47,000 employees would be furloughed if an agreement isn't reached.
If enacted, the House bill would be the 21st extension. Senators introduced their own bill on Wednesday without the airline subsidies provision, but it was unclear if they would have time under Senate rules to pass it before Friday night.
Democrats say they won't let a bill pass the Senate with the subsidies provision, which is seen as a way to force them to negotiate with the House on a labor issue.