WASHINGTON (AP) -- The Senate is scrambling to pass a $105 billion bill designed to improve air safety and improve customer service for air travelers before the law governing the Federal Aviation Administration expires at midnight on Friday.

If senators can't resolve a series of disputes over the measure by then, around 3,600 FAA employees could be furloughed, potentially exacerbating existing staffing shortages.

The FAA says no one in "safety critical" positions -- like air traffic controllers -- would be affected if the deadline is missed, and the safety of the flying public would not be at risk. But failure to pass the popular bipartisan bill by May 10 would be the latest setback after months of delays on the measure, and another example of Congress struggling to pass major legislation, even when it has broad support.

The bill stalled in the Senate this week after senators from Virginia and Maryland objected to a provision that would allow an additional 10 flights a day to and from the heavily trafficked Reagan Washington National Airport. Other senators have tried to add unrelated provisions, seeing it as a prime chance to enact their legislative priorities.

Senators from both parties were working to clear objections Thursday and move the FAA bill, or a short extension, before the Friday deadline.

The legislation was negotiated by Republicans and Democrats who lead the House and Senate committees overseeing the FAA. The agency has been under scrutiny since it approved Boeing jets that were involved in two deadly crashes in 2018 and 2019. The legislation would govern FAA operations for the next five years.

The House passed a similar bipartisan bill last summer. Senators introduced their revised 1,069-page measure last week, with days to go before the deadline.

The legislation, which also comes after a a series of close calls between planes at the nation's airports, would increase the number of air traffic controllers and require the FAA to use new technology designed to prevent collisions between planes on runways. It would require new airline planes to have cockpit voice recorders capable of saving 25 hours of audio, up from the current two hours, to help investigators.

It would also try to improve customer service for flyers by requiring airlines to pay a refund to customers for flight delays -- at three hours for a domestic flight and six for an international one. Lawmakers tweaked the bill this week to make it even easier for customers to receive refunds, revising language that would have put most of the onus on the customer to request them. The change put the Senate bill more in line with new regulations issued by President Joe Biden's administration last week.

In addition, the bill would prohibit airlines from charging extra for families to sit together and triple maximum fines for airlines that violate consumer laws. And it would require the Transportation Department to create a "dashboard" so consumers can compare seat sizes on different airlines.

Opening the Senate on Thursday, Senate Majority Leader Chuck Schumer urged senators to come to agreement soon. "Absolutely nobody should want us to slip past the deadline because that would needlessly increase risks for so many travelers and so many federal workers," he said.

Virginia Sens. Tim Kaine and Mark Warner, both Democrats, have said they would object to a short-term extension of the legislation passed by the House on Wednesday unless Schumer agrees to a vote on their amendment to block the additional long-haul flights at Virginia's Reagan National. They say the airport is restricted in size and too busy already, pointing to a close call there between two planes earlier in April that they said is a "flashing red warning light."

Several Western lawmakers have argued for more flights at Reagan National, saying it is unfair to consumers that there is a restriction on long-haul flights there. The provision's chief proponent is Texas Sen. Ted Cruz, the top Republican on the Senate Commerce Committee, who has argued that San Antonio should have a direct flight from the airport.

Airlines are also split on the idea. Delta Airlines has argued for more flights at Reagan National, while United Airlines, with a major operation at farther-out Dulles Airport, has lobbied against the increase.

The House last year rejected a similar provision after intense, last-minute lobbying from the Virginia delegation -- a bipartisan vote on an amendment to the FAA bill that saw members aligning not by party but geographic location. Lawmakers use the airport frequently as it's the closest Washington airport to the Capitol, and Congress has long tried to have a say in which routes have service there.

"We understand that some of our colleagues have to travel a long way to get to D.C.," Kaine and Warner said in a joint statement with Maryland's two senators, Democrats Ben Cardin and Chris Van Hollen. "But the convenience of a handful of members is not as important as the safety of the 25 million people who use DCA every year. Last month's near-miss at this airport is a startling reminder of what's at stake if Congress jams even more flights onto the busiest runway in America."

The FAA says that if the law expires on Friday, the 3,600 employees would be furloughed without guarantee of back pay starting at midnight. The FAA would also be unable to collect daily airport fees that help pay for operations, and ongoing airport improvements would come to a halt.