RICHMOND, JAN. 29 -- A Virginia Senate committee approved a bill today that would double the fines airlines pay for flying noisy aircraft into National Airport at prohibited times, rejecting a much tougher version that would have allowed penalties up to $50,000.

The legislation, sponsored by Sen. Clive L. DuVal 2d (D-McLean) and strongly backed by the Metropolitan Washington Airports Authority, would raise the fines for violating noise regulations from the current $2,500 per incident to $5,000. The fines can be levied on loud aircraft that land at or take off from National between 10 p.m. and 7 a.m., when a noise curfew is in effect.

The bill, approved 8 to 6 by the Senate Committee on Local Government, will be voted on by the full Senate this week, and if approved, will be sent to the House for consideration.

Meanwhile, in an unrelated matter, a special Senate Finance subcommittee huddled in closed session with three Cabinet secretaries to shape an offer designed to persuade United Air Lines to build a huge maintenance facility at Dulles International Airport, which could create about 16,000 jobs in the next half-dozen years.

DuVal, chairman of the subcommittee, said it was necessary to bar the public from the session because "we can't expose our hands. We are in competition with other states" for the facility.

The assembly is weighing a package of bills designed to lure United. One of the incentives being considered would allow the Loudoun County Board of Supervisors to give the airline facility a special, lower property tax rate. Another would create a special transportation tax district, similar to that approved to finance Route 28 improvements, to allow the issuing of $30 million in bonds to widen Route 606 to four lanes at the facility site.

DuVal said the final package must be "enough to attract United, but below the anticipated tax benefit," which, according to documents given to the subcommittee, would be $21 million a year in income and sales tax for the state and about $6 million a year in real and personal property taxes for Loudoun County.

Gov. L. Douglas Wilder is expected to inform United about details of the proposal in the next few days. The offer must be made public Sunday, when the Senate and House unveil their final budget plans.

On the measure to increase noise fines at National, DuVal said, "We'll have to fight it in the House side. Maybe we can toughen it up there. The fine is a lot lower than the airports authority wanted, but it's better than being defeated."

The airports authority aggressively lobbied for a $50,000 maximum fine, sparking an angry backlash from airline representatives. They said the penalty would be excessive at a time when their industry is suffering layoffs, flight cancellations, bankruptcies and even the closing of major carriers.

They also noted that the number of noise violations at National declined from 70 in 1989 to about 35 last year. They said there have been no violations so far this year.

Nonetheless, in a subcommittee meeting on the bill yesterday, Edward S. Faggen, an attorney for the airports authority, said that the current $2,500 fine is "not a meaningful deterrent" because the fine is so small that airlines would rather pay it than comply with the regulations.

"The noise rule at National is the toughest in the nation, bar none," said Roger Cohen, of the Air Transport Association of America. "The existing fines are the toughest in the nation, bar none."

The noise ordinance at National is tough, Faggen said, because "it was designed with the community in mind so we could keep the airport open 24 hours a day for airlines that comply."

Efforts at compromise failed, and Sen. Charles J. Colgan (D-Prince William) finally suggested doubling the existing fine.

Colgan said he backed off from the tougher proposals because six committee members believed that even the $5,000 fine was too hefty.