The machines are built by L-3 Communications of New York City, which received a federal contract in April to provide up to 500 more machines.

"It breaks down and you want it to keep going," said Susan Hallowell, who directs the Transportation Security Administration's research into new ways to stop bombs and guns from getting on airplanes.

The company says its machines have been vastly improved and that any problems occurred in older models.

The government plans to meet a Dec. 31 deadline for inspecting airline passengers' checked baggage for explosives by using 1,100 of the minivan-sized explosive detection equipment built by both L-3 and InVision Technologies of Newark, Calif., as well as 4,700 smaller machines that detect traces of explosives.

The Transportation Security Administration on Friday awarded a $508 million contract to a division of the Boeing Co. to install explosive detection machines at airports.

No problems have been reported with InVision's machines, officials said.

The company will buy the machines from both suppliers.

Congress has required the government to buy the L-3 machines, which are built in the congressional district of House Appropriations Committee chairman C.W. (Bill) Young, R-Fla. A spokesman for Young, Harry Glenn, said the committee staff and not the lawmaker pushed for the provision.

LAA has spent $938,000 over the last two years to hire lobbyists such as former Deputy Federal Aviation Administrator Linda Daschle, wife of Senate Majority Leader Tom Daschle, D-S.D., records show.

The provision, part of the 2000 spending bill, required the government to buy an equal number of explosive detection machines from both companies. "We wanted to ensure there was some competition," said Rep. John Sweeney, R-N.Y., a member of the House Appropriations transportation subcommittee.

Hallowell and Facility Manager Ronald Poullo said the earlier L-3 machines had been unreliable but the company made repairs. To see if the problems have been fixed, newer equipment is undergoing tests at the Transportation Department's William J. Hughes Technical Center, at the Atlantic City, N.J., International Airport, Poullo said.

No problems have been reported with InVision's machines, officials said.

Both the National Research Council and the Transportation Department inspector general have found problems with L-3's equipment.

Inspector General Kenneth Mead reported last October that a machine at Dallas-Fort Worth International Airport broke down an average of once every four days and took an average of six hours to fix each time. The airport no longer has the machine.

The National Research Council said in April, "The machines themselves are considered too unreliable to deploy in airports, where downtime for unplanned maintenance can wreak havoc with flight schedules."

Paul Hudson, a member of a Federal Aviation Administration advisory committee on airline security, said he has also heard of problems with L-3. "They're not really proven the way the InVision (machines) are," he said.

Still, the chairman of the National Research Council's committee reviewing aviation security technologies, Thomas Hartwick, said he expected the L-3 machines to become more reliable.

"It's like a new automobile the first year out," Hartwick said. "They'll get corrected in time."

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On the Net:

L-3 Communications: http://www.l-3com.com

Transportation Department: http://www.dot.gov