In an airplane hangar north of Fort Worth, technicians are preparing to mount a fire-hydrant-shaped device onto the belly of an American Airlines Boeing 767. It is an effort that could soon turn into a more than $10 billion project to install a high-tech missile defense system on the nation's commercial planes.
The Boeing 767 -- the same type of plane that terrorists flew into the World Trade Center -- is one of three planes that, by the end of this year, will be used to test the infrared laser-based systems designed to find and disable shoulder-fired missiles. The missiles have long been popular among terrorists and rebel groups in war zones around the world; the concern now is that they could become a domestic threat.
The tests are being financed by the Department of Homeland Security, which has been directed by Congress to move rapidly to take technology designed for military aircraft and adapt it so it can protect the nation's 6,800 commercial jets. It has so far invested $120 million in the testing effort, which is expected to last through next year.
Yet even before the tests begin, some members of Congress, and several prominent aviation and terrorism experts, are questioning whether the rush to deploy this expensive new antiterrorism system makes sense.
Homeland Security officials have repeatedly cautioned that no credible evidence exists of a planned missile attack in the United States. But there is near unanimity among national security experts and lawmakers that because of the relatively low price and small size of the missiles, as well as the large number available on the black market, they represent a legitimate domestic threat.
The concern is not just for the lives that would be lost in the shoot-down of a single plane, proponents say. It is for the enormous economic consequences that would result if the public were to lose confidence in flying.
''We are long overdue for a passenger aircraft to be taken down by a shoulder-launched missile,'' said Representative John L. Mica, Republican of Florida, who is pushing for the systems to be installed. ''We have been extremely, extremely lucky.''
But a significant contingent of domestic security experts say the administration's focus on these missiles may be misdirected. They cite the broad range of ways that terrorists might strike next and point to studies showing that shoulder-fired missiles -- the most popular of which are American-made Stingers and Soviet-made SA-7's -- present less of a threat at airports than do truck bombs or luggage bombs.
''People have probably assumed that these kinds of weapons would work with much greater certainty,'' said K. Jack Riley, the director of the public safety and justice program at the Rand Corporation, a nonprofit research organization that has studied threats from shoulder-mounted missiles. ''This is not as big a threat as people might think.''
Northrop Grumman and BAE Systems are competing to build the devices, which rely on plane-mounted sensors that detect heat-seeking missiles and then automatically fire infrared lasers to jam or confuse the missiles' guidance systems. The defense would be used for about a 50-mile area around airports, while planes land or take off.
The American Airlines Boeing 767 and two jets owned by Northwest Airlines and FedEx will be tested to determine whether they remain as airworthy with the new technology aboard and to figure out if, in simulated attacks, the defense system is reliable. For now, no passengers will be aboard.
Shoulder-fired missiles were introduced by the Americans and the Soviets in the 1960's to protect ground forces. A recent Congressional study found that more than 350,000 existed in government arsenals worldwide. But they also are a favorite of rebel groups and terrorists. At about six feet long and 50 pounds, they are easy to transport, and older models can cost only a few hundred dollars.
Calls for putting the defense systems on commercial planes took on new urgency in 2002. That year, two missiles were fired at a Boeing 757 in Kenya that had been chartered by an Israeli airline. Both missed.
And in November 2004, an Airbus A300 cargo plane flown by DHL was struck by a missile on takeoff from the Baghdad airport. The plane lost hydraulic power but was able to land.
''This is one of the greatest dangers we face in the air,'' said Senator Charles E. Schumer, Democrat of New York, who has helped lead the push in the Senate for the deployment of missile defense systems.
Northrop Grumman and BAE Systems have briefed members of Congress, urging them to invest in the systems, and Northrop has commissioned a poll in an effort to demonstrate public support for the program. One Northrop briefing featured photographs of men in long, loose robes taking a missile launcher out of a car and firing a round into the air.
As the tests proceed, Homeland Security officials are looking for ways to lower the price tag. It has been estimated that it could cost $10 billion to install the systems on all commercial jets and as much as $40 billion over the next two decades, once maintenance and operational costs are added in. By comparison, the entire budget proposed for aviation security in the United States is $4.7 billion for the coming fiscal year.
Homeland Security has asked BAE and Northrop Grumman to design systems that would cost no more than $1 million per plane, rather than the roughly $1.6 million some industry experts had expected. And the devices must work without need of repair for 3,000 hours, instead of the 300 hours required for military jets, according to specifications set by Homeland Security.
But even if the contractors can lower the costs, airline industry representatives and some terrorism experts say the price is hard to defend.
The Air Line Pilots Association, Boeing and the Air Transport Association of America are urging that more emphasis be placed on alternative defenses, like controlling areas around airports, limiting the international supply of missiles and making less expensive changes that would allow an airplane to fly even if its hydraulic system was lost.
''The cost versus the benefit here does not play out,'' said Jim Proulx, a spokesman for Boeing. The presumption is that the government would pay to install the systems on existing planes, but the airlines would assume the ongoing maintenance and fuel costs, which could exceed $1 million a year per plane.
In a recent study by Rand, which examined security threats at Los Angeles International Airport, a shoulder-mounted missile was characterized as a ''lesser threat'' in terms of potential deaths than a truck bomb or a luggage bomb. In fact, the study suggested that the threat posed by a shoulder-fired missile was not much greater than that of a sniper who might fire a .50-caliber rifle at a plane from outside the airport.
A separate study, financed by Homeland Security in 2004, said that the infrared systems would be useless or only marginally effective against several types of shoulder-mounted missiles.
Part of the reason for the relatively low ranking of missiles among threats is that large passenger airliners are designed to fly after the loss of an engine, even if that engine explodes, industry experts said.
Even before the current tests are complete, the Bush administration and Congress are moving to set aside $110 million for the next phase. Mr. Schumer and Mr. Mica also want to require that the devices be installed immediately on certain new commercial jets.
And others, including Representative Steve Israel, Democrat of New York, and Senator Barbara Boxer, Democrat of California, want Congress to set aside $10 billion to install the systems on existing planes, perhaps first on the 1,000 aircraft that make up the Civil Reserve Air Fleet, which transports troops in times of war. The cost could be great, they said, but most likely smaller than the damage to the economy that would result from a single shoot-down.
''We have been warned over and over again by the people who know,'' Ms. Boxer said, referring to classified briefings by the F.B.I. and the C.I.A. ''It is a race against time.''
Yet Representative Christopher Cox, a California Republican who is chairman of the Homeland Security Committee, said the missile defense program was being driven too much by politics and lobbyists.
''This is not the result of considered analysis of potential threats, terrorist capabilities or intentions,'' Mr. Cox said. ''It should be.''