The House aviation subcommittee on Thursday approved a bill designed to counter the threat of shoulder-fired anti-aircraft missiles (search) to U.S. airliners and airports by speeding the development of aircraft defense systems, among other measures.

The subcommittee's chairman, Florida Republican John Mica, noted that Israel is installing anti-missile devices on its commercial airliners. The United States, though, faces a bigger challenge because it has far more aircraft than El Al Israel Airlines (search), he said.

"The cost and complication associated with installing these systems on commercial aircraft could be staggering and also time-consuming," Mica said. "The United States must also move forward with other domestic and international efforts."

The bill would require the Federal Aviation Administration (search) to expedite approval of missile defense systems. It also encourages President Bush to continue efforts to reduce the number of weapons around the world. It also requires the Homeland Security Department to report to Congress within a year on its assessment of airports' vulnerability to attacks from shoulder-fired missiles.

Last week, Israel said El Al Israel Airlines would begin testing a system to guard its 30 aircraft against the shoulder-fired, lethal missile launchers. In 2002, Al Qaeda (search) claimed responsibility for an attempt on an Israeli chartered jet with anti-aircraft missiles in Mombasa, Kenya.

Singapore in February said it would install anti-missile defenses on its national carrier within two years. Regional militants linked to Al Qaeda are believed to have tried to target Western sites in Singapore.

"Other nations are going faster with their studies," said James Shilling, a full-time pilot for a major cargo carrier and spokesman for the Coalition of Airline Pilots Association (search). "Their systems are very much on a par with our systems. Let's not study and look and ponder so six years down the road somebody shoots something."

In the United States, security experts say an attack by an anti-aircraft missile is statistically improbable, potentially catastrophic and very expensive to prevent.

Homeland Security officials believe the biggest threat lies outside the United States.

"We remain most concerned about this threat overseas," said Homeland Security spokesman Brian Roehrkasse. "We do not have threat information that indicates Al Qaeda is planning an attack at a specific location in the United States."

Brian Jenkins, founder of the Rand Corp.'s (search) terrorism research center, said the threat is real but remote.

"If it were to occur tomorrow, we would be shocked, but we could honestly argue we wouldn't be surprised," Jenkins said.

If terrorists downed an airliner with a shoulder-fired missile, Jenkins said, the cost to the airline industry would be tens, if not hundreds, of billions of dollars.

But the cost of prevention is also high. Air Transport Association (search) President James May said on Wednesday that the cost of outfitting the commercial aviation fleet — about 6,800 planes — with missile defenses would be between $50 billion and $100 billion over 20 years.

He urged the government to go slow on developing the countermeasures because they haven't been proven effective.

"The whole system is driven to one goal: drive the civilian fleet to a technology that doesn't work," May said.

The Homeland Security Department selected three companies in January to determine whether technology used by the military could be adapted for civilian aircraft to counter the terrorist threat. The military's defenses include infrared jammers that redirect heat-seeking rockets away from aircraft engines.

After six months, one of the three contractors will be chosen to conduct more rigorous testing for up to 18 months.