However, the report by the RAND research corporation said the installation of such systems on the United States' some 6,800 airliners could be justified at a later date if the price of such technology declines.

"Installing such systems on the nation's fleet of commercial airliners would cost an estimated 11 billion dollars, with operating costs ramping up to 2.1 billion annually upon full operational capability," the report said.

"Over 20 years, the cost to develop, procure and operate these systems would amount to an estimated 40 billion," it said.

The US airline industry has been struggling to survive since the September 11 attacks on New York and Washington. United Airlines and US Airways are vying to emerge from bankruptcy.

The availability of shoulder-fired ground-to-air missile launchers on the black market has fed concerns here in national security circles that attackers might use such weapons in an attempt to down a commercial jet.

In November 2002, assailants attempted to hit an Israeli airliner in Mombasa, Kenya in a failed missile strike.

RAND researchers found there are still too many unresolved questions about how defense systems would operate on commercial airlines, including issues such as the number of false alarms that may occur and whether attackers could find ways to sidestep such safeguards.

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