It's a terrifying scenario and a big reason for the beefed-up perimeter security at Los Angeles International Airport. Al-Qaida has already targeted an Israeli passenger plane with shoulder-fired missiles, in November 2003 in Kenya. In Baghdad, a DHL cargo plane was hit and forced to turn back.
"We think it is a real and significant threat," says Rand analyst James Chow.
But Chow says outfitting each commercial aircraft in America with a missile defense system would be cost prohibitive. According to a Rand study, it would cost $11 billion to equip 6,800 aircraft and another $2 billion each year in operating costs.
By comparison, the total transportation security tab is now $4.4 billion.
"We think the expense to deploy these countermeasures at this time would not be prudent," says Chow.
That's not to dismiss the threat. Experts say thousands of shoulder-fired missiles and launchers are missing. And they remain very concerned at the Department of Homeland Security, where they're spending $122 million on an urgent push to come up with a missile defense system for commercial aircraft.
Two companies, Northrup Grumman and BAE Systems, are working on laser defense systems, competing to present proto-types to the government in 12 months.
Regardless of the cost, some lawmakers say it would be cheaper than the economic toll of an attack.
"Do you think anyone would fly for six months?" asks Sen. Charles Schumer, D-N.Y. "The economy would go into a total tailspin."
But the airline industry, already on its knees, does not want to pay billions to retrofit aircraft.
"I agree there's a threat, and it's serious and it's one we have to pay attention to, but it's not a $40 billion-$100 billion threat," says Jim May with the Air Transport Association.
But the question remains — how much do you pay to address a threat that virtually everyone agrees is deadly serious?