US Airways planes occupy gates and taxi for takeoff at Sky Harbor Airport on Wednesday, July 25, 2007 in Phoenix. Another major terrorist offensive on the U.S. air transport system, on the scale of the Sept.11, 2001 attacks but next time perhaps involving shoulder-launched anti-aircraft missiles, could cost the U.S. travel economy from $214 billion to $420 billion, a new study by University of Southern California researchers finds. Credit: AP Photo/Ross D. Franklin
