Tomorrow marks the 50th anniversary of Operation Skyshield, one of the largest air drills in U.S. history. Operation Skyshield, which took place on Sept. 10, 1960, tested the ability of the U.S. to detect and defend against an assault by air. During the joint U.S.-Canadian military exercise, all civilian aircraft in North America were grounded. The exercise was repeated twice more–on Oct. 14 and 15, 1961, and a third time on Sept. 2, 1962. As part of Skyshield, the North American Air Defense Command (Norad) and the FAA notified the airlines to prepare and coordinated with all airports in the U.S. and Canada to shut down operations, grounding all airliners and general aviation aircraft. The results of the national test were, and still are, classified, though the U.S. and Canadian governments assured citizens that its defenses were “99 percent effective.” U.S. airspace was shut down during real air attacks nine years ago this Saturday, when terrorists flew hijacked airliners into the World Trade Center towers in New York and the Pentagon in Washington, D.C. A fourth hijacked airliner crashed in a central Pennsylvania field, though its intended target was believed to be either the U.S. Capitol or White House.