A VCI distress signal was an emergency coding system installed on all commercial airliners by Day 5. Its intended use was to alert air traffic control in the event the plane was hijacked and would be used to target installations on the ground.
Jack Bauer stowed away on Diplomatic Flight 520 in pursuit of an audio recording that implicated President Charles Logan in David Palmer's assassination. When one of Logan's co-conspirators, Graem Bauer, discovered that Bauer was in possession of the recording, he decided that Logan had no choice but to order the plane shot down. Graem had broadcast a simulation of a VCI distress signal to Flight 520's transponder code (which would give Logan the pretext to shoot down the plane). The signal was picked up by Air Traffic Control, who alerted the Air Force. Rear Admiral Kirkland — as per Logan's approval — scrambled an F-18 Hornet out of Point Mugu with orders to shoot down Flight 520. Bauer, however, forced the plane's co-pilot, Scott Evans, to make an emergency landing on the 118 Freeway. With the plane no longer deemed a threat, the F-18 was recalled. ("Day 5: 3:00am-4:00am")