As the nation haltingly takes to the skies following last week's terrorist attacks, it will witness the most dramatic security overhaul in aviation history. Only ticketed passengers are allowed past security checkpoints. All knives have been banned. Screeners search carry-ons more thoroughly. Armed agents are stationed in terminals and officials vowed Monday they would soon be on flights.

　　Overall, airport security stands at its highest level since the Persian Gulf War in 1991. Airlines are asking passengers to arrive as many as 4 hours before flights to check in.