In the 25 years since hijackers took control of four commercial aircraft and turned them into weapons, the air travel process has been overhauled in ways that few other industries have.

From federalizing airport security to hardening cockpits and vetting passengers to screening baggage, the process in no way resembles the one that existed at the turn of the century. It has evolved repeatedly over the past two and a half decades to meet and counter emerging threats as technology improves and new types of aviation-aligned attacks are thwarted.