Airport security is not a designed system. It is a sediment of responses, each layer added after a specific event. Reading it in order makes the queue considerably more comprehensible.
1973: universal screening
Before this, nobody was searched. The hijacking wave of 1968–1972 — over a hundred incidents in the United States, at times more than one a week — ended with the FAA mandating screening of all passengers and carry-on baggage. Metal detectors and X-ray machines appeared at every American airport, and the practice spread globally. Hijackings fell immediately.
1988: hold baggage and explosives
Pan Am 103 was destroyed over Lockerbie by a bomb in checked baggage, killing 270. The response was baggage reconciliation — the rule that a checked bag may not fly unless its passenger is aboard — and the deployment of explosive detection systems for hold baggage. Passenger–baggage matching is why a delayed passenger causes a bag to be pulled from the hold.
1994: liquids and assembled devices
The Bojinka plot demonstrated a liquid explosive carried aboard as toiletries and assembled in flight. Detection research began, though visible restrictions came later.
2001: access to the flight deck
September 2001 shifted the objective. Screening had been designed to stop weapons; now the target was preventing anyone reaching the controls. Reinforced cockpit doors became mandatory, air marshal programmes expanded, and in the United States screening was federalised under a new agency. The list of prohibited items expanded significantly.
2001: shoes
In December 2001 Richard Reid attempted to detonate explosives concealed in his shoes aboard a transatlantic flight. Shoe removal at screening dates directly from this single incident.
2006: liquids
A plot to destroy multiple transatlantic aircraft using liquid explosives carried as soft drinks was disrupted in the UK. The response was the 100 ml rule and the transparent bag — the most visible and most complained-about security measure in the world, and the most traceable to a single event.
2009: body scanners
Umar Farouk Abdulmutallab attempted to detonate explosives concealed in his underwear aboard a flight approaching Detroit. Metal detectors cannot find non-metallic explosives on a body. Millimetre wave and backscatter body scanners were deployed rapidly afterwards.
2010: air cargo
Explosive devices concealed in printer cartridges were intercepted in air cargo shipments bound for the United States. Cargo screening requirements were substantially tightened, an area that had received far less attention than passenger screening.
What the pattern shows
Aviation security is reactive by construction. Each measure closes a method that has already been used, which is why the queue looks arbitrary until you know the order.
The more interesting development is the recent move in the opposite direction: computed tomography scanners now entering service can resolve a bag in three dimensions well enough that liquids and laptops no longer need to be removed. Airports fitted with them are quietly rolling back two of the most disliked rules — not because the threat changed, but because the detection finally caught up.