- Domestic flights resumed on September 12, 2001.
- International services followed on September 13.
- The remaining restrictions were removed on September 14.
The US reopened its airspace on September 13, but flying had already entered a new era. Airports could no longer treat security as a quick inspection before passengers wandered to the gate with half their extended family.
Before the attacks, passengers taking domestic flights in the US could often reach their gates without showing identification. Friends and relatives were generally allowed through to departure areas, while some travellers connecting from domestic to international flights faced no additional screening.
Those arrangements disappeared almost immediately.
Why were identity and baggage checks expanded?
In November 2001, President George W. Bush signed the Aviation and Transportation Security Act into law. It required passengers and baggage to undergo checks conducted by federal officials and created the Transportation Security Administration, better known as the TSA.
Access to departure gates became restricted largely to ticketed passengers. Identity checks grew more systematic, with travellers screened against government databases. Baggage inspection also became far more extensive, even though airports had used X-ray machines before the attacks.
The Federal Air Marshal Service expanded, placing more armed officers on selected flights. Meanwhile, cockpit doors received an overdue rethink. Before September 2001, they were often left unlocked during flights.
New rules required reinforced doors that could be locked from inside the cockpit. Bullet-resistant materials were widely introduced, although not every reinforced door is technically bulletproof. Airlines outside the US quickly adopted similar measures, with Virgin Atlantic among the first British carriers to announce reinforced cockpit doors in October 2001.
They are now standard across commercial aviation, including on older aircraft modified to meet the new requirements.
Which attempted attacks produced more rules?
The security system kept evolving as attackers found new places to hide explosives. Each attempt produced another airport ritual, usually involving passengers removing or separating something they had previously been allowed to leave alone.
Knives and box cutters were banned from carry-on baggage immediately after 9/11. Before the attacks, the Federal Aviation Administration had allowed passengers to bring knives with blades shorter than 10cm, or four inches, aboard aircraft.
In December 2001, British-born Richard Reid attempted to detonate explosives concealed in his shoe on an American Airlines flight from Paris to Miami. Passengers restrained him, and the bomb failed to explode. The incident led the TSA and some airlines to request voluntary shoe removal during screening. The TSA made it mandatory in 2006.
Another change arrived in 2003, when the first pilots were certified through the US Federal Flight Deck Officer programme. Trained and approved pilots could choose to carry handguns to help defend commercial aircraft.
Where did the 100ml liquids rule come from?
In 2006, British authorities disrupted a plot to detonate liquid explosives aboard flights travelling from London to cities in the United States and Canada. The UK government said the planned attacks could have caused more deaths than September 11.
The US initially banned liquids, gels and aerosols from carry-on luggage. It later relaxed the restriction, allowing containers of up to 100ml, or 3.4 ounces, provided they fitted inside a small transparent bag for inspection.
That limit became one of aviation security’s most recognisable rules, ensuring that a full-size shampoo bottle could create complications beyond its apparent strategic importance.
Technology has since allowed some airports to loosen the restriction. The 100ml limit remains widely used in the US, while airports in countries including the UK have begun allowing containers of up to two litres where newer scanners are operating. Implementation varies by airport, so passengers still need to check local rules before arriving with a large drink and misplaced confidence.
Why did airports introduce full-body scanners?
On Christmas Day in 2009, Umar Farouk Abdulmutallab attempted to detonate explosives hidden in his underwear during a flight from Amsterdam to Detroit. Al-Qaeda in the Arabian Peninsula, a Yemen-based group, claimed responsibility.
The device failed, but the attempt accelerated the introduction of full-body scanners. The TSA began installing machines using advanced imaging technology to detect objects concealed beneath clothing. Similar scanners later became common across Europe, Japan, the UK and other regions.
A further technological shift began in 2017, when some airlines tested facial recognition systems that allowed a passenger’s face to function as a boarding credential. Those trials developed into biometric boarding programmes now used at major international airports.
That same year, security procedures increasingly required passengers to place electronics larger than a mobile phone into separate trays for X-ray screening. Laptops and tablets joined shoes, liquids, belts and jackets in the expanding collection of belongings temporarily removed from their owners.
How much longer did airport security become?
The new checks significantly reduced the number of passengers that screening lanes could process. Research published in 2007 by Alexander G. de Barros and David D. Tomber found that throughput fell from roughly 500 to 600 passengers per hour per lane before September 2001 to 100 to 150 passengers afterwards.
Passenger numbers in North America dropped by as much as 30 percent immediately after the attacks, according to research published in 2003. Traffic eventually recovered as travellers adjusted to longer waits and stricter procedures.
By 2005, the number of passengers boarding domestic and international flights had reached 735.1 million, according to the US Bureau of Transportation Statistics. That exceeded the 665.4 million recorded in 2000.
The added scrutiny was not experienced equally. In a 2007 Pew Research Center survey of 1,000 Muslim Americans, 30 percent said airport security had singled them out for questioning or searches because they were Muslim.
Visa processing continued, but background checks became tougher. In 2009, the United States introduced the Electronic System for Travel Authorization for visitors using the Visa Waiver Program, allowing authorities to assess travellers before they boarded.
Which airport restrictions still apply?
Many of the central measures introduced after the attacks remain standard practice:
- Reinforced, internally locked cockpit doors on commercial aircraft
- Extensive X-ray screening of checked and carry-on baggage
- Passenger identity checks against government records
- Departure gate access generally limited to ticketed travellers
- Physical screening before passengers reach their gates
- Bans on knives, box cutters and other dangerous objects in hand luggage
- Armed air marshals on selected flights
Other requirements have gradually softened. Shoe removal is no longer universal at US checkpoints, although steel-toe footwear and items that interfere with scanners may still need separate X-ray screening. Most European airports also let passengers keep their shoes on unless an alarm or scanner prompts further checks.
The TSA has also announced limited arrangements allowing some people without plane tickets to pass through security and accompany relatives to a gate, provided they register through an approved PreCheck-based programme.
The result is a system that remains far stricter than anything most passengers encountered before 2001, even as newer scanners and biometric tools reduce some of the inconvenience. Security queues may move faster than they once did, but the days of casually walking to an aircraft gate without identification are not coming back.
