After four hijacked commercial aircraft were used in coordinated attacks on New York and Washington, DC, on September 11, 2001, air travel changed almost overnight.
Two of the planes, controlled by al-Qaeda hijackers, were flown into the Twin Towers at the World Trade Center in Lower Manhattan. The buildings collapsed, killing nearly 3,000 people.
A third aircraft struck the Pentagon, while a fourth, believed to be bound for the White House, crashed in a field in Pennsylvania after passengers and crew fought back against the hijackers.
The attacks remain the deadliest terrorist incident in US history. All four aircraft had been boarded and seized with relatively little resistance.
The response reshaped aviation security, airport procedures and the experience of millions of passengers around the world.
Immediately after the attacks, all air traffic across the US was grounded. Canada closed its airspace, although military, law-enforcement and humanitarian flights were permitted to operate, according to a government document.
Airports elsewhere were also closed and flights to the US were cancelled, leaving many travelers stranded for days.
Canada reopened its airspace gradually. Domestic flights resumed on September 12, international services followed on September 13, and the remaining restrictions were lifted on September 14.
The US reopened its airspace on September 13.
Which new travel rules were introduced after 9/11?
ID and baggage checks
Before September 2001, passengers flying domestically within the US could proceed directly to their gates without presenting identification. At most American airports, friends and family were also allowed to accompany them through security and see them off.
In some cases, travelers transferring from a domestic flight to an international departure did not undergo additional screening.
Those practices ended after September 11. In November 2001, then-US President George W. Bush signed the Aviation and Transportation Security Act into law. It required federal screening of every passenger and piece of checked baggage and led to the creation of the Transportation Security Administration, or TSA.
Armed marshals and reinforced cockpits
The Federal Air Marshal Service was expanded, placing more armed officers on commercial flights. Cockpit doors, which had sometimes remained unlocked during flights, were required to be reinforced, fitted with bullet-resistant materials and kept locked from the inside.
Airlines and aviation authorities worldwide quickly adopted the new cockpit standard. Virgin Atlantic was among the first carriers to announce the change in October 2001.
Reinforced cockpit doors are now standard on commercial aircraft, although not every door is specifically designed to be bulletproof.
Knives and sharp objects
Knives and box cutters were prohibited in carry-on luggage immediately after the attacks. Before September 11, the Federal Aviation Administration had permitted passengers to bring knives shorter than 10cm (4in) aboard US aircraft.
Shoe checks
In December 2001, British-born Richard Reid attempted to detonate explosives concealed in his footwear on an American Airlines flight traveling from Paris to Miami.
Other passengers restrained him before the device could be triggered. The failed attack prompted the TSA and several international airlines to begin asking passengers to remove their shoes voluntarily for screening.
Shoe checks became mandatory in the US in 2006.
Pilots carrying handguns
In 2003, the first pilots qualified under the US Federal Flight Deck Officer program were authorized to carry handguns aboard commercial aircraft after completing specialized defensive training.
Limits on liquids
In 2006, UK authorities uncovered and disrupted a plot to explode liquid explosives on aircraft traveling from London to cities in the US and Canada.
According to the UK government at the time, the potential death toll from the attacks could have surpassed that of September 11.
The US initially banned all liquids, gels and aerosols from carry-on baggage. In September 2007, it relaxed the restriction, permitting containers of no more than 100ml (3.4oz), provided they fit inside a small, transparent bag for inspection.
Full-body scanners
In 2009, Umar Farouk Abdulmutallab attempted to detonate a bomb concealed in his underwear on a flight from Amsterdam to Detroit. Al-Qaeda in the Arabian Peninsula, based in Yemen, claimed responsibility for the attack.
Although the attempt failed, the TSA began installing full-body scanners using advanced imaging technology to detect items hidden beneath clothing.
These scanners are now used at airports across Europe, Japan, the UK and many other countries.
Facial recognition and electronics screening
In 2017, some airlines began testing facial-recognition systems that would allow a passenger’s face to function as a boarding credential. The trials eventually became part of biometric boarding systems used at major airports internationally.
That same year, rules required travelers to place electronics larger than mobile phones in security trays for X-ray screening.
How did the changes affect air passengers?
More extensive baggage checks and security screening mean travelers generally spend more time at the airport, lengthening the overall journey.
A 2007 research paper by Alexander G. de Barros and David D. Tomber found that, a few years after the new procedures were introduced, passenger screening rates had fallen from approximately 500 to 600 people per hour per lane before September 2001 to 100 to 150 people afterward.
Passenger numbers in North America also dropped by as much as 30 percent immediately after the attacks, according to a 2003 research article on ScienceDirect, a platform for peer-reviewed publications.
Demand gradually recovered as travelers adapted to the new procedures. By 2005, US domestic and international boardings had reached 735.1 million, surpassing the 665.4 million recorded in 2000, according to the US Bureau of Transportation Statistics.
Expanded screening also meant that more passengers faced additional questioning and searches.
In 2007, the US-based Pew Research Center found that 30 percent of 1,000 people interviewed had been singled out by airport security because they were Muslim.
Visa screening also became far stricter while applications were being processed.
In 2009, the Electronic System for Travel Authorization, or ESTA, was introduced for travelers from countries participating in the US Visa Waiver Program. It enabled authorities to conduct background checks before passengers boarded flights.
Which restrictions have been relaxed, and which remain?
Some post-9/11 measures have been loosened, but several remain central to aviation security.
Rules that are still in place include:
- Reinforced cockpit doors, many of which are bullet-resistant, remain standard on commercial aircraft worldwide. Older planes have also been retrofitted.
- X-ray screening of checked baggage continues, with inspections far more extensive than before 9/11.
- Passengers must pass identity checks against government databases.
- Access to boarding gates is generally restricted to passengers.
- Travelers must pass through a physical security checkpoint, often before reaching the departure gate and, in some airports, again at the gate itself.
- Dangerous items, including knives, remain prohibited in carry-on luggage.
Other restrictions have been relaxed:
- The 100ml liquids limit remains in the US. In the UK, passengers may carry up to two litres in total, subject to individual container limits.
- Shoe-removal requirements have been eased in some places. Steel-toe boots may still need to be removed because they can interfere with scanners and must be X-rayed separately. Most European airports allow passengers to keep their shoes on unless the screening equipment raises an alarm.
- The TSA has permitted some people without plane tickets to use TSA PreCheck security lanes and proceed to departure gates when they are registered and otherwise eligible to escort relatives.