Airport security procedures were shaken awake, slapped round the face and forced to undergo a rigorous transformation in the wake of the terrorist attacks of 11 September 2001. And the authorities have been playing catch-up ever since, updating and tweaking security screening measures to address subsequent attempted attacks on aircraft using shoes, liquids, printer cartridges and even underwear.
Responsibility for airport security screening in the USA was swiftly put into federal hands after 9/11, with the formation of the US Transportation Security Administration. Targets were set for all US airports to screen 100% of checked baggage for explosives; passengers were subjected to much more vigorous screening procedures - which continue to evolve; and watch lists were drawn up to prevent people suspected of having terrorist links from flying.
The latter culminated in the Secure Flight programme, which the TSA says now "conducts terrorist watch list matching of passengers on 100% of domestic and international airlines for flights within, inbound and outbound from the United States".
But despite the myriad changes that have taken place in the past decade, critics argue that airport security is still not all it could be. "For airport security pre-9/11, the technology was 1970s. Post-9/11, it is 1990s, but it is not yet in the 21st century," says Jeff Price, founder of consultancy Leading Edge Strategies and author of Practical Aviation Security. But he makes the point that "you can't protect against every single thing unless you want no freedom".
And it is this emphasis on freedom in the USA that has led to the latest round of highly-publicised controversy over airport screening procedures - the TSA's aim to replace all passenger screening units with advanced imaging technology (AIT) equipment. These machines use either backscatter X-ray or millimetre-wave technology to provide a graphic image of a passenger's body beneath clothing which can detect both metallic and non-metallic threats, including weapons and explosives.
There are almost 500 AIT units at 78 US airports, and the TSA plans to deploy 500 more in FY2011 and 275 in FY2012. Passengers who object to the machines on privacy grounds can opt for an enhanced pat-down instead, although this can be much more intrusive. "Nothing is left untouched," says Price.
He points out that some states have passed legislation to make these enhanced pat-downs illegal, but says that as "state law can't trump federal law", this is more of a publicity stunt than serious legislation.
To address privacy concerns, the TSA plans to install new software on the millimetre-wave AIT machines, which it says will "auto-detect items that could pose a potential threat using a generic outline of a person for all passengers".
The software was tested in February 2011 at three US airports, and the TSA now plans to install it on all millimetre-wave units. It will begin testing similar software for AIT units that use backscatter technology this autumn.
But some observers believe that making the machines more palatable to the American travelling public could hamper their ability to detect potential threats. "Whenever you alter an image, you lessen the ability of that technology to find what you're looking for," says Nevada-based aviation security consultant and former Northwest Airlines security director Douglas Laird. "There are enough false positives now with millimetre wave that to burden it with that software makes it even less reliable."
However, L3 - the company that manufactures ProVision AIT scanners and has developed the ProVision ATD (Automatic Target Detection) unit that uses the new software - says the ATD is "more operationally efficient" than the original machine because it "reduces the need for analysis personnel".
L3 adds: "Data is captured from a two-second scan and analysed in a computer resident within the system. Sophisticated software algorithms analyse the raw data to determine if any suspect items are present. The data remains in a numeric state and images are never generated.
"If a potential target is detected, an outline highlighting the area(s) of concern is placed on a generic mannequin that is displayed on the operator's screen. Based on local regulatory requirements, the security staff can then perform a directed search to assess the situation."
Safran-owned Morpho Detection believes body imaging scanners offer a "partial solution to identify some concealed threats", but should be used in conjunction with explosives trace detection units to provide a "layered solution". Morpho says it can offer new systems capable of detecting 'bombs in bodies', which could be available within a year "if a focused effort is applied to commercialisation and deployment through a government-industry partnership".
The decision to increase the use of AIT scanners in the USA was prompted by an incident on Christmas Day 2009 on board a Northwest Airlines flight from Amsterdam to Detroit, when Nigerian national Umar Farouk Abdulmutallab allegedly tried to detonate an explosive device that was sewn into his underwear.
Whether an AIT machine would have detected this explosive device remains the subject of debate, but the TSA finds itself caught between a rock and a hard place. When it tries to introduce security measures that might protect against evolving threats to airliners, it comes up against a civil liberties outcry.
"When the TSA does what is right, it upsets the American public," says Laird, explaining that this is because the TSA has "failed in making the case to the American public that we live in perilous times".
A number of other new security measures implemented at airports across the world since 9/11 were triggered by failed attacks. For instance, passengers are now frequently asked to remove their footwear at security checkpoints following Richard Reid's December 2001 attempt to ignite explosives hidden in his shoe on board an American Airlines flight from Paris to Miami.
And the limitation on the amount of liquid passengers are allowed to carry onto flights has its origins in a 2006 plot uncovered in the UK to blow up transatlantic flights using liquid explosives concealed in soft drinks bottles.
"We can only anticipate to the extent our imagination can carry us," says Price. "When good guys try to imagine bad things, we're not very good at it. You need to talk to [terrorists] and get the intelligence out of their heads - then maybe we'd be chasing our tails less. We're plugging holes in a dyke and there are lots of holes. Instead of plugging every hole, let's build a dam."
Another problem with gradually introducing new layers of airport security is that it leads to inconsistencies from country to country, and even from airport to airport, which can cause confusion among travellers.
Price adds: "There are a lot of inconsistencies in the world - sometimes you have to take your shoes off, sometimes you don't, sometimes you have to take your laptop out, sometimes you don't - and there are mild inconsistencies across the USA. All it's doing is making travel inconvenient."
© Rex Features
Passenger profiling is another area that has been at the centre of debate. The TSA is in the process of pilot testing a new behavioural profiling programme at Boston Logan airport, under which TSA officers are trained to "engage in conversations with passengers to verify or dispel suspicious behaviour and anomalies", says the security agency.
Price believes this is "the right way to go", but is concerned that the training may not be as thorough as it needs to be. "The real training that the Israelis pioneered is higher level and it leads to a gut feeling," he says. "But we tend to take a really good idea and try to institutionalise it. A couple of weeks' training to a lowly-educated individual is not realistic." The TSA describes the behavioural profiling pilot as "part of its ongoing risk-based, multi-layered security strategy".
The UK is also looking towards a more risk-based approach to airport security. The country's government has opened a consultation into proposals to replace its 'direct and inspect' approach with what it calls an outcome-focused, risk-based approach. The aim is for the aviation industry "to design security processes that deliver specified security outcomes rather than having to follow detailed rules", says the UK Department for Transportation.
"The new approach is about moving to a system that further prioritises inspections based on risk and incentivises the industry to collectively raise overall security performance," it adds.
This will include transferring certain aviation security functions currently carried out by the DfT to the UK Civil Aviation Authority, which the DfT says is "consistent with the 'user pays' principle, which is already applied to aviation safety and other aspects of aviation security".
But David Reynolds, flight safety officer at the British Airline Pilots' Association, is concerned about the impact this could have on the CAA's ability to remain independent. "Since 9/11, the stakeholders have been airports and airlines and this is worrying for us," he says. "It puts the independence of the regulator in question."
In terms of how aviation security is likely to evolve in the future, Price sees it as being "more of the same" tail-chasing as new measures are introduced to address new and emerging threats. "There will be a continuation of AIT and then something will happen that will change security," he says. "We need to refocus our efforts - quit X-raying people and look at better technology down the road."
Price lists three areas he believes are not being addressed sufficiently: surface-to-air missiles, airport perimeter security and threats posed by airport "employee insiders".
Laird has a more chilling view of airport security going forward. "If you're a terrorist and you want to sabotage an aircraft, there are thousands of options," he says. "It's hard to go after everything, but the TSAs of this world have to do something because the public demands it. It's all smoke and mirrors, and a well-educated terrorist knows it's all theatre."