Recent events such as the 2007 vehicle attack at Glasgow Airport and the plot to detonate peroxide (TATP) devices on aircraft departing the UK have highlighted the need to incorporate terrorist attack mitigation into the design of civil aviation facilities, both in the UK and abroad.
Indeed, many experts now believe that security should be considered one of the central drivers in airport terminal design and not an afterthought or a necessary evil.
In the short-term these measures are highly visible, in the form of concrete block anti-ram vehicle barriers at terminal forecourts and long queues at departures security search. However, a number of additional measures can – and are being – incorporated into the design of new terminals in the UK that provide a high degree of protection whilst not compromising functionality or degrading the passenger experience.
The UK is possibly a world leader in this area due mainly to the former threat of the Irish Republican Army (IRA), which between 1970 and 2001 used weapons such as car bombs and other explosive devices in a long running terror campaign against the British Government. Its attacks included firing mortar bombs on to a runway at Heathrow in 1994, which luckily failed explode.
As a result of the terrorist threat, since 1996 the UK’s Department for Transport has issued guidance to the operators and designers of airports in the document Aviation Security in Airport Development (ASIAD).
This covers security screening of passengers and aircraft hold baggage, and design measures to be applied to airport terminal buildings. Guidance includes the location of car parking facilities, glazed terminal facades and terminal structures.
ASIAD has recently been revised and re-issued and now covers forecourt design to defeat hostile vehicle attacks. This has been applied at Heathrow Terminal 5 and measures are also being incorporated into designs at Gatwick Airport and Heathrow East Terminal. ASIAD requires a 30-metre zone around the landside terminal forecourt into which no unauthorised vehicle may enter. This is delineated by structures such as bollards, planters or other landscaping elements to stop unauthorised entry into the area, or attack, by vehicles at speed.
Entry and egress from this zone for emergency vehicles is facilitated by moveable barriers rated to the same performance as the fixed barriers. These barriers are located in pairs to create ‘airlocks’ to maintain the integrity of the secure zone.
Airlocks are required at the entry and egress points of goods delivery routes to the terminal. Typically these run beneath the building to loading docks below the concourse. The consequences of a detonation inside a building are much worse than with an external blast, as the confining effects of the building make the damage much more severe.
Ideally the need for large vehicles to enter the terminal, and hence the risk, should be eliminated in the terminal design, perhaps by the use of a ‘logistics’ building away from the terminal to which external deliveries are made and shipped to the terminal by controlled and secure vehicles.
The barriers and secure zone ensure that a vehicle bomb cannot penetrate the terminal, however, a detonation at 30 metre stand-off range will still cause a blast wave to impact the terminal.
The intensity of the blast is a function of the size and type of explosives and the stand-off range of a van bomb detonation, for example, at seven metre range will typically yield a pressure of one ton per square foot, whilst for the same detonation at 30 metre range, the pressure reduces to one-tenth of a ton per square foot.
The primary aim of counter-terrorist design is thus to maximise the stand-off between the bomb and the target. In the UK a design stand-off of 30 metres is applied for car bomb devices, as at this range frame buildings designed to British Standards and UK Building Regulations are generally capable of withstanding the blast without collapsing.
The terminal building
Contemporary terminals often have extensive glazed facades to provide an open and attractive passenger environment. This could be a source of deadly fragments in a blast event, and so considerable research and testing has been undertaken over the last 30 years into the behaviour of glazing under blast. Out of this, glazing blast assessment tools and computer models have been developed, as well as blast-enhanced glazing systems.
These facade systems all use laminated glass in deeply rebated frames to ensure that, although the glass panes shatter under blast loading, the fragments remain attached to the plastic interlayer rather than being projected into the terminal where they could injure or kill.
The laminated panes are silicone bonded or rigidly clamped into the supporting frames to ensure that they do not get blown in en masse and to enable the plastic interlayer to stretch and absorb the blast energy. This membrane effect reduces the intensity of the load applied back into the terminal structure, which enables attractive structures to be designed.
Doors in the terminal facade are enhanced to the same standard as the facade itself and, in the UK, research and testing has produced commercially available blast-enhanced doors for airport use, including sliding doors.
The terminal structure and facade may be enhanced to safely withstand an external vehicle bomb blast but such measures do not prevent a package or person-borne device from entering. They may be intercepted at the landside/airside security check, but that still leaves the landside concourse vulnerable to attack.
To prevent this, a security search facility would have to be created outside the terminal. Such facilities have been provided to buildings with low people flows but would most likely disrupt passenger flow if applied at an airport.
Instead, the risk of a landside package or person-borne explosive detonation is accepted by ASIAD and measures applied to limit the effects and injuries from such an attack. These include:
• Limiting the extent of glazed balustrades and screens and designing those remaining with laminated glass to limit the creation of hazardous fragments
• Providing securing restraints to large suspended signs and high level suspended ceiling panels to prevent them becoming detached and falling onto people
• Avoiding glazed screens at check-in desks
• Requiring retail units and ticket/information booths to comply with blast mitigation design criteria
• Shielding off high-density public areas, such as check-in zones, from each other to limit the extent of blast effects.
Parking and public transport
Parking structures are clearly a potential vehicle bomb location and so ASIAD requires that they be located at least 30 metres from the terminal. Customer interface areas within them are designed so that any glazed elements are blast-enhanced, albeit to a notional level.
Because the consequences of an explosion in a confined space, such as an underground car park, are greater than those of the same explosion in the open air, structural damage and human injury will be much more severe and the impact on the operation of the airport will be greater. The answer is to avoid the need for underground parking (eliminate the risk, for instance) or to provide effective security access and egress control to these spaces, including the use of anti-ram barriers.
Public transport interchanges such as bus and railway stations should be designed with similar enhancement measures to those applied to the terminal, which they serve.
In order to protect public areas and buildings against attack, security issues must be at the forefront of any decisions made when constructing new airports or terminals. The provision of counter-terrorist measures to civil aviation is by necessity one of compromise and pragmatism.
To provide complete protection against terrorist attack would inevitably result in a non-functioning terminal and so a reasonable level of protection is provided commensurate with the perceived risk to aviation and the public at large.
The design measures described here should be seen as part of a larger counter-terrorist strategy that includes the work of the airport operators, police, security services and other agencies.