Cargo Screening: technological options

Although bomb attacks against commercial airliners date back to the early days of aviation, air cargo security in itself is a relatively young topic. It has gained increasing importance over the last 15 years, most recently as a result of the October 2010 terrorist plot in which printer ink cartridges, filled with explosives, were infiltrated onto flights originating in Yemen. The plot was unsuccessful due to intelligence that led to the discovery of the explosives at the eleventh hour. In the months subsequent to the ‘Yemen Plot’ there has been considerable industry discussion as to which technologies might be effective tools for screening cargo in the future; Amir Neeman and Joy Banerjee set out the options.

Most bomb attacks on airliners since the 1980s have not involved air cargo, but rather found their way onto the aircraft as checked baggage or were carried on board by passengers.  As a result, available air cargo screening technology is that which has evolved from checked baggage and passenger screening technologies to focus on detecting explosive devices that could be hidden in air cargo parcels.  However, air cargo shipments can also be used by potential hijackers to gain access to the aircraft as stowaways, thereby creating another threat vector within the industry.

How is Air Cargo Different?

The characteristics of air cargo shipments differ significantly from those of checked baggage or passengers. For instance, air cargo does not follow the “typical” dimensions, commodities, weights or volumes (i.e., number of pieces) encountered at baggage or passenger operations. Commodities transported by air range from machine parts, electronics, pharmaceuticals and fresh vegetables to bee hives, various chemicals, and many other items. Shipment weights range from less than a kilogram to several hundred tonnes.  Similarly, shipment sizes vary from a few square inches to hundreds of cubic metres, and parcel counts within shipments range from one to thousands of pieces. Air cargo can be transported on dedicated freighter aircraft or in the cargo holds (“bellies”) of passenger aircraft, adding yet a different dimension in the means of transportation. Various combinations of these characteristics create a very complex environment.

Air cargo security on passenger aircraft has focused primarily on explosive detection rather than the threat of stowaways.  This is because bellies on passenger planes are usually not suited for stowaway hijackers due to the lack of access from belly holds to the main deck.

Until recently, the biggest threat from air cargo transported on freighter aircraft was thought to come from hijackers hiding in cargo pieces. However, the “Yemen incidents” have shown that terrorists will also target freighter aircrafts with improvised explosive devices (IEDs), either aiming to destroy the aircraft itself, use the aircraft to target population centres on the ground or to cause harm further down the supply chain (for instance when freight is delivered to the consignee).  Therefore, mitigating threats from potential stowaways and explosives detection for cargo transported on freighters are equally important.

Industry and Regulatory Considerations

Under the Implementing Recommendations of the 9/11 Commission Act of 2007 (Public Law 110-53), all cargo transported on passenger aircraft in the US must be screened at the piece level, “commensurate” with passenger baggage. In addition to the complex characteristics of air cargo pointed out above, particular challenges arose from the fact that this mandate was unfunded and that on-airport cargo facilities often were not designed to accommodate such a full-scale screening process. The Transportation Security Administration (TSA) addressed these challenges by allowing supply chain players upstream from the airport to become certified screening facilities.  Although certified shippers, freight forwarders, third party screening facilities and airlines do not have to subscribe to the use of screening technology – physical search remains an option both for primary and secondary level screening – TSA supports the process by vetting and publishing approved technologies on the Air Cargo Screening Technology List (ACSTL). Currently, there are four different types of technology on the ACSTL which are described below.

It is important to mention that cargo may undergo scrutiny for various purposes, such as the detection of stowaways, currency, narcotics or other illegal substances.  Depending on those purposes different screening methods may be applicable. However, as mentioned above, IEDs represent one of the principle threats facing the air cargo industry, and the following sections highlight screening technologies aimed at the detection of explosives hidden in air cargo shipments.

Technology-Based Current Screening Solutions

There are two main categories into which explosives detection technologies fall: trace and bulk inspection. Both types of inspection technology are currently in use or approved for use as communicated in the TSA ACSTL and are discussed in this section. Trace detection is based on the physical transport of explosives particles or vapour from the source to the inspection system, allowing the detection of the presence of explosives, without indication of size or quantity. Bulk detection technologies, such as Non-Computed Tomography Transmission X-ray and Explosive Detection Systems sense the physical (or chemical) composition of an object remotely, indicating the type and amount of explosives. Electronic Metal Detection is a bulk inspection technique that does not identify the presence of explosive substances at all, but rather looks for metal components of IEDs.

In addition to the United States, other governments and countries have approved these technologies and use them for air cargo screening purposes.

Explosive Trace Detection (ETD)

ETD is likely the most popular technology used by airlines and freight forwarders to screen air cargo.  With ETD, samples of particles are collected from the pieces of cargo being screened and then analysed for traces of explosives or vapours which explosives may give off.

ETDs are relatively inexpensive (typical purchase prices are in the tens of thousands of dollars rather than hundreds of thousands), which has contributed to their widespread popularity.  However, the amount of consumables is proportionate to the usage and has to be taken into the overall cost considerations.  The analysis of samples presented to the ETD may take only a few seconds, but the overall sampling and analysis process can be labour- and time- intensive.

ETDs have a small footprint and can be used in either a stationary installation or a mobile setup. However, due to the sensitive technology used in the machines, frequent calibration is required and changing environmental conditions – such as fluctuations in humidity and air pressure – may impact the performance of the machine.

A wide range of commodities can be screened by ETDs, but for accurate results cargo needs to be broken down and screened at its smallest packaging component.

Non-Computed Tomography Transmission X-ray (Non-CT X-ray)

X-ray technology is well established and many off the shelf solutions are available. With currently 60 systems available from seven different original equipment manufacturers (OEM), X-ray forms by far the most diverse category among TSA-approved cargo screening devices. An advantage of X-ray machines is the capability of processing cargo consisting of multiple pieces.  It is not necessary to break down freight to its smallest packing component.  Some of the larger systems can readily accommodate skidded freight (48x48x60 inches), which increases the overall throughput and efficiency of this technology for cargo screening.

Non-CT X-ray machines are relatively expensive (several hundred thousand dollars per unit) and usually only economically suitable for relatively large cargo operations. While there is a reduced need for consumables compared to several other screening methods, efficient operation of an X-ray machine typically requires 2-3 people. An X-ray machine is installed in a fixed location and requires a designated area beyond the machine footprint to accommodate efficient operations and maintenance activities.

One important limitation of X-ray technology is related to the density of materials being screened.  An X-ray machine generates an image of an object by emitting X-rays, which pass through and bounce off the object.  These X-rays are then detected by an array of sensors and an image is created.  For dense materials, the X-rays emitted may not be able to pass through the object, and therefore the machine may not render images of satisfactory quality.  Given the limitation in penetrating dense materials, X-ray technology is not well suited for screening all types of freight. While some commodities with relatively low density and homogenous content are well suited for such technology, other commodities such as metal machine parts or high water content may require screening with alternative screening technology. Use of X-ray technology for screening electronic equipment is technically feasible, but may produce ambivalent images that could pose difficulties for the human operator and could increase the false alarm rates.

Explosive Detection Systems (EDS)

EDS machines are complex systems consisting of an apparatus that examines the physical characteristics of an object and a software component that processes the images and data to determine if there are explosive substances. EDS machines use computed tomography (CT) and are designed to automatically determine the mass and density of an object.  A rotating X-ray source is used to take hundreds of images from many different angles of the object in question. Specialised software then assembles the images to give a visual presentation of the objects contained in the examined piece.

Explosives occupy a unique density range against which the characteristics of the examined object are compared by means of algorithms and an alarm is issued if a match is found. A human operator is needed to assess the validity of the alarm by inspecting the digital images.

Since cargo has to be inducted into the EDS machine, the overall footprint is considerable (compared to those of Non-CT X-ray machines) and needs to be taken into account when planning the screening operation. EDS solutions are often more expensive compared to other alternatives: the price of EDS machines often far exceeds those of Non-CT X-ray systems and operating and maintenance costs are significantly higher as well. However, the automated process of explosive detection is less labour-intensive and provides robust detection capabilities. EDS aperture is another consideration since most EDS machines have an aperture designed for checked baggage, which can limit the size of cargo parcels that can fit into an EDS.

Given all of the above-mentioned challenges in using EDS for cargo screening, it is currently not a very common technology used for this purpose.