On international departures from the UK, all bags checked in are automatically screened for terrorist devices. Since 1995 ERA Technology has been helping the BAA to incorporate automatic X-ray screening machines into their baggage systems.

Initially, ERA was involved in a design review and risk assessment of an in-line hold baggage screening system at a BAA trial site in Slough. The aim of the work was to demonstrate that in-line screening could be used safely to identify bags that had not been cleared by the X-ray screening process. The generic layout of this trial system has since become the basic design for installations at BAA's airports in the UK.

Specifically, the baggage control system has to identify and clear bags before loading onto aircraft. Unclear bags are also identified and removed from the system. ERA's independent task was to assess whether there was a sufficiently robust control process to ensure all the unclear bags were being removed.

Programmable Logic Controllers (PLC), which are used to control baggage system conveyors, interact with X-ray machines to establish the result of the screening process. From the point of screening to the separating of unclear bags from clear bags, the position of each bag is closely tracked using photoelectric cells, which register the presence of bags, and by monitoring the speed of each conveyor.

Unfortunately, bags vary in shape and size: they have wheels and hooks and are prone to movement on the conveyors. The bag tracking logic must be designed to be failsafe so that, if there is any doubt about the identity of a bag, the system treats the bag as unclear. However, since there are thousands of bags, which are screened in an airport each day, it is also important that not too many bags are mistracked.

The PLC software design had to be reviewed to check whether sufficient failsafe functionality was provided, whilst still limiting the number of mistracked bags. ERA analysed the design of the layout of the conveyor system, controls and software, using system hazard analysis and risk assessment techniques, to provide a prediction of the chance of missing an unclear bag. The success of the trial and the use of an in-line, automatic screening process significantly reduces the number of screening machines and the amount of time to load bags onto aircraft, compared to off-line screening solutions.

Since their involvement in the trial, ERA has assisted BAA in the procurement of 100% Hold Baggage Screening Systems at Stansted, Gatwick and Edinburgh Airports. This work has included development of performance specifications, audit of control suppliers, review of design specifications and software, witness testing and confirming that PLC designs include the failsafe functions to ensure that all unclear bags are removed for further screening.

For further information contact: safetyengineering@era.co.uk