Least-Risk Bomb Location: the physics, construction and proof of concept

Least-Risk Bomb Location: the physics, construction and proof of concept

Aircraft are uniquely attractive targets for those with terroristic intent, and one of the biggest concerns for crewmembers on commercial flights is the smuggling of an improvised explosive device (IED) onboard. Crewmembers are aware of the risk their airline is exposed to and receive specific training in case they hear the frightening words: “There is a bomb onboard”. They are trained in bomb stack design and learn the Least Risk Bomb Location (LRBL) Standard Operating Procedures (SOPs). Alexia Lequien discusses the physics of a blast, the rationale for having the LRBL and the importance of crewmember training.

Ensure that the suspect item is safe to be<br />moved to the LRBL
Ensure that the suspect item is safe to be
moved to the LRBL
Create a base. Bags can be used beneath<br />the stack, but not on top of it.
Create a base. Bags can be used beneath
the stack, but not on top of it.
Place the suspect item on the stack and<br />cover with plastic.
Place the suspect item on the stack and
cover with plastic.
Dampened blankets are placed on top<br />before the stack is completed.
Dampened blankets are placed on top
before the stack is completed.

Pre-flight aircrew and passenger security measures continue to evolve in response to developing threats. Part of this evolution is the availability of novel, affordable detection solutions. Technology may be effective but IEDs remain particularly difficult to identify. Recently, terrorists have used their ingenuity to circumvent security and smuggle IEDs onto aircraft. This has been achieved by designing devices that appear non-threatening or hiding them in everyday objects like shoes, toys, drink cans or, perhaps more surprisingly, meat grinders. In this way, terrorists are showing their aptitude to evolve and to bypass the technologies designed to protect passengers and crewmembers.

The race to stay one step ahead of saboteurs is endless and, as is often the case in aviation security, a chain of unforeseen failures can result in a deliberate and catastrophic explosion. We adopt a layered approach to security and trust that, even if one layer fails, overall the multiple layers will provide the protection needed to prevent an attack being successful. This is best illustrated using James Reason’s ‘Swiss Cheese’ model, whereby every pre-flight security measure represents a slice of Swiss cheese. The holes in the slices represent weaknesses at each security checkpoint and, to make matters worse, these are continually varying in size and position across the slices. The system produces failures when a hole in each slice momentarily aligns with the hole in another slice, permitting, in Reason’s words, ‘a trajectory of accident opportunity’. In our case, this is when an IED passes through the holes in all security checkpoints, leading to a failure and, in the worst-case scenario, loss of life.

The more barriers we create, the more likely it is that we will be able to detect threats and prevent a tragedy, despite the reality being that the likelihood of an IED being smuggled onto aircraft is extremely low in the first place. In fact, according to plane crash statistics, sabotage (which includes hijacking, being shot down and IEDs) is the cause of only 9% of fatal accidents. The most likely cause is pilot error, which is the root cause of 58% of fatal accidents. So, if the odds are so low, why do we need a LRBL onboard an aircraft?

The answer is straightforward: because it is believed to improve the chances of aircraft and passenger survival if a potential IED is identified in flight. The LRBL is defined by the Federal Aviation Administration (FAA) as the ‘location on the airplane an explosive or incendiary device should be placed to minimise the effects to the airplane in case of detonation’. The aircraft design itself has a significant role to play in preventing tragedy resulting from an inflight attack. In several incidents, the aircraft fuselage has proven to be robust enough to resist an inflight explosion, allowing the pilot to land safely and minimise the loss of life. This was the case on the 1982 Pan Am flight, which safely landed in Honolulu, United States and, more recently, in 2016 when a Daallo Airlines flight was bombed shortly after take-off from Somalia. These positive outcomes are outweighed by the negative ones. Between 1971 and 2016 there were 58 IED attacks onboard an aircraft and 35 of them brought the aircraft down. The most memorable for many is Pan Am flight 103, which was bombed over Lockerbie, Scotland, and claimed the lives of 270 people.

In 2015, Sheffield University participated in tests conducted on  aircraft blast-resistance at Cotswold Airport  (Credit Sheffield University)
In 2015, Sheffield University participated in tests conducted on
aircraft blast-resistance at Cotswold Airport (Credit Sheffield University)


As it is the last line of defence, the LRBL must be well understood by crewmembers. This starts with knowing the fundamentals of bomb blasts. These differ depending on the type and quantity of explosive used and the internal pressure of a cabin that generates the differential pressure to the atmosphere outside. Explosions are chemical reactions that generate huge energy at a high rate, in turn compressing the surrounding air as the energy accelerates from its source. After a compression phase, a blast wave expands from the source of the reaction and it is this wave that causes damage. The extent of the damage secondary to this blast wave depends on the location of the explosion. IEDs that are detonated close to fuel tanks have a greater chance of rupturing the fuselage and igniting jet fuel, causing an additional explosion and damage. As in the case of the failed Northwest Airlines attack on Christmas Day 2009, terrorists attempt to maximise the potential of igniting jet fuel by positioning themselves over the wing in a window seat. In terms of aircraft search, or even pre-flight security checks, crewmembers should be particularly vigilant for IEDs in these areas of the fuselage.

It is believed that the force of a blast wave is amplified when an explosion occurs in a fully pressurised aircraft because the pressure differential between the cabin and atmosphere is at its highest. In effect, the outer shell is already under tension and, as such, an additional localised force is more likely to breach the structure. In most circumstances, it is preferable to reduce the cabin-atmosphere pressure differential to zero. Reduction of cabin pressure with or without a rapid descent is considered by many to be an extremely effective way to minimise structural damage in the event of detonation. Contrary to this, experiments studying the effect of pre-pressurisation on plastic deformation, when placed next to blast-loaded square aluminium plates that were designed to mimic fuselage, show that a pressure differential does not make a crucial difference. The principal conclusion from this study is that, for clamped aluminium plates under four different blast load scenarios, no significant increase in plate deformation was detected as static pressurisation increased from 0.0 kPa to 62.1 kPa. This result was not entirely expected and challenged conventional wisdom. This shows that we do not know everything about the impact of blast waves on aircraft fuselage. What we do know is that an onboard explosion has costs to passengers’ and crewmembers’ lives, and financial costs to airlines, partly due to the long-term reputational impact.

“…LRBLs were initiated voluntarily by aircraft manufacturers in around 1972…”

Given the uncertainty about the real impact of an onboard IED detonation, crewmembers need to have a LRBL plan. Retrospectively conceived procedures for LRBLs were initiated voluntarily by aircraft manufacturers in around 1972. Since 2008, a FAA regulation made it compulsory for any commercial aircraft holding over 60 passengers or weighing over 100,000lbs to have one designated. Since aircraft manufacturers are not obliged to test the LRBL themselves, tests have been conducted by the U.S. Army in Aberdeen, Maryland on behalf of the Department of Homeland Security (DHS). This project was called the ‘Army and DHS Scenario-Based Security/Threat and Mitigation Assessment of Commercial Aircraft LRBL’ and it informed the regulation. The mandate requires aircraft manufacturers to include the amplifying effects of the cabin-atmosphere pressure differential at cruise altitude, which is believed to be the worst-case scenario for a bomb blast. The research was designed to produce essential information for crewmembers about where to place a suspected IED to significantly reduce the harm caused by a detonation. However, the effectiveness of the LRBL is not verified by testing every model of aircraft, nor each one that is manufactured. The historical approach of developing the LRBL procedure soon after the basic design of the aircraft was completed is being challenged by the new rules, which intend to get aircraft manufacturers to include the LRBL in their designs earlier.

The Fly-Bag European Research Project examined the use of blast-resistant textiles to contain blasts in cargo holds and inside the passenger cabin (Credit: European Commission)
The Fly-Bag European Research Project examined the use of blast-resistant textiles to contain blasts in cargo holds and inside the passenger cabin (Credit: European Commission)

The Bomb Stack

If a suspected IED is found on an aircraft, it is best for pilots to descend and depressurise the cabin before the suspicious item is placed in the LRBL. Cabin crewmembers have the critical role of identifying the LRBL and moving the suspicious device to this location. The suspected IED should be placed as close as possible to the fuselage and we assume that a portion of the aircraft will be lost should the device detonate. The designated location is usually one furthest away from critical structures (e.g. cockpit and fuel tanks) and in an area designed to ‘open’ (e.g. a door).

The suspected IED should be moved by a crewmember, escorted by two others, one in front and one behind. Simultaneously, other crewmembers must start collecting items such as seat covers and suitcases for placement at the LRBL. A strong base at the bottom, built upwards in the shape of a pyramid and as thick as possible, is needed. With suitcases and other heavy materials surrounding it, more of the force from the bomb should be pushed outside, instead of inside, the aircraft; the shock front travels better through denser materials. Dampened blankets are placed immediately above the suspected device, and separated from it by a sheet of plastic; in the event of a blast, the water uses up heat energy by converting it into steam. Softer materials, such as cushions and clothing, are then placed above the blankets, as they are poor mediums for transmitting shock whilst also diminishing the effect of any fragmentation.

If the IED does not detonate, upon landing a trained EOD unit (bomb squad) will need to have easy access to it. The position of the suspected IED within the stack should be identified by using an item such as a rope or a cable. When on the ground, passengers should be evacuated as safely and quickly as possible.


For security reasons, the specific location of the LRBL on the aircraft is not marked in the same way that an emergency exit is; crewmembers must be able to readily identify the LRBL from an onboard aircraft diagram or, better still, know the location of the LRBL for the aircraft types they operate without there being any need to reference any manual.

“…the effectiveness of the LRBL is not verified by testing every model of aircraft…”

The bomb stack assembly is a long operation that requires extra physical exertion, especially because time is of the essence. It is imperative that crewmembers act quickly and promptly. To do so, they need to know the LRBL procedure perfectly. Thankfully, this procedure is not done often on board an operational flight; therefore, crewmembers need effective simulation and refresher training to perform this. Competency should also be tested as part of yearly exams. It is particularly important to deliver simulations of the protocol so that crew are familiar with the physical movements required. To facilitate the correct actions and save precious time, and as is done in fire and medical drills, every crewmember should be designated a specific role and responsibility, defined by the order in which they arrived at the scene of the threat. Cabin crew are required to fill certain roles: the mover, two escorts, bomb stackers and a communicator to liaise with the flight crew.


There is a very low chance that crewmembers will face an IED threat on their aircraft, thanks to the security procedures and technological improvements that precede each flight. However, the job of cabin crew is to be ready for all eventualities by knowing how to react effectively. If an IED is detected on an aircraft, the most effective response is clear, albeit time consuming. To reduce the reaction time, crewmembers’ training must be regular and, importantly, easily integrated with aircraft design.

LRBLs were not considered in the design of most aircraft and there is scope for prospective safety improvements rather than the retrospective considerations currently in place. In particular, the incorporation of a bomb containment area or casing and/or pressure relief panels would both reduce the time to effective containment of an IED by crewmembers and reduce the impact of a blast wave. New containment bags such as a ‘FlyBag’, which is made from flexible, millimetre-thick Kevlar that absorbs the pressure from an explosion, could be tested on passenger flights. ‘FlyBags’ are used in cargo but could easily be produced in smaller sizes to be discretely stored in an overhead locker in the cabin.

Crew training and protective measures in both aircraft design and equipment can save precious time and effort when identifying the LRBL and containing a potential blast. Money and, most importantly, lives will be saved in the event of a catastrophe during passenger flights. Given the right tools, crewmembers can become even more capable of protecting the passengers they serve.

Alexia Lequien is a former senior crewmember for both British Airways and Emirates airlines and an aircrew trainer. She can be contacted at: lequienalexia@gmail.com