Powders: understanding the threat

Powders: understanding the threat

Restrictions on the carriage of powders in cabin baggage have been imposed by several regulators and, as always, as a consequence of a terrorist plot. Alejandra Gentil explores the question that was left hanging: what threat do powders actually pose to aviation?

Many travellers in Australia, New Zealand and the United States were surprised in 2018 by new restrictions on what they could carry on board an aircraft. Passengers were already used to limitations on the amounts of liquids, aerosols and gels (LAGs) and the removal of laptops and large electronics from cabin baggage for the purposes of screening, along with the divestment of shoes, coats, scarves, hats, coins, watches, mobile phones, belts and the assortment of other items people carry while travelling. Then, they were further inconvenienced by the subsequent introduction of fresh restrictions placed on the carriage of powders.

The TSA and Canada’s CATSA limited the carriage of powder-like substances in carry-on baggage to 12 ounces (approximately 350ml) per person. Australia ruled that for international flights, ‘inorganic powders must be in containers of 350 millilitres (volume), 350 grams (weight) or less, the total volume of inorganic powders must not exceed 350 millilitres, 350 grams per person’ and ‘passengers cannot tip powders out to fall under the 350ml threshold as the restriction is calculated on total container volume’. New Zealand imposed similar limitations however, so far at least, the restrictions have not been rolled out globally.

“…their handler was reportedly a senior ISIS operative who mailed them the ‘explosive DIY kit’ from Turkey and provided instructions on how to assemble it…”

We know that aviation security, despite efforts towards proactivity, is still reactive in nature. A serious incident normally results in surges of activity by regulators and/or politicians, and the introduction of countermeasures. Not because the threat wasn’t always there, but rather that now they are forced to face it and assuage the public’s fears. Risk assessment is trumped by fear. When Umar Farouk Abdulmutallab tried to set off his explosives on board a flight from Amsterdam to Detroit and instead set his underpants on fire – earning him the moniker, ‘The Underpants Bomber’ – the system’s response was to introduce body scanners. In all honesty, body scanners are a much-needed upgrade from the legacy metal detectors, but crucial questions were obscured by the panic that ensued. Among them, why were LAGs restricted in the aftermath of the 2006 London plot, which was said to involve binary liquids, but powders not banned after Abdulmutallab set his pants on fire?

That would have to wait until an incident which occurred in Sydney in 2017. Two men attempted to have their unsuspecting brother check in for an Etihad flight with a bag containing an explosive device hidden inside a meat grinder. According to open source reports, the weight of the baggage exceeded limitations, so the plot was thwarted since the bag could not be carried on board. The Australian police stated that the explosive device was “technically sophisticated”, which implies it was not the work of lone wolf terrorists. Australian Federal Police Deputy Commissioner Michael Phelan said, “We have been saying for a long time that it is not only low-capability lone actors we have to worry about, we have to worry about sophisticated plots”. Although the would-be bombers in this case were inexperienced, their handler was reportedly a senior ISIS operative who mailed them the ‘explosive DIY kit’ from Turkey and provided instructions on how to assemble it.

When this plot didn’t work, the would-be terrorists’ ISIS handler turned to thinking about dispersing hydrogen sulphide, a toxic chemical, into a crowded city-centre space. This plan didn’t advance much further as they were promptly apprehended by the Australian police. However, the aviation security response to the meat grinder explosive device was to impose restrictions on powders.

“…questions arise as to whether Ibrahim al-Asiri, the expert bomb-maker from al-Qaeda in the Arabian Peninsula, who fashioned the courier bombs, had been collaborating with ISIS in masterminding the meat grinder bomb…”

Types of Powder Explosive

Abdulmutallab, the Underpants Bomber, was carrying powder-based explosives. He was wearing custom-made, ridged underwear filled with pentaerythritol tetranitrate (PETN) and, some say, triacetone triperoxide (TATP) as well. Both are powerful explosives. Both are powders or crystal-like substances. Both were used before in terrorist bombings. Neither of them was new to aviation security practitioners or authorities. The Sydney meat grinder improvised explosive device (IED) allegedly used PETN as its main explosive, in a similar fashion to the printer bombs sent by courier from Yemen in 2010. In fact, the similarities between the two plots are so remarkable, question arises as to whether Ibrahim al-Asiri, the expert bomb-maker from al-Qaeda in the Arabian Peninsula (AQAP), who fashioned the courier bombs, had been collaborating with ISIS in masterminding the meat grinder bomb.

Explosives that have a powder-like texture are not new. Screeners, however, are taught to detect plastic explosives, which can be moulded into any shape. However, they have that consistency because plasticisers are added to them. In their original form, most are powders!

In their book, Aspects of Explosives Detection, authors Marshall and Oxley explain that, ‘[b]ecause powders do not readily hold a shape and TNT is the only common melt-castable explosive, most of the explosive powders (RDX, HMX, PETN,1,3,5-triamino-2,4,6-trinitrobenzene (TATB)) are plasticized to make a mouldable material, for example, C-4, Semtex H, PE4, sheet explosive’. These are all military-grade explosives, which can be found in their original powder form as well. As an example, PETN was used, not coincidentally since both were the brainchild of AQAP, in the underwear plot and in the printer bombs. In the latter, the PETN was made to look like printer toner. However, unlike the failed underpants bomb, the fusing system in the printer bombs was much more sophisticated and probably would have worked had the plot not been interrupted by intelligence working in the way it should. In any case, detection through conventional means had already faltered and the printer bombs were well on their way. In the Sydney plot, a ‘DIY explosive kit’ had been sent to the terrorist cell from Turkey via mail. This included the explosive, PETN.

“…ammonium nitrate – a fertiliser – mixed with fuel oil, sugar, aluminium powder or nitromethane, has been repeatedly used in terrorist acts. Although not exactly a powder – rather small white crystals – it is relatively easy to make with readily available materials, requiring little expertise…”

Explosives are usually referred to as high or low (or secondary and primary, respectively). In very general terms, the high explosives explode, rapidly decomposing into heat and gas, while the low explosives deflagrate – that is, they decompose by burning. When it comes to aircraft bombings, high explosives are the weapons of choice because a small amount causes maximum damage, while low explosives are used in the detonators that set them off. However, it is thought a Canadian Pacific flight was brought down in 1965 by a passenger mixing gunpowder and acid in a lavatory, intentionally causing an explosion. Low explosives, if confined, also cause explosions. Consider, for example, a pipe bomb. When burning, the low explosive releases gas that builds up until the pipe cannot withstand the pressure exerted and an explosion occurs. This is caused by low explosives such as gunpowder and smokeless powder. These too, as their names imply, are powders.

In addition to the military-grade high explosives, ammonium nitrate – a fertiliser – mixed with fuel oil, sugar, aluminium powder or nitromethane, has been repeatedly used in terrorist acts. Although not exactly a powder – rather small white crystals – it is relatively easy to make with readily available materials, requiring little expertise. Timothy McVeigh used over 20 tonnes of ammonium nitrate to blow up the Oklahoma City federal building. It was also used in the Brussels Airport bombing of 2016 and was the explosive of choice of the Irish Republican Army (IRA). Urea nitrate, a mix of urea fertiliser and nitric acid, is a similarly popular explosive that takes on a crystalline or powdery form. The bomb that detonated in the basement of the World Trade Centre in 1993 was made of urea nitrate. The prevalence of the use of both ammonium nitrate and urea in the making of explosives has sparked restrictions or bans on their sale in various countries. Afghanistan, for example, banned ammonium nitrate in 2012 but the availability of it in Pakistan means the ban has had little to no effect. Other countries have been more successful, so presently the actual risk depends both on the availability of fertilisers in any given country and whether the restrictions on their production and sale are effectively imposed and monitored.

Home-made explosives (HME) are relatively easy to make from readily available materials and instructions found on the internet. In addition to the above fertiliser-based explosives, the hydrogen peroxide-based TATP and hexamethylene triperoxide diamine (HMTD), although used since 1980, have become very popular in the last 20 years. Richard Reid, the shoe-bomber who tried but failed to detonate his explosives on board an American Airlines flight from Paris to Miami in December 2001 used TATP as his main charge. So did the 7/7 London and Manchester Arena terrorists, amongst others. All of these explosives, from ammonium nitrate to acetone-based substances, are powders that may look like otherwise harmless products or illegal drugs. In the case of peroxide-based explosives, their danger lies in the availability of the materials used to make them and the relative ease of ‘cooking’ them when considering their blast strength. It’s a winning cocktail for both savvy and amateurish terrorists, and one that is being very widely used. Screening points that incorporate peroxide vapour or trace detection are in a position to identify explosive devices that use TATP or HMTD. However, it is important to understand that not all trace detection equipment or canine teams have this capability. Airport security management should be cognisant of this when conducting risk assessments.

Inorganic Explosives

Talking about risks, a relatively new one has reared its ugly head. Despite not being a novelty as compounds, inorganic explosives are increasingly being used in the theatres of war by non-state actors. Chlorate-based explosives and aluminium powder additives to otherwise organic explosives are now en vogue. They are powder or crystal-like substances and, of course, being inorganic means the X-ray machine doesn’t show them as orange but as green. This is a concern because screeners are taught that explosives are orange in colour because they are organic; they are looking for that indicator. That is the reason laptops are screened separately; the presence of orange in a laptop X-ray image indicates the existence of organic material – drugs or explosives. But these explosives blend well with the laptop’s inorganic material and are virtually undetectable for traditionally trained screeners. It is like training a dog to detect nitro explosives and then wondering why it did not detect a peroxide-based one. Questions also arise as to whether our automated explosive detection systems are designed to detect them considering the software may be programmed based on material discrimination and volume. Adding to this already grim scenario, explosive trace detectors (ETDs) are also unable to detect chlorates.

“…explosive trace detectors (ETDs) are also unable to detect chlorates…”

Moving Forward

It is evident that the Syrian, Iraqi and Afghan wars have done much to spearhead non-state actors’ technical and tactical savviness while aviation security practitioners resort to old-fashioned, dogmatic approaches to infrastructure protection. The risk arising from explosives is one that the industry is well aware of. Despite this, the knowledge of them is appallingly low. Keeping up with current threats – especially those being tried and used by non-state actors within theatres of war – is essential to our risk management systems.

Explosives in powder form are not new. They have been around since the invention of gunpowder. In fact, the first attempted use of powder explosives in what can be termed a ‘terrorist attack’ can be dated back to the Gunpowder Plot of 1605, in which Guy Fawkes aimed to blow up the House of Lords in London and kill King James I using 36 barrels of gunpowder. The plot is marked annually on 5 November in the United Kingdom when Guy Fawkes’ Night is ‘celebrated’ with firework displays across the country. Incidentally, what thwarted the plot was what we continue to rely on today: intelligence. This should be a lesson in and of itself.

For those in the frontlines of aviation security, it is the way in which explosives are used and concealed, and the potentially new HMEs, intentionally configured to avoid detection, which pose challenges. It is important that incidents are critically assessed to understand terrorists’ modus operandi, and whether our countermeasures are sufficient to deter or detect them. If terrorists innovate – and they do – it is our duty to innovate too. Risk management frameworks play an important role in this and should hence constantly incorporate the information gained from terrorist events worldwide.

So, are powders a risk? Understand the capability of your detection equipment and the training of your staff, the capability and intent of the adversaries, and their concealment methods and preferred explosives. Then you will have answered the question.


Alejandra Gentil has more than 25 years of experience in aviation security, specialising in regulatory compliance, capacity building and counterterrorism. She has worked extensively in AVSEC operations, consultancy and training in the Americas, the Middle East and Africa, providing technical assistance and training to airlines, airports, regulators and other aviation stakeholders.