Controlling Explosions – By Timers, Barometric Devices, Switches and More

How does the terrorist of 2010 ensure that their device detonates at the intended time or location? Andy Oppenheimer studies the various control mechanisms that can be used and what they might look like on physical inspection or under X-ray examination.

On Christmas Day 2009, an attempt by 23-year-old Nigerian Umar Farouk Abdulmutallab to blow up Northwest Airlines Flight 253 bound for Detroit failed – most likely due to a faulty detonation sequence. The would-be bomber, whose device incorporated a cocktail of explosives hidden in his underpants, is said to have tried to initiate it by injecting an acid into the main charge of Pentaerythritol (PETN), a proven, stable high-explosive combined with the peroxide-based homemade explosive Triacetone Triperoxide (TATP). It is believed the injected acid was intended to ignite the TATP in order to, in turn, detonate the PETN – but the normally highly volatile TATP did not ignite as planned, as the acid injected into the explosives may not have produced enough heat.

This botched attempt at mass murder in the skies (and the failed Times Square, New York car bomb attempt on 1 May 2010) illustrated a gap in the terrorists‘ learning curve with regard to detonation sequences and more generally, the body of knowledge and expertise on making bombs explode at exactly the right time, place, and to the maximum effect desired. Unless an airborne improvised explosive device (IED) has all the components functioning at once, failure is possible – and the more links in the chain of initiation, the more likely it is to malfunction. Such devices, although appearing crude, are complex enough to require testing in real time and in the actual setting for the attack – a virtual impossibility for most terrorist groups.

Learning Curve

While suicide terrorists tend not to need advanced delay timers and remote-controlled initiation, the Taliban and al-Qaeda are moving towards greater control of IED explosions for maximum effect, more precise targeting, and avoidance of pre-emption. Ad-hoc self-starters, disparate groups and fanatical individuals who may try to hijack and blow up an aircraft may not have access to expertise, command, control, and supply of materials. The current less definable perpetrators have less formal and recognisable structures and supply chains. These terrorists must therefore resort increasingly to homemade explosives (HME) and mechanisms.

They also face modern deterrence and pre-emption. Had the Christmas Day bomber had the luxury of choosing the best type of detonator for his mission, he would have used a blasting cap or an exploding wire detonator to set off the PETN in his device, which would have sent a rapid shock wave into it. However, commercial detonators may well show up on a standard airport scanner Therefore, although it is hard to detect PETN and some other reliable conventional, solid explosives at the security check, the standard scanning system for checked baggage – enhanced worldwide following the Lockerbie aircraft bombing – may have provided a vital deterrent to effective detonation.

Of growing concern, however, is the likelihood that the bombers will adapt to the cat and mouse game of countermeasures. If they can’t smuggle commercial detonators on board, they could attack the terminal, inside or outside, with a commercial-detonator IED or improvised chemical device (ICD) before searches take place. This threat reared its head when terrorists, having failed to set off car bombs in London in August 2007, drove a vehicle loaded with gas cylinders into the terminal at Glasgow Airport.


the ‘key’ to all bombs Commercial electrical detonators work by sending a rapid shock into the main explosive charge. They are necessary to set off stable high explosives; less so in instable HME such as the peroxide mixes.

Detonators often malfunction. When they become detached there is no explosion, and many explosives ordnance disposal (EOD) operations depend on disrupting or detaching the detonator (and the timing and firing mechanism) from the explosive charge.

Faulty initiation seems to be the main cause of several recent failed attempts. The London 2007 incidents, although not airborne or at an airport, demonstrated this. As in many other attacks around the world, the terrorists attempted mobile phone initiation to blow up two cars parked near a nightclub. The ‘detonators’ were highly improvised – consisting of two mobile phones wired to a light bulb surrounded by match heads. A phone call was supposed to trigger the homemade device igniting the vapours swirling inside the cars. Only one detonator sparked, but even that was quickly snuffed out because the mixture of petrol and gas was too thick and there was insufficient oxygen to trigger the devices.

The Times Square bomb suspect, Faisal Shahzad, told police that he lit a fuse which he expected would detonate the car bomb within five minutes as he walked away. The timing device was a simple alarm clock. However, the bomb malfunctioned, emitting smoke that attracted the attention of the street vendor who reported the suspect vehicle. Investigators believe Shahzad purposely made the device out of less potent fertiliser and M88 firecrackers to avoid detection.

Chemical detonation: the growing threat

Today’s airborne terrorists are more likely to use chemical detonation, as attempted by the Christmas Day bomber. These are less rel iable methods than standard commercial detonators. Where a commercial detonator cannot be used, the peroxide-based HMEs, TATP or Hexamethylene triperoxide diamine (HMTD) – used in the London July 2005 bombings – may be the detonator.

“…a phone call was supposed to trigger the homemade device igniting the vapours swirling inside the cars…”

TATP was selected as a detonator by the so-called ‘shoe bomber’, Richard Reid, when he, on 22 December 2001, tried to blow up American Airlines Flight 63 only months following the 9/11 attacks, by trying to ignite with matches two TATP-PETN devices hidden in his shoes. The device had been made with a small thread of TATP running through 100g of PETN, attached to a powder fuse running through the shoelace. As with the Christmas Day bomber nearly a decade later, TATP was seen as an unusual trigger for PETN, but it was acknowledged as a sophisticated combination to avoid detection. The complex initiation sequence for the devices intended to down seven transatlantic airliners in the 2006 liquid explosives plot revealed a sophisticated attempt at chemical detonation. The terrorists planned to use soft drinks containers and replace their original contents via a syringe with a homemade liquid explosive – concentrated hydrogen peroxide mixed with a powdered fruit drink which would help detonation.

“…the 2006 liquid explosives plot revealed a sophisticated attempt at chemical detonation…”

A small amount of HMTD – was intended to chemically detonate the peroxide soft drinks mix and was disguised in hollowed-out batteries. An adapted miniature light bulb or similar heat source was to provide the electric element, with its tail leads and with the element exposed, and would have been connected to a disposable flash camera to initiate the detonation of the liquid explosive. The IEDs were intended for easy assembly and detonation during the flights and the separate components – the batteries, drinks bottles, and cameras at least – could arguably have passed through security as innocent items.

Timing is of the essence

Timing mechanisms evolved through necessity – the need to fire off a bomb at a designated time and place and the need to avoid being too close to the detonation being uppermost. The IRA, which also usually tried to minimise civilian casualties, developed most of the timing mechanisms used by the Taliban and others today, from watches and analogue and digital clocks to central-heating, VCR and electric fence timers. They also targeted aircraft with timed IEDs.

However, one of the IRA’s miniaturised timing mechanisms failed in its most serious attempt to down an aircraft, on 23 July 1974, when a flight bringing several MPs and the Royal Ulster Constabulary’s Chief Constable from Belfast to London was diverted to Manchester following an IRA phoned warning that four bombs were on board. After evacuating the aircraft, a 1kg device was found under a seat. Its analogue wristwatch timer had failed because a drawing pin fixed to the watch’s face was covered by a thin layer of paint. This had acted as an insulator and prevented electrical contact with the hand of the watch.

Once luggage scanning became more widespread, terrorists tried to use less detectable plastic or liquid explosives detonated by miniaturised and benign-looking timers. By the mid-1990s this strategy for smaller devices was epitomised by the so-called Bojinka plot, when three men were accused of planning to put 12 bombs on US airliners carrying 4,000 people across the Pacific Ocean to US west coast gateway airports in January 1995. Each device would have included a timer crafted from rewiring a Casio digital watch, which was to be connected to a stabilised form of liquid nitroglycerin, stored in a bottle that appeared to contain contact lens solution. The timer was crude – a metal rod taped to the foot of one of the conspirators.

Under pressure: barometric timers

Barometric timers work in response to the level of air pressure inside the aircraft – which is lower than sea-level pressure. Once the plane ascends and atmospheric pressure falls below 950mB a barometric or altimeter triggering device, which is sensitive to high-altitude atmospheric pressures, will move a diaphragm which pushes a switch connected to a battery, and which by electric charge blasts the cap and detonates the bomb.

The Semtex bomb brought aboard Pan Am flight 103 which blew up over Lockerbie on 21 December 1988 was said to have been set off by a barometric switching process coupled with a long-delay timer, a fragment of which was found in the wreckage. As the plane cruised at about 30,000 feet, the barometric sensor is said to have completed a circuit which started the timer.

“…its analogue wristwatch timer had failed because a drawing pin fixed to the watch’s face was covered by a thin layer of paint…”

Blowing up an aircraft at cruising altitude (29,000-45,000 ft) is like bursting an inflated balloon with a pin-prick. Only a small amount of explosive would be needed to blow a small hole sufficient to destroy it or at least cause it to crash.

Compact enough to be concealed in a radio cassette player, the Lockerbie IED was reconstructed and said to have incorporated a basic commercial detonator and a circuit board for a small timer. Reconstruction of the device showed that inside the cassette player case a large battery was wired to a barometric sensor – the bellows unit from an aneroid barometer – concealed under the cassette-player’s motor. This in turn was set to start the MST-13 countdown timer, which in turn was wired to a detonator to set off the 350g of foil-wrapped Semtex slotted into space for the loudspeaker. There is dispute as to whether the Lockerbie bomb was a dual device containing a barometric switch and a timer, or a single trigger device, which was activated by only a timer.

The advantage of barometric timers for terrorists is that they are only activated once the plane is airborne; the bomb will not go off on the ground if the plane is delayed. The aircraft has to take off and gain height so that some seven or eight minutes into flight, the air pressure drops enough to activate the barometric timer, which would be set to go off 30 minutes later, that is, 37 or 38 minutes after the flight took off. The time delay on the Lockerbie bomb timer (claimed as a MST-13, but this remains controversial), had been set at about four hours to allow the airliner to be far above the Atlantic. This was meant to guarantee that everything on board would be lost without trace. But as Flight 103 departed late, the explosion took place over Scotland and remains Britain’s worst terrorist event.

Spotting the components

Nowadays most of the components in the Lockerbie device would show up on scanners and be spotted by operatives trained to find odd wires and switches.

Some commercial detonators such as blasting caps, and conventional detonators based on fulminate mercury or lead may show up on X-ray scanning. However, smaller, thin-walled detonators (as described by Bruce Koffler in ASI, February 2009 issue) may be more easily concealed or made to resemble other items in the luggage. Paper-shelled and other improvised detonators and are even harder to detect.

The main fear is that terrorists adapt their methods and tactics to evade scanners and other deterrents by not using conventional timers and detonators. The Christmas Day attempt did not include timers or standard detonators but was still not detected as it was concealed in the bomber’s underwear. This has prompted the need for selective body scanning and spectroscopic scanning for liquid chemicals, in addition to the current restrictions on liquids. The growing use by terrorists of liquid explosive mixes in small devices, as evidenced in the liquid explosives plot, and attempts at chemical detonation is still a challenge to scanning systems worldwide. Therefore, security staff in any airport must be well trained and extra vigilant in spotting other factors alerting them to a possible suspect device.

© Aviationsecurityinternational
Issue October 2010

About The Author:

Andy Oppenheimer

Andy Oppenheimer is an independent UK-based specialist in CBRNE (chemical, biological, radiological, nuclear weapons and explosives) and counterterrorism. He is Editor of Chemical & Biological Warfare Review and G2 Defence Intelligence & Security, former Editor of Jane’s Nuclear, Biological and Chemical Defence, NBC International, and Jane’s World Armies, and is a Member of the International Association of Bomb Technicians and Investigators. His book IRA: The Bombs and the Bullets – A History of Deadly Ingenuity (Irish Academic Press, 2008) is regarded as the seminal work on the military campaign of the Irish Republican movement.