Ann Murphy, a 32-year-old Irish chambermaid, arrived at the El Al checkpoint at Heathrow Airport, London, on April 17, 1986. She was about to fly to Israel in the belief that she was to meet the mother of her Jordanian boyfriend and then get married. She was already five months pregnant. Her boyfriend, Nezar Hindawi, said he would follow in another plane because he had got a separate ticket through his work.

Ann Murphy queued up with the other passengers to board the Boeing jumbo jet which was to carry 375 people to Tel Aviv.

A security officer asked her some routine questions and put her suitcase through an X-ray machine which showed nothing unusual.

He then emptied the case and found that it was ‘quite heavy for an empty bag’. Alerted by the suspicious weight, he pulled at the bottom of the case and discovered a secret compartment containing 31b (1.4kg) of plastic explosive. A pocket calculator among Ann’s clothes contained a timer and detonator which would have exploded the bomb at 1pm when the airliner was flying at 39,000ft over Austria.

Nezar Hindawi had given her the suitcase – already packed with explosive because he said her own was too heavy, and had put in the calculator which he said was for a friend. On the way to the airport in a taxi, Hindawi had put a battery in the calculator which armed the bomb.

Hindawi, a Palestinian terrorist backed by Syrian intelligence, was caught and sentenced to 45 years in prison. Ann Murphy – who gave birth to his baby girl before the trial – was described in court as the victim of ‘one of the most callous acts of all time!’

The airliner would have been destroyed in midair with all its passengers and crew but for the alertness of the security man and the thoroughness of El Al’s system of checking all passengers and their luggage.

El Al, the Israeli airline, has the reputation of being the most security-conscious in the world. Passengers have to check in about three hours before departure, and undergo a thorough body search. Their entire luggage is examined by hand.

The nightmare of terrorist action on a packed airliner hangs perpetually over everyone responsible for airline security. It is a nightmare that sometimes becomes hideously real, as when a Pan American jumbo was blown out of the sky over the Scottish town of Lockerbie on December 21, 1988, killing 259 passengers and crew and 11 residents of the little town.

Crime in the air, in particular hijacking and sabotage, goes back to 1930 when an airliner was hijacked for the first time – a Peruvian Airlines plane hijacked in Peru. Since then there have been more than 600 incidents, 90 per cent of them since 1968.

Hijackers usually demand money, publicity or political action. And terrorists treat airlines as a symbol of a nation whose policies they oppose.

Each tragedy brings tighter security at airports, but security will always have its limitations. As new ideas and new technical advances succeed each other, security officials play a deadly game of leapfrog with terrorists. And there is always a conflict between the need for security and the need to process passengers quickly.

Though companies are wary of discussing details, there are half a dozen main forms of security at airports.

Calculated death-bid

Unknown to her, Irish maid Ann Murphy had a time-bomb in her suitcase for a flight from London to Israel in 1986. It had been planted by her Jordanian boyfriend Nezar Hindawi, who was later sentenced to 45 years imprisonment for attempting to blow up the plane which carried 375 people. Among the evidence at his trial were a gun, bullets, a bag, a passport, and a calculator for detonating the explosive.

Checking the staff

Airports are huge areas employing thousands of people, and with many vulnerable points. Catering and cleaning staff, for instance, have smuggled weapons and explosives onto aircraft.

To tighten security, airlines may introduce ‘covert tagging’ – treating uniforms, vehicles and credentials with a chemical that can only be detected with special reading equipment.

X-raying luggage

Examinations of luggage by X-ray machines are carried out at airports throughout the world. An X-ray Rapide Monitor shows that a suitcase under inspection contains – as well as such things as sunglasses and scissors – a handgun.

Low-grade X-ray machines, commonplace in the 1970s, have been improved with solid-state circuitry to provide images sharp enough to spot electrical wire finer than a human hair.

But X-ray checks are only as efficient as the guards watching the images. Most people would spot a handgun in profile. But seen end-on it is harder to recognise.

Plastic explosive – like the Czech-made Semtex – is invisible to X-rays. The battery, detonators and wiring used to set off the explosion can easily be incorporated in a calculator, as in the El Al case, or a radio, as in the Pan-Am disaster.

Metal detectors

Machines which create magnetic fields have been widely used from the early 1970s to detect metal objects in luggage. Between 1973 and 1980, in the USA alone, they spotted 20,000 firearms.

To avoid setting off the alarms unnecessarily, however, operators often turn down the sensitivity, thereby increasing the risk that a small weapon will be missed.

And metal detectors may eventually become obsolete. Security experts fear that it will one day be possible to build a gun from plastic.

Tagging explosives

Some explosives manufacturers include in their products `taggants’, tiny colour-coded chips of plastic which reveal the place and time of origin, thus allowing the buyers to be traced. Though these only become useful after an explosion, their inclusion could deter terrorists by making detection more certain. International agreements may extend the use of taggants.

Searching passengers

Almost all airports now search some passengers and their baggage. El Al search them all. But airport authorities say it would be far too expensive and slow for every airline to search every single bag and person. Searches are usually random, unless there is reason to suspect a particular flight or passenger. Searches are now routinely backed by questionnaires to establish who has packed passengers’ bags and whether anyone has given them something to take with them.

Remote sensing

Sniffer dogs are used to detect explosives and several types of vapour sampler are also used.

No machine or animal, however sensitive, can detect odourless or hermetically sealed explosives. Several techniques, however, are being developed. One is gamma-ray radiography, which passes mildly radioactive radiation through the baggage. Certain wavelengths are partly absorbed by the contents, giving the beam a ‘signature’ that identifies explosives.

Another device is the thermal neutron analysis machine. This bombards baggage with neutrons (subatomic particles), which react with the nitrogen used in most explosives, releasing a detectable gas. The machines are being introduced into some major airports specifically to detect plastic explosives.

Until new devices are developed, the best defence against terrorists is efficient and sensitive surveillance – sensitive, because thorough searches may alienate even the most patient passengers.

It was alertness that led officials to the bomb in Ann Murphy’s bag and so probably saved the lives of hundreds of innocent people.

For the immediate future, the terrorist’s best ally is a bored, slapdash security inspector.