The threat is manifestly persistent, but unlikely to extend to knocking aircraft out of the air or even striking at those taxiing with a load of passengers; after all, some might be American. The IRA recognises that indiscriminate acts of terror are counter-productive; a lesson it has learnt principally by its incompetence. The possibility of massacres at airports cannot, therefore, be ruled out. Even so, security measures should be aimed primarily at preventing the use of weapons that will cause damage to property, including aircraft and traffic facilities, rather than people.
Immediately, the focus is upon mortar attacks. This has proved a successful weapon because multiple barrels, fired simultaneously, have thrown bombs successfully across the perimeter fence. The type of bombs projected are inherently inaccurate - thus increasing the random danger - but would not substantially damage reinforced concrete. Parked aircraft are another matter; shrapnel strikes on jumbo jets could result in huge repair bills.
IRA mortars are crude in construction. Ranges are likely to remain short and their power limited. This means that the firing base will be within a few thousand yards of an airfield perimeter. Even so, this would appear at a glance to create a considerable potential launching zone round the extensive circumference of a major airport.
Fortunately, a good deal of this zone lacks that potential in an area such as Heathrow. For example, the greater number of dwellings, shops and offices do not offer the necessary space for bomb flight, strength of flooring or secrecy of operation. The number of open spaces, including car parks, may be numerous, but modern scanning devices offer a means of raising the probability of discovering preparations for an attack at an early stage.
A weakness of our situation is that the public are incurious. Lorries - rather better and thus to date preferred as launching vehicles to cars - are sometimes left on side roads for several days, without being remarked on except as a parking nuisance. But mobile police patrols can cope with this problem - and will do so more readily if members of the public make a point of reporting abandoned vehicles and, indeed, any other matters that make them suspicious.
At root, much of the danger of mortar attacks at Glasgow, Manchester or Birmingham, and other major airports, will be reduced by measures which help the police to reduce crime generally. The etching of car numbers on windows, even the locking of car doors, would make life more difficult for thieves, terrorist or otherwise. On balance, therefore, given detailed study and the appliance of surveillance technology, the risk of mortar attacks on airports will diminish considerably. The problem of acts of terrorism at these and other busy, complex and expensive sites remains, however.
The IRA will continue to operate wherever risks are low and success offers high cost to the state and wide publicity. The range of counter-measures planned must thus be widened. Notwithstanding the use of advanced security equipment, some of these measures will involve, essentially, additional manpower. This raises the question as to whether troops should be drafted to assist the police in these duties.
In one sense, the Army is already well trained for such work as a result of operations in Northern Ireland over 25 years, and this includes the ability to contribute its own unique skills to joint police and military operations. Moreover, it has learnt a good deal about the sensitivities of the police as the primary upholders of law and order. Chief constables in Great Britain need not become alarmed over such matters.
But the numbers of soldiers are diminishing. If they are to be employed in this way, the Government must stop further reductions in the service, or must provide funds for additional police manpower.
The very fact of deploying the military at a major airport such as Heathrow would be a deterrent to terrorists, as the IRA is very well aware. Moreover, the Army is as experienced in rural operations as in security work inside cities, and its techniques would be suitable to securing areas like airports.
The Army could assist the police by deploying night vision devices and infra-red scanning devices that are capable of picking up, for example, heat-emanations. The heat reflected by objects varies widely, and weapons - such as the array of mortar tubes that were left in that patch of scrub at Heathrow - would be readily detected, even if covered over with camouflage. They would reflect a different heat signal from that of the surrounding material. Such objects would be less easy to detect if they were on a lorry, because they would be in the heat field of the lorry, but in the middle of fields and other open spaces, they would be found.
The Army has other equipment, too - the character and capability of which it does not disclose for security reasons.
The presence of the military is likely to reduce the chances that an attack would be made and if it were, that it would succeed. Full surveillance need not even be carried out every day or every night. The very fact that it was done, now here, now there, would be a discouragement to any terrorist.
There is nothing, of course, that can provide security that is 100 per cent effective in an open - or even in a closed - society, but I think that the public generally understands that there is no such thing as full security, just better security.
Such military operations would be expensive, but the expense is containable. It would not even approach the replacement cost of a building blown up in the City.
For the involvement of the military to be effective, however, one individual must be placed in complete overall command of the fight against terrorism. He - or she - must not simply direct operations; they would have to be given the power, authority and political backing to do the job. It could be a civilian, a senior policy or a military officer. That is simply a matter of selecting the right person.
But the attacks at Heathrow do not merely call for counter-measures at airports. They remind us, if we still need reminding after all that has happened since 1969, that the IRA has no intention of abandoning violence until its aims are achieved. It will not be prevented from doing so without a wholehearted response from the state - and the sooner the better for us all.
General Sir Anthony Farrar-Hockley is a former Nato Commander-in-Chief and was the first Commander, Land Forces, Northern Ireland.
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