The attack may well have involved months of planning and many reconnaissances of the area.
The IRA would have had only one chance of attacking Heathrow, as security would then have been tightened. Mortars offered the best chance of inflicting damage and scoring a propaganda success.
Since February 1989 there have been more than 140 republican bombing and shooting incidents in England. Some of these, such as the placing of incendiary devices or small 'nuisance' bombs, have been minor affairs, but others have required a great deal of planning and execution.
Mortar attacks such as that on Heathrow and on 10 Downing Street three years ago clearly come into the latter category. After the Downing Street incident, the IRA said it had been planned over a period of months.
At Heathrow, the IRA would have wanted to inflict casualties on security personnel at the airport, and to damage the runway.
Actually hitting an aircraft and causing multiple casualties in the process would have been a catastrophe of historic proportions for the IRA, given its repeated assurances that it seeks to avoid civilian casualties.