The IRA for example never targeted civilian aircraft
So what about the attacks on LGW & LHR in the early 90's?
Al Qaida used the methods on 9/11 for major impact, and they acheived it. They have no reason to ever use that method again, that single attack has resulted in major expenditure for airlines and airports and major inconvenience for the travelling public. The after effects of the attacks of 9/11 will be felt for many years to come. As an act of terrorism it was probably the most successful in history. Even if Al Qaida never carry out another attack their entry into the terrorist hall of infamy is assured.
Why would they try to attack another aircraft, the risk of detection is far to great now to risk such an attack. There are numerous other methods they could adopt to create just as much chaos. Imagine the effects of exploding a super tanker filled with crude oil or chemicals in Boston harbour for example.
