You are more correct than you think! Terrorism is not about MAJOR actions. It is about the slow building of fear and disillusionment with the ability of the government to protect you. The ultimate goal is to "convince" you to make the government change thier policies (e.g. surrender to terrorist demands).
Thus even "failed" attempts (when publicized) can greatly benifit the terrorists as the fact the "they are still there, plotting" and "it's only a matter of time" both aid the terrorist - they want us to live in fear and to dread the next newscast!
So, yeah, targeting "one plane" - thus proving that they CAN is entirely possible.
I've long thought that what two fellows with a beat up old car and a .30-06 rifle did to the capital was horrifying. The odds of any individual getting shot at any given time was infinitesimal, on sniper in an entire metropolitan area, but everyone was steppin and fetchin, serpentine walking and all the rest. Just astonishing how terrorists can use weak bellied liberals and the media to make frenchmen out of all of us.