Those minimum wage rent-a-cops at the airport do a good job deterring the nutcases and crackpots. Actually, it is the metal detectors that do the screening versus the rocket scientists standing there chewing their cud.

Someone who is serious about taking out an airliner can very easily do so without using crude handguns or bombs. SA-5's and SA-7's (surface to air missiles) are readily available throughout the CIS/Africa, and can be had for $15k to $20k. These can be brought into the country via boat, or along either the Canadian or Mexican "border". Then, just standoff a mile from the runway, point and pull the trigger. I assure you that a lumbering airliner makes an incredibly easy to hit target for such a device.

It is the CIA that is providing the true airport security; via buyoffs, threats, bribes and a million other venues (that we will never know about) to ensure that the above scenario does not play itself out via the fringe groups throughout the world.