But screening will never be perfect. We can’t keep weapons out of prisons, a much more restrictive and controlled environment. How can we have a hope of keeping them off airplanes? The way to prevent airplane terrorism is not to spend additional resources keeping objects that could fall into the wrong hands off airplanes. The way to improve airplane security is to spend those resources keeping the wrong hands from boarding airplanes in the first place, and to make those hands ineffective if they do.
Exactly two things have made airline travel safer since 9/11: reinforcing the cockpit door, and passengers who now know that they may have to fight back. Everything else — all that extra screening, those massive passenger profiling systems — is security theatre.