While it might be harder to get things on planes today, it doesn't mean with planning, that it can't be done. There is the fact that sprays don't always affect the person doing the spraying the same as those being sprayed. Contents under pressure right at the nozzle do not react the same as when exposed to air over a distance. Passengers aren't going to be prepared for the chemicals while the hijackers were. By the time the passengers get organised, the hijackers could be in control of the plane. Cockpit crews do have better doors now and they do have the cockpit guns in many cases for added defense. Link Removed