Why don't airlines put security guards on their planes?

Question

I've been watching quite a few aviation disaster documentaries lately, and it got me wondering something.

So many disasters involve terrorist attacks, and while some are resolved peacefully with no casualties, some - like the 9/11 attacks - claim thousands of lives.

However, in my opinion, nearly every single one of these attacks could have been prevented with one very simple measure - aircraft security guards. I can not understand why even now, in post 9/11 times, this still hasn't become a standard measure.

Nearly every form of martial art or combat training has some method of dealing with knives - which are usually the only weapons that a terrorist can manage to get aboard a plane. I myself am a practitioner of Aikido (a martial art derived from samurai fighting arts), and I know for sure that if a skilled Aikidoka had been aboard any of the planes involved in 9/11, that particular plane would have been saved.

So why not just put a few trained guards aboard all planes?

Answer 1

Security guards with stun guns and stun batons would be nice to have on board. I've heard that the 9/11 attack was caused by terrorists with box cutter knives. Stun guns would be more than sufficient.