Just a thought - wouldn't they be better creating some sort of toxic gas instead. Given that the idea was to just blow a hole big enough to crash the plane why not just make some sort of deadly gas type concoction instead(you can tell I failed A level chemistry here). Planes tend not to fly too well if the pilots are all dead.

I seem to remember at school we were constantly being rushed out of the science lab for mixing two liquids together and making some sort of nasty gas - chlorine was one of them I think.