you wanna smuggle enough c4 or semtex onboard a jumbo jet, enough to blow it out of the sky? easy.. take one crazy motherfu**r, who is willing to die for his 72 virgins in paradise (easy to find) and send him into surgery.. crazy motherfu**er doctor removes one of the dudes' lungs..in place, is a plastic bag with an equal amount of explosive, I'll bet you could cram about 10 pounds, at least into the thoracic cavity plus the area where a lung was removed...a simple interjugular catheter would serve as the fuse. of course the dude would be in a world of hurt but it matters not as he is going to friggen die...they wheel him on the plane as a medical case..probably getting special treatment from the attendants...then at the right time, he pulls out his catheter and lights the cannon fuse that protrudes from the caps...takes his last breath and BOOOOM..that would be enough explosive to take out a tank, let alone a jet...
not that I've given it any thought...