Before going through TSA security, check in a bag. This bag contains an explosive device with one of many wireless technologies attached. Go through security. Get on the plane. Hit the detonator at a time of your choosing. Seems easy enough.
I can't imagine it would be too hard to get some kind of explosive device in a checked bag onto a plane. For example, they let you check in bottles of liquid.
Heck, you could just have a timer on it or an altimeter. Nobody says you have to actually get on the plane.
For ultimate irony, have your device scan for wifi devices. When the number decreases significantly (when the people on the plane turned their phones off), then detonate.
Checked bags are usually searched, either by hand (small airports), or by x-ray (larger airports). I would imagine they go through an explosives sniffer too, so hiding an explosive in a checked bag is nearly impossible these days.
So far as not checking in for the flight, the airlines have checks and balances to stop that. Everyone who checked a bag had better be on board that aircraft, or the plane doesn't leave the gate until that bag gets pulled. Sorry, no unattended baggage on board.
This is part of the reason why Al Quaeda was trying to sneak explosives inside of printers, on a cargo flight to the USA. The idea isn't to kill a bunch of people, but to inflict
TERROR on a bunch of people. To them, they don't care whether they kill 2 or 2,000, so long as they've terrorized a shitload of people.
And actually, the easiest way, if you want to blow up a plane, is to hit it with some mortars, while they're lined up (like ducks) waiting to take off, at the end of the runway. Shoot, I've seen 8 to 10 jets waiting to take off at once. Hit the first one, hit the last one, then start walking your mortars up and down the line. You could take out hundreds of people, on planes full of jet fuel, that are completely helpless.
I thought that scenario up, the last time I flew home to visit Mom, while I was in my plane, waiting at the end of the runway/taxiway area, for our turn to take off. Hopefully someone at TSA has thought of it too! :hmm: