Knowing something about explosives, having been paid to play with explosives in a professional sense in the past, I'd disagree on the plan being viable. To me, viable means being able to assemble and place the explosives and have them have a reasonable chance of detonation at the desired time.

The plot, as described, does not feature those attributes IMO. They are far (far!) more likely to have blown themselves up prematurely while trying to assemble the final device - basically, a squib detonation that would likely only burn (maybe kill, if they were unlucky (or lucky, depending on perspective)) them and do minimal damage otherwise. In the context of a plane, still potentially dangerous, but no more so than the idiot shoe or undies bombers. If they tried to assemble it airside in the sterile area then no real threat at all.

I would strongly suggest you never go into the field of risk assessment if that's your definition of "viable".