A bombed B747F crashing onto a highly populated town/city would be a massive publicity coup for Al Qaeda.
And how on Earth would you know the routing? And how would you detonate it?
If you have the technology to make lead azide and PETN, then surely you would use a GPS or barometric device. Not a mobile phone circuit.
I'm still struggling to see the logic in this.
So they were either going to blow a Rabbi's printer up, which is highly unlikely, or try and "down" an aircraft over a city, which also incredibly unlikely with that amount of PETN.