Normally you must actually
commit an offence to be convicted of it.
However, circumstances commonly arise where it is desirable to have a legal avenue to stop an offence that has not yet been committed, while still punishing the offender with a suitably serious sentence.
Thus we have "attempted" crimes and "conspiracy" crimes.
I cannot remember the details of the case but there is a precedent where an Irish separatist group were conspiring to murder one of the English Royals through some daft and improbable scheme that (for whatever reason) would never have succeeded.
They were found guilty of conspiracy nonetheless.
Our friend made threats to the safety of the aircraft and (without reading the legislation) that, I suspect, is an offence in and of itself. The means by which he proposed to carry out his threat is somewhat immaterial.
Cheers
