Sounds like he can be convicted only once there is the proverbial smoking hole in the ground.
Not really, checks are performed on landing as well. Any pilot who tests positive right after landing the plane will definitely get a conviction as well as lose his job and license. That said, not doing preventative checks but only checking after the fact simply because that way you can get more convictions (as if they were some sort of personal trophy
) would in itself be borderline criminal. After all, actually improving flight safety should really be the Number 1 priority...