Until it turns out that the behaviour was due to a brain tumour, prescription drugs, hypo or hyper glycaemia, hyokalaemia etc etc etc.
Of course but that is going to be a very rare occurrence. One presumes in this case that if they were arrested after being reported by other crew members, blood or urine tested positively (this is the evidence that will convict) and charged, then there is enough evidence to show the crew-member had committed a gross breach their terms and conditions of employment.