Mate - off course it doesnt.
First of all the crew member would be suspended etc, offered duties elswhere.
"Asked to leave" - so they resign enabling them to have a 'clean' CV rather than being sacked (I have seen that one sooo many times!).
An investigation would take place formal/informal etc etc, it really is an arduous process and if the employee really has a problem he/she will cooperate and help will be offered (and rightfully so), in most cases the "asked to leave" will apply as the 'piss takers' will rather take the easy way out.
Another thing people do in that case is go 'sick' straight away, go to the GP (again say they are an alcoholic) as whilst this is going on the person in question can not be sacked......
I have sat on both sides of the table (1st as Union rep, now as manager) it really is a mine field!.
I have seen so many piss takes and as an employer you really need to jump through all the hoops to avoid exactly that unfair dismissal scenario.