Unless the rules have changed, you can opt for a short service commission 12 yrs (with an option after 8, these are the aircrew periods). The six years refers to a return of service post OCU, BUT they have recently changed this to invoke the 6 years even if you fail the OCU. I don't agree with buoy15's idea that you won't fly for all 12/16 years, I have and so have almost all my mates. If you want to stay flying (esp these days) you will.