It's called the 18 day schedule since you are on the hook for 18 days. They don't necessarily get use you all 18 (I did work all 18 about half the time while I was on it). Seniority isn't gonna make any difference for how much you are used here, unless you are super A-team and can still find a way to bribe the schedulers to work only on your days off for extended pay every day you work (not very likely anymore). They can keep you out up to 7 in a row and if they keep you the full 7 they have to give you at least 4 days off afterwards. If they keep you anywhere from 1 to 6 days, they must give you at least 3 days in a row off after (I never did less than a 3 day trip). Out and backs are not done like at the airlines and you would have to be super lucky to do it more than once or twice a year, but they would definitely assign you duty for the following day instead of releasing you for your 3 days off following a work tour. I have had their schedules fall apart and gotten to show at home for work days, but that isn't too common either. You will have 2 sets of 2 hard days off in a row to use for each month and they can be put together for 4 in a row off, but if not put together you need at least 7 days in between the pairs. They used to put out prospective schedules on the crewops website, but they stopped doing it last summer. I switched to 7 and 7 now and I'm not sure if they are putting up the prospective schedules again or not. Too many people made plans around their prospective schedules (not including the hard days off) and then complained when their schedules changed. It is pretty clear in the contract that they can change your schedule, other than hard days and required days off following a tour, so the complaints were unwarrented and pissed them off enough to stop giving us the courtesy of a probable schedule (they were usually almost dead on for days on and off).
