I used to fly 121 Supplemental, and the duty time rules are a tad different - i.e. your duty time can be broken up into chunks over a 24-hour period in such a way that can horribly disrupt your normal sleep/work cycle (I don't remember the regs in detail off the top of my head, but If I remember right there isn't an actual "8-hour rest lookback rule" like there is in Domestic and Flag). Add that to the fact that we were unscheduled, and things got real interesting.
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I have a question about the part 91 reposition flight. The way I read the rules you can do all the 91 flying you want AFTER the 121 shift is over.
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That's the dirty little secret that I didn't know about until I started flying 121 freight. The common interpretation of the rules is that any "tail-end ferry" legs after your 121/135 legs doesn't count toward the 8-hour limit. A ferry flight to PICK UP your passengers/freight does count, though.
Keep in mind that whenever there is a "grey area" in duty/rest regs, everybody seems to have a different interpretation depending on their goals, and then you have conflict. Pilots always want to be rested and legal, managers and ops people want to make profit and those two things don't always go together.