You're assuming your pairings wouldn't change if you had a duty rig. Very rarely do we see any credit with our 2:1 duty rig, it really only becomes a factor if we get stuck on a day with low credit.
Because we do pretty much the same type of flying for the same carrier, I would say from my experience, your 14 hour duty days would shrink to an average of 8-10 like they are here. The duty rig is there to basically force the company to create pairings that are (more) efficient.
As far as over blocking 4 hours in 4 days...holy jesus. Do you guys go out and shut the cargo door and close the pax door to get the acars started, then open the doors back up??? I think the best 4 day I ever had barely made it past 4 hours overblock. I've completed 3 4-days in september and overblocked *maybe* 1 hour total, and it's not if we're not trying
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