Worked great for our family trek too this trip. Funny how a BP (one with STA) is not an issue but a little hex key can be. Not complaining about that so... But hey, a heavy camera lens probably would make a much more effective weapon than a small hex key also.... and we don't talk about what one could do with first stages on long hoses as we really like to keep taking those on board...I have carried on my backplate on several trips without incident.
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But in this case I only did it to be sure it's there with our flight because we had to get 1 island away yet from the airport and get on a boat the day after.
4 people, 2PB&Ws, 1 BCD, 1 transpac, 4 reg-sets, 4 pairs of fins and booties, 4 suits, 4 DCs, 1 day of cloth, some stuff, all camera gear (OMG that was easier with a compact), 2 laptops (stupid us), other e-gizmos, tons of cables and batteries and chargers... all in carryon and personal bags. on the way there and sooo happy to stick BP&Ws, BCD, Transpac and Suits, fins and booties (the lower value per volume stuff (at least as we bought it) into extra duffels for the return flights as hauling it for 4 flights would have sucked and as we were actually on a 2 checked bags per person allowance. So happy not to haul that stuff on a rather lengthy return treck (not that getting there was short). But putting it in carry on still is the only way I know how to assure it is there when we are...