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Having only ever used a jacket style bcd, it is difficult to understand if a hard plate has any benefit other then weight. I guess stability could be an issue with the soft travel plate?
Re "bulk of AL BP;
There isn't much bulk, but I used to pack the plate last as a hard cover for the (wrapped in undergarment) regulators/gauges/inflator hose etc. as if my whole carry-on was like a turtle shell. If "they" want to inspect the bag,
its easy to just peel back the top of the shell, let them see regs, and put the shell back on.
Having only ever used a jacket style bcd, it is difficult to understand if a hard plate has any benefit other then weight. I guess stability could be an issue with the soft travel plate?