Adding to the above, the equivalent tank helmet piece for that dungeon is the Aetherial Mythril Celata. If you compare the two you'll notice that they have difference defence stats and as a tank, you really don't want to be wearing the dragoon gear (or DPS gear in general even though often it lets you).

Aetherial Mythril Barbut: http://na.finalfantasyxiv.com/lodest...m/a3e16d9c760/ (Defence 53, Magic Defence 39)
Aetherial Mythril Celata: http://na.finalfantasyxiv.com/lodest...m/0a96c03aaa1/ (Defence 73, Magic Defence 73)