A 90-degree female flare swivel adapter in AN10, for the high-volume fuel, oil cooler and dry sump lines where hose is far too stiff to bend tightly. Forcing a large-bore hose around a corner reduces effective flow and puts constant strain on the hose end, so making the turn at a rigid elbow is the better engineering answer. The swivel allows the assembly to be sealed on the 37-degree flare and then rotated so the outlet faces exactly the right way, without unwinding the joint to get there. Plan spanner access before final assembly at this size, and check the parts you are joining use the AN 37-degree flare seat.