There's a helmet through the grating to the left. Helmet 5 - In the area with the huge fan on the ceiling in the back of the room with a raise control panel to the left, exit BT and move up the stairs to the right of the entrance.To get there, look for the large pipe near the ceiling with a red light on the upper left, and wall-run and jump to get onto the back part of the pipe.
Helmet 6 - When you exit BT, go through the large shaft with pipes and meet up with the militia forces in the next chamber.Move up the second set of stairs on the right wall, and climb the large cylindrical vat. Helmet 7 - Outside of the combat area, there's a large chamber with acid on the floor.Up on the left is a catwalk that you can double-jump and wall run to for the helmet.
Helmet 8 - Enter the room with the two drones.Turn left through the set of doors, and there's a helmet on a pipe in the back left corner. Jump up to the yellow pipes above and opposite the entrance. Helmet 9 - In the next room, jump onto the first inner ring to get to the upper level of the sewage drainage tract.Wall-run across the Inlet 13 sign, then against the opposite wall, then double-jump to the high catwalk. On the upper level, wall-run then double jump over and over until you get to the run-off drain near the ceiling where this helmet is.