This glitch is done on the level Metropolis. First get in the scorpion. go through the level like you normally would until you reach the part where you are going to enter the command post with the marines inside it. go to the second bridge then wait.a pelican will land there. jump up and hold x. you should be inside it. after that the pelican will crash and you will float in midair! walk up to the pelican and it will say: "Press and hold X to board banshee. to get back to the game paus then revert to last checkpoint.