In the level Covenant, after disabling the tower Sergeant Johnson was meant to do, run as fast as you can avoiding the Flood. Then head to where the Warthog and Mongoose are. Jump on to the Warthog as the Pelican is dropping of the Scorpion. Jump up as close as you can to the back of the Pelican and hold RB. You will magically hop inside the Pelican, even though the door was closed.