In the second to last part of the level where Sergeant Avery Johnson destroys the door with a Scarab, wait for him to shoot the door then fly into it. Though not required, it is advisable that you destroy the two wings of the Banshee before you enter the door. You can still make it with the wings, but you'll just damage them and it will take a longer time. After you get inside the door, start flying upward and then forward. You will fly over one wall and then get stuck in between two walls. When this happens, fly downward. Your Banshee will end up in front of the door.