From Halopedia, the Halo wiki
Directions: When you reach the area where the Scorpion Tank is, finish off all of the Covenant forces. After clearing the area of all hostiles, leave the Scorpion alone or get out of it if you used it in the clearing effort.
Next, walk down the small hill and look up while doing so. You should then notice a small rock bridge followed by a small platform coming out from the canyon wall. Keep walking slowly until you get a perfectly clear view of the platform. Take out your Rocket Launcher and aim for the middle of the platform. If you have a Sniper Rifle, zoom with it all the way and adjust your aim to the middle of the platform, switch to your Rocket Launcher and fire. If your aim was perfect, a Banshee will fall off the platform and tumble to the canyon floor. This is the Banshee that would have been used against you by an Elite. This may take a few tries and some re-aiming, but it does work. You can actually use the Banshee, so have a blast and take advantage of a time saving hint later on. Later, when you exit the tunnel, there will be a large tower. Fly to the top and there will be another Banshee sitting on top waiting for you to steal it before the Elite spawns. And in the second canyon, if you fly there before the enemy forces spawn, there will be a third early Banshee where you can take it or flip it over so it can't be used.
This trick can also be a great headache saver. When you get to the second canyon, fly the Banshee over the towering rock building in the middle. Then just keep flying very high until you get to those twin bridges. Land on the second bridge (the one further from you on approach). Simply go through the doors, but make sure the arrows are pointing the right way. You can finish the level with no more Covenant forces to deal with.
Another way to do this is near the end of the level, at the beginning of the chapter If I Had A Super Weapon.... There are two Banshees sitting on the bridge that you walk out to. It is possible to obtain one either by killing the pilot (the Elite which tends to run towards the Banshee), simply by rushing up and getting in (there is an Active Camouflage module just before the chapter begins), or if you have a rocket launcher, you can blow the Banshee off the bridge (this can flip it over too), and you use the spire to get to the ground. This saves you from having to kill all the Covenant surrounding the control room, and you can have additional firepower on your way to the room itself. Halo PC
Method Single If you have DevTrainer, when he door opens on the rocky bridge above the control room freeze time and go to one of the Banshees and unfreeze time(since you can't be in a vehicle while time is frozen) and get in the Banshee. Or you can turn Medusa on and walk out threre and get the banshee.
If playing on Co-Op, do the same as the above. When you have the Banshee, have one person get into the Banshee and fly it to the entrance to the Control Room.
Once the player has landed, have the other played who did not drive the Banshee commit suicide. They will then respawn next to the player with the Banshee. You can then continue on playing normally.
It is recommended to have the player with the Banshee to have important items (ex: Rocket Launcher, Sniper Rifle) as the other player will lose them when they die. Alternatively, you can fly back down to retrieve them in the Banshee.