If you get close to it, 'Press X to flip Banshee' appears on your screen. You and the marines will be ejected safely, and the pelican will spin erratically. If you don't get out, the pelican will take you to the edge of the map and drop you off on a large concrete 'platform'. You should get in, along with a few marines.
Look up while the pelican is hovering above you and hold the 'X' button. A pelican should fly to your position and unload marines. When you are at the top of the stairs (on the ramp) get out and jump ON TO the turret. Grab a Warthog and drive up the stairs of the ramp closest to the command post. You should notice two ramps accessible by stairs in the plaza where you just destroyed the wraiths.
Take them out and the door to the command post should open. On the level 'Metropolis' when you get in front of the marine command post, you are asked to take out the Covenant Wraith tanks.
