When you get to the part with the Scarabs there will be a pelican near the Scarab to the left.

Go over to it (make sure you are a little bit higher than it), go near the door and jump out of your Hornet and quickly press RB.

You could also do this on co-op with one player standing on top of the Hornet.

Warning: once you get in the Hornet, you can't get out unless you are willing to die.