Sector Y? You need to perform a couple of tasks in Corneria to get there. 1. Rescue Falco when he's being chased by the ships after the checkpoint, so that he doesnt get shot down! 2. Fly through all the arches above the water, (In the section when you're near the boss)until you hear Falco telling you to follow him, then you'll turn automatically towards a waterfall, which you need go through.