That's what I thought. I don't remember much about the junkyard, but I do remember the air is sneaky to get. If I remember correctly, the air plate should be on one of those piles or mountains of junk. It's like how you access the cave/dungeon by pushing the pile. It's in the second part of the junkyard top left near where you access the dungeon.
Idk if this is how I got it, but pushing the pile will open a shortcut so you don't have to keep running around back and forth while avoiding the flying monster insect. :3