This video shows you how to get the skull that is hidden in Bad Piggies When Pigs Fly Level 3-16. The skull is way over on the right side of the level, and not easily reached. Build something that looks like a pickup truck and very slowly make your way through the lowest chasm. If you manage to get close, pump the bellow like crazy!
If you have a different strategy for completing this level feel free to leave a comment below.