halacha - Why do plastic bags and not fuselages protect against impurity?

In a picture on this article, we see a kohen who has enclosed himself in a plastic bag so as to avoid becoming impure as a result of flying over a cemetery. Why is it that a pressurized fuselage does not protect against this impurity, but a plastic bag with a hole in it does? What factors specify what barriers appropriately protect against this impurity?

EDIT - In light of some answers I consider incomplete, I'd like to make something clear: a proper answer should explain the logic behind why metal is an ineffective barrier but plastic is not. Also, I thought of something else relevant: if plastic is an effective barrier, then would this plastic bag not be necessary in a 787, which is made of carbon-fiber reinforced plastic?

Answer

Anything which can contract impurity cannot block impurity from passing through it (Megillah 26b, Shulchan Aruch YD 371:1). A vessel can only contract impurity if it is made from cloth, sackcloth, leather, bone, wood, metal, or earthenware (Rambam Keilim 1:1, see Leviticus 11:32-33 and Numbers 31:22). Plastic therefore cannot contract impurity, so it can block effectively. The metal of the plane very possibly can contract impurity (depending if "metal utensils" means any kind of metal or just the six listed in the verse (gold, silver, copper, iron, tin, and lead)), so it can't block effectively. I note that even a vessel which ordinarily does not contract impurity can sometimes become susceptible if some metal is present and essential to its function, but these laws are quite complicated and beyond the scope of this answer.