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.