Just to make clear, I don’t know the halochos of tuma, I just have a question for those who do.
Lets say that the actual airplane is not an ohel by itself because it is an Ohel Zaruk (as noted above, I don’t begin to know the sugya), and the plastic bag stops the tuma because of “tzomid posil” (as noted above). When one is sitting in the airplane there are many different layers between the aircraft outer metal wall and the plastic interior. Why can’t the platic (or fiberglass or whatever it is) also make the “tzomid posil”? There are layes and layers of insulation, wiring, airducts, etc, in those walls, that are (IMHO) not part of the Ohel Zaruk.
Again, I maybe wrong. I never learned the sugya. Maybe someone who did, can enlighten us all.