This is the law: if a man dies in an ohel, anyone entering the ohel and anything in the ohel shall be unclean for seven days. (Bamidbar 19:14)
The Torah forbids kohanim (priests) to become defiled from contact with a dead body (Leviticus 21:1). Maimonides (d. 1204) explains that there are three ways in which tumah (spiritual impurity) could be conveyed; touching, carrying and entering an ohel (tent or covering) with a dead body (Laws of Impurity 1:1) which includes proximity to a dead body.
A number of questions arise as a significant amount of the earth’s surface probably contains dead bodies, from ancient graveyards to former battlefields. In addition, while Maimonides ruled that a non-Jew does not impart tumah via an ohel (Laws of a Mourner 3:3 based on Yevamot 61a), according to the Shulchan Aruch (Code of Jewish Law published in 1565), even non-Jewish cemeteries should be avoided (Yoreh Deah 372:2). This would make it impossible for a kohen to go anywhere.
Yet the Talmud rules that in a case of indeterminate or unspecified tumah in a public area, we may rule that the area is tahor (pure) and a kohen may enter (Nazir 57a).
Maimonides adds that tumah defiles a person who is directly above a grave regardless of their distance (Laws of Impurity 1:10). This implies that it is forbidden for a kohen fly on an aeroplane over a Jewish cemetery unless there is some chazizah (barrier) blocking the tumah. The chazizah must be something which cannot become defiled itself.
Nonetheless, Rabbi Moshe Feinstein (d. 1986) held that only the metals listed in the Torah (Numbers 31:23) are subject to tumah. Since aeroplanes are made from aluminium and titanium, the plane’s fuselage is enough to prevent tumah from defiling the kohen on board (Iggerot Moshe Yoreh Deah II 164).
While it appears that this leniency may be relied on, some who wish to be stringent resort to temporarily wrapping themselves in plastic (which being manmade is not affected by tumah) on aroplanes which fly over the Holon cemetery. This may cause some embarrassment to others, but perhaps we should admire someone who is willing to risk derision and mockery for the sake of upholding their convictions and principles.