The Gemara in Erubin (30) discusses a famous debate between Rebbi and Rabbi Yosse regarding a case of ‘Ohel Zaruk’, which literally means a "movable tent." This is referring to a moving enclosed domain. If a Kohen is situated inside this kind of Ohel, like in a box, does the box interject (separate) the Kohen from the Tum’a (ritual impurity) of a cemetery? If we consider a moveable Ohel an independent domain, then the Tum’a will not penetrate the box, and the Kohen would be permitted to cross through a cemetery by being carried in a box. Rebbi maintains that a transportable Ohel does not have the status of an Ohel, and the Tum’a therefore penetrates the box and the Kohen becomes Tamei (ritually impure). Rabbi Yosse, however, claims that this kind of Ohel is considered a self-contained Ohel, and it would therefore be permitted for a Kohen to pass through a cemetery in this manner.

The Rambam follows the position of Rebbi, that an Ohel Zaruk does not block the Tum’a from penetrating the enclosed area. We follow Rambam’s opinion and Kohanim today do not travel in an Ohel Zaruk in a cemetery.

The question arose as to whether a Kohen may fly in an airplane if its flight path takes it over a cemetery. A plane is a classic example of Ohel Zaruk – an enclosed area that is transported. Halacha says that the Tum’a of cemeteries extends indefinitely to the sky above. Therefore, since the airplane is an Ohel Zaruk, which as we have seen does not block the penetration of Tum’a, seemingly a Kohen therefore would be forbidden from flying in an airplane that passes over a cemetery.

The Halacha is that if the Kohen knows with certainty that a given flight’s path takes it over a cemetery, then he must ensure not to take that flight. If, however, he is unsure, then Rabbi Pinchasi says that he may be lenient and travel on that flight.

For a time, El-Al’s flight path took it over a cemetery. The Chachamim (scholars) intervened, and the company rerouted the path such that the planes would not fly over cemeteries, and Kohanim would therefore have no problem flying on these flights.

Of course, this entire discussion pertains strictly to Jewish cemeteries, which generate Tum’a. Non-Jewish cemeteries do not transmit Tum’a, and thus there is no problem for a Kohen to fly over a non-Jewish cemetery.

In conclusion, if a Kohen knows with certainty that a plane will fly over a Jewish cemetery, he may not travel on that flight. If he does not know this with certainty, then Rabbi Pinchasi rules that he may take that flight.