Is there a problem with a kohen flying in an airplane that may fly over a Jewish cemetery? If so, how can a kohen be sure that a flight does not go over a Jewish cemetery?
There are a number of issues:
The fact that the airplane MAY fly over Jewish Graves would probably be because it's like flying over a Beis Haperas (which most Achronim permit nowadays [see Taz 4 & Shach 2 on Yore Dai'ah Siman 369]).
There is also a Sofek Sfeka: