If one plans to fast on Tuesday and one also plans on going to Israel, leaving at 12:00 noon american time, and arriving on Wednesday morning 4:25 am Israel time b”h, when then does the fast end?
The fast ends wheneven nightfall arrives at the place over which one is flying.
Assuming you’ll be somewhere over Europe on Tuesday afternoon, the fast will end at whatever time nightfall is in the relevant location.
A number of sources indicate that while on a journey, one bases halachic times on the place where one is — irrespective of the point of embarkation and destination. This applies both to the question above, and to the (more involved question) of crossing the international dateline. See Shoel U’meishiv 4:2:154; Or Le-Zion 1:14;Minchas Shlomo 23; and see at length Vedarashta Vechakarta, p. 612ff.