I will be flying stopover on Shiva Asar B’Tamuz from Israel to New York. My flight leaves at 5:15 AM, once the fast has already began in Israel. Do I fast Israel time or must I wait until Tzais Hakochavim in New York after I land?
The halachic times that a person must observe on a flight depend on the land over which he is flying.
Therefore, if there is no place you’ll be flying over that has reached nightfall (as can be expected based on your flight times), you will have to fast continuously until nightfall New York time.
The significance of nightfall for fasts is the rabbinic equivalent of nightfall for Shabbos and other Torah matters, and the principles are therefore the same.
See: Yerushalayim Be-Mo’adehah, p. 232, in the name of Rav Shlomo Zalman Auerbach; Iggros Moshe, Orach Chaim Vol. 3, no. 96.