If one is flying on a fast day and is extending or hastening the sunset, when is he to break his fast?

He is to break his fast as soon as Tzeis Hakochavim arrives in the area that he is currently in. Thus, if he flew from the U.S. to Israel and is hence hastening the coming of Tzeis Hakochavim, he may to break his fast as soon as Tzeis Hakochavim arrives on his plane. If, however, he flew from Israel to the U.S. and is hence delaying the time of Tzeis Hakochavim, then he is to fast until Tzeis Hakochavim arrives while he is on the plane.[1] Some Poskim[2] however rule that when flying to the west, one may be lenient to break his fast in accordance to the area that he left from, upon Tzeis Hakochavim arriving there.[3] 

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[1] Igros Moshe 3:96; Betzeil Hachochmah 1:31; Chavatzeles Hasharon 1:43; Yaskil Avdi 8:31; Piskeiy Teshuvos 562:2; See Nitei Gavriel Bein Hametzarim 4:5

The reason: As there is no precedence in Poskim to allow one to break one’s fast based on the area that he started his fast, and hence since the law states that the fast does not end until Tzeis Hakochavim, one may break the fast when it reaches Tzeis Hakochavim in his area, whetehr this is earlier, or later, than his original location. [Igros Moshe ibid]

[2] Shevet Halevi 8:261; See Nitei Gavriel ibid

[3] The reason: A the Sages never decreed for one to fast more than the normal hours, and hence one may break his fast as soon as the fast is over in his original location. [ibid]