If, at daybreak of the 14th of Adar, one will be in a place that celebrates Purim on that day, he is obligated both to hear the Megilla reading the previous night, as well as to hear it again on the 14th during the day.[2] This is true even for a resident of Jerusalem. (Intention to travel elsewhere on Purim itself sometimes makes a difference, but not in this case.) This is deduced from a pasuk[3] that states that people who “are sitting in an un-walled city,” in addition to residents of such cities, observe Purim on the 14th. One who is located in an uninhabited area (including a plane over the ocean) at daybreak of the 14th also reads the Megilla on the 14th, as this is Purim for anywhere that did not have a wall at the time of Yehoshua.[4] If such a person is subsequently in Jerusalem at daybreak of the 15th, the Talmud Yerushalmi[5] apparently rules that he is obligated to hear the Megilla on the 15th (night and day) as well.
Not all agree with this ruling that one can be obligated to hear the Megilla on both days. According to the way that the Korban Netanel understands the Rosh,[6] the Bavli argues with the Yerushalmi and maintains that the place where one is at daybreak of the 14th sets his status and determines on which single day he will be obligated to read or hear the Megilla. Accordingly, Rav Ovadia Yosef says that one who is outside Jerusalem at the beginning of the 14th and returns to Jerusalem by daybreak of the 15th reads the Megilla with a beracha on the 14th but without a beracha on the 15th.[7] Rav Frank[8] goes further, saying that even the aforementioned Yerushalmi required hearing the Megilla on the 15th after hearing it on the 14th only in the case of one who is a resident of Jerusalem or who moved there permanently in between the respective reading times, not for one who merely visited Jerusalem. In any event, we do not know of any opinion that exempts one who is outside Jerusalem on the morning of the 14th from hearing the Megilla on that day due to plans to hear the reading in Jerusalem on the 15th.
The plan to arrange to hear the Megilla when you get to Jerusalem toward the end of the 14th raises a few halachic complications. One is that only someone who himself is obligated in reading on the 14th can read for you on that day. This is based on the idea that someone who is not obligated in a certain mitzva cannot facilitate the fulfillment of that mitzva for one who is obligated.[9] Thus, reasons the Yerushalmi,[10] since a Jerusalemite has no obligation to read the Megilla on the 14th, he cannot read for someone who does have an obligation on that day. Rav Frank[11] cites a minority opinion that the Bavli disagrees with this Yerushalmi, with the logic that the general obligation to read the Megilla along with the concept of responsibility for a fellow Jew’s religious obligations suffice for all Jews to be considered obligated. Although there is some additional basis to claim that all are considered obligated regarding the 14th,[12] the consensus is that the ba’al korei for the 14th should be someone who is obligated that day.[13] (If the reader is from the correct place, the fact that he has already fulfilled the mitzva earlier is not a problem.[14])
Needing to hear the Megilla reading in the afternoon of the 14th will also raise an issue regarding eating, as it is forbidden to eat a meal before fulfilling the obligation to hear the Megilla,[15] both at night and during the day.[16] Although one is allowed to snack before the reading, some say this is permitted only in a case of significant need.[17] Moreover, while a “snack” sometimes means anything less than a k’beitza of bread,[18] some maintain that one should not eat more than a k’beitza of anything.[19] Certainly you would not be allowed to have the Purim seuda, in which you will be obligated, until after Megilla reading. (For mishlo’ach manot and matanot la’evyonim, appointing an agent in advance is likely a wise step.[20])
Due to these complications, most people would probably prefer to avoid a trip such as the one you are planning or to at least try to arrange to read or hear a valid reading from a kosher Megilla on the plane.