What is the considered time for performing the Fajr prayer for a traveler on an 8-hour plane journey: the time of the country of departure or the time of the country of arrival?

The validity of prayer hinges on verifying that the prayer time has begun. A traveler on an airplane cannot rely on calendars because there isn't a calendar that specifies prayer times for their current location. Therefore, they must verify the beginning of prayer times using the known signs. As for Dhuhr and Asr, they should strive to determine their times, seeking assistance from the airline crew if possible. Whoever exerts effort (ijtihad) and prays is not required to repeat the prayer unless it becomes clear that they prayed before the time; otherwise, their obligation has been fulfilled.