[The ruling on one who broke his fast, then boarded a plane and saw the sun]

Answer: The fast of the person mentioned is valid. He fasted the entire day and only broke his fast after completing it. Seeing the sun afterwards does not invalidate his fast, because in the town where he fasted and broke his fast the sun had set, the day had ended, and Allah had permitted him to break his fast. Allah the Exalted said: “Then complete the fast until the night.” [al-Baqarah:187]. This fasting person did what Allah commanded.
The scholars of legal theory said, in defining “valid”: it is “that which accords with the command of the Lawgiver.”
If it be said: Should he pray Maghrib while the sun has not set?
We say: If that man who broke his fast in his town had prayed Maghrib before the plane took off, then his prayer is valid, because he prayed it in its time. But if the plane took off before he prayed Maghrib, then he should not pray it so long as the sun has not yet set; for Maghrib is only valid within its time, and its time is after the sun has set.
If it be said: If he had prayed Maghrib before the plane took off, then it took off and he saw the sun—must he pray Maghrib again?
We say: He is not required to pray Maghrib again. What Allah has prescribed upon the Muslim is five prayers in the day and night; and in the ḥadīth: “There are no two ẓuhrs in a day.” There is no disagreement about that.
Source : Min Thimār al-ʿIlm wa al-Ḥikmah vol.1