Praise be to Allah.
If the sun sets and he breaks his fast on the ground, then when the plane takes off he sees the sun, he does not have to refrain from eating and drinking, because he has completed his fast in full, and there is no need to repeat the act of worship after he has finished it.
If the plane takes off before sunset, and he wants to complete that day’s fast whilst traveling , then he should not break his fast until the sun sets in the place where he is in the air.
It is not permissible for the pilot to take the plane down to a level where the sun cannot be seen so that the people can break their fast sooner, because this is a kind of trick. But if he descends for a valid technical reason and the disk of the sun disappears, then he should break his fast. (From the fatwas of Shaykh Ibn Baz. See the booklet Sab’un Mas-alah fi’l-Siyam (Seventy Issues related to Fasting).
The Standing Committee said:
“If the fasting person is on board an airplane and he finds out through the clock or the telephone that a nearby city has broken the fast, but he can see the sun because the plane is high up, then he is not allowed to break his fast because Allah says (interpretation of the meaning): “then complete your Sawm (fast) till the nightfall.” [al-Baqarah 2:187]
This does not apply in his case so long as he can still see the sun.
But if he broke his fast in that city after the day ended for him, then the plane took off and he saw the sun, then he should continue to break his fast because he comes under the ruling of the city from which he took off, where the day ended whilst he was still there.”
In another fatwa, the Standing Committee said:
“If a person is in an airplane during the day in Ramadan , and he is fasting and wants to continue fasting until night-time, it is not permissible for him to break his fast until the sun has set.” (Majmu’ Fatawa al-Lajnah al-Daimah, 10/136-137)
And Allah knows best.