Answered by Shaykh Faraz Rabbani
Question: What is the ruling of praying on a plane? How does one determine the time for prayer and does one have to pray standing?
Answer: In the Name of Allah, Most Merciful and Compassionate
It is obligatory to pray on the plane. When it is reasonably possible for a person to pray standing or sitting (with actual prostration) then he must do so.
The prayer time should be determined by making a reasoned estimate based on one’s location. [Ibn Abidin, Radd al-Muhtar; Tahtawi/Shurunbulali, Hashiyat Maraqi al-Falah] Determining Maghrib is straightforward as it is when the sun has completely set below the horizon. Isha is when it is totally dark. Fajr is when light appears on the horizon. Zuhr time does not vary much from one place to another. Asr is when the shadow of something is equivalent to two full shadow lengths, and one should pray it half-way between Zuhr and Maghrib to be safe.
And Allah alone gives success