Praise be to Allaah. 

If a traveller who is on board a plane wants to pray a naafil prayer, then he can pray no matter what direction he is facing; he does not have to face the qiblah, because it was narrated that the Prophet (peace and blessings of Allaah be upon him) used to pray whilst riding his camel, no matter which direction it was facing, if he was travelling. But in the case of the obligatory prayers, it is essential to face the qiblah and to do rukoo’ and sujood if possible. So if a person is able to pray on board the plane, then he should do so. If the prayer which falls due whilst he is on the plane can be joined with another prayer later on, e.g., Zuhr, which he can delay and join with ‘Asr, or Maghrib which he can delay and join with ‘Isha’, he can do that. He has to ask the stewards about the direction of the qiblah if he is on board an airplane which does not have a sign to show the direction of the qiblah. If he does not do that, then his prayer is not valid.