Standing and facing the qiblah during an obligatory prayer are two of the essential parts of the prayer, without which it is not valid unless one has an excuse. Among the excuses that the scholars have mentioned in this regard is one who prays in an airplane and is unable to stand or face the qiblah, if he is afraid that the time for prayer will end and the prayer is one that cannot be joined to the prayer that comes before it or after it.

Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) said: If the traveler in a plane wants to offer a nafil (supererogatory prayer), then he may pray in whatever direction he is facing and he does not have to face the qiblah, because it is proven that the Prophet (peace and blessings be upon him) used to pray on his camel no matter what direction it was facing, when he was travelling. But with regard to obligatory prayers, it is essential to face the qiblah and to bow and prostrate if one is able. Therefore if a person is able to pray properly when he is in the plane, then he should do so., but if the prayer whose time has come when he is in the plane is one which may be joined with the prayer that comes after it, such as the time for Zuhr, he can delay it and join it with ‘Asr, and if the time for Maghrib comes when he is in the plane, he can delay it and join it with ‘Isha’.  He should ask the flight attendants about the direction of the qiblah if he is in a plane where the direction of the qiblah is not shown. If he does not do that then his prayer is not valid.

On the other hand, purification is an essential condition of prayer being valid. Wudu is necessary for performing salah and our own personal experience proves it is really easy to make it on planes.