Salaah on an Aeroplane
Important laws and guidelines from Jamiatul Ulama KwaZulu-Natal.

Requirements for Salaah
For the validity of Salaah, certain conditions and actions are necessary. For example:
Facing the Qiblah
Qiyaam (performing Fardh & Waajib Salaah standing)
Making Sajdah (by placing the forehead on the ground).
It will be necessary for one to carry out these actions / fulfil these conditions even when performing Salaah on the plane.
Arranging space with the crew
It is necessary to try one's best to do so. If there's a need, one must request the air crew to facilitate the same.
If prevented from standing and facing the Qiblah
However, in the unfortunate event where one was prevented by the crew from performing Salaah by standing & facing the Qiblah, only then will one be allowed to perform one's Salaah on the seat.
In this case, one will perform Salaah on time, even though one may not be facing Qiblah. One will perform the Ruku by bending forward slightly and bending forward more than that for Sajdah.
The Salaah which was performed on the seat in this manner, due to the crew not allowing one to perform Salaah standing, will have to be repeated once one lands.
Source & references
By Jamiatul Ulama KwaZulu-Natal, South Africa.
References: Imdaadul Fataawa, Fataawa Mahmoodiyyah.