Q: Can I sit and perform Salaah in an aeroplane? Do I need to repeat my Salaah afterwards?
A: If you are able to stand and face Qibla whilst aboard in an aircraft, then no, it will not be permissible for you to sit and perform Salaah. One should endeavor to ascertain the correct direction of Qibla and find a suitable place on the aircraft and perform Salaah standing and facing towards the Qibla.
As a precautionary measure, it would be preferable that when you reach your destination you repeat your Salaah as there is a difference of opinion amongst the Scholars with regards to whether all the conditions for the validity of Salaah are established whilst aboard in an aircraft.
Alternatively, if you are unable to stand and face Qibla due constrains in space or for any other reason and do not hope to land before the termination of that Salaah time, then yes, it would be permissible to sit in one’s seat and perform Salaah. Naturally in this instance, it will be necessary to repeat the Salaah when reaching your destination. (Fataawa Mahmoodiya-Farooqiya V7/ 53)