Answer: One should perform his/her Fardh Salaah standing up on the plane. However, if this is not possible due to dizziness then will perform Salaah whilst seated whilst making Ruku and Sajdah. It is incorrect to perform Salaah on one’s seat. In the case where one does, Ulama recommend that it should be repeated as a precaution. (Fatawa Darul Uloom Zakariya vol 2 pg 111, Fatawa Sirajiyah pg 79)
Note: One should be courteous of others whilst praying Salaah on the plane. Experience has proven that by politely asking the air hostess, one is easily accommodated to perform Salaah.