بسم الله الرحمن الرحيم
If your plane lands at a time that you may offer salah easily on ground then you should offer salah after you get down from the plane. However, if you fear that your salah will be missed then you should offer it in the plane. If you feel too weak to stand, and offered Salah facing the direction of Qiblah and sitting on floor (away from seat) then even your fardh salah will be valid and you will need not to reoffer it. In case you offered your salah sitting while you were able to offer salah standing then your fardh salah will not be valid.
Allah knows Best!
Darul Uloom Deoband
This answer was collected from the official ifta website of Darul Uloom Deoband in India.