For an obligatory prayer the default remains: stand, face the qiblah, and bow and prostrate normally. So where possible, pray the farḍ on the ground before boarding or after landing while its time still remains, or pray in an open space on board if the flight allows, starting towards the qiblah as best you can determine it. If none of that is possible, pray seated: face the qiblah at least for the opening takbīr and stay with it as far as you can, bow with a slight inclination and lower your head further for sujūd, and your prayer is valid — for Allah says, "So fear Allah as much as you are able" (64:16), and He permitted prayer while walking or riding in a state of fear, "And if you fear, then pray on foot or riding" (2:239). As a traveller you may also combine Ẓuhr with ʿAṣr and Maghrib with ʿIshāʾ, which often removes the difficulty altogether. Voluntary prayers are easier still: Ibn ʿUmar reported that the Prophet (ṣallallāhu ʿalayhi wa sallam) used to pray the nawāfil on his mount in whatever direction it was heading, and would pray witr on it, but would not pray the obligatory prayers on it — so a passenger may pray voluntary prayers seated, in any direction.
Q&A · Prayer
How do I pray on an aeroplane or in a car?
Evidence — the daleel
On deen2u
Informational, not a personal fatwa. Consult a qualified scholar for rulings on your situation.