Is it permissible to pray Dhuhr and Asr combined and shortened a little before the plane takes off, knowing that the time of takeoff is the time of Dhuhr prayer in the city from which the plane departed?

A traveler is permitted to combine prayers, but it is not valid to pray before its prescribed time, by the consensus of Muslims. Such a prayer must be repeated if performed. If the prayer time begins and the traveler on the plane knows that they will not land until after the time expires, then they must pray on the plane according to their ability, standing, bowing, prostrating, and facing the Qiblah. It is not permissible for them to pray sitting if they are able to stand.