Wudhu of one who sits and sleeps in a plane or car seat

In regard to sleeping while sitting, the ruling in the Hanafi Mazhab is that if one sleeps while sitting in such a way that his/her buttocks are firmly placed on the ground, then the wudhu will not break, even if one has a dream. However, if one sleeps for a long time, then it is better for one to renew his wudhu.

And Allah Ta'ala (الله تعالى) knows best.