Technically, a commercial jet can stay still in the air provided we cause the wind to blow at it from the opposite direction at over 600Kmph. This wind must blow towards the nose and through its wings to cause it to get lift force and thus not make any displacement relative to the ground. This wud means it has zero ground speed. An aircraft has two speeds namely groundspeed and airspeed. Airspeed is measured by the amount of air rushing thro' its sensors placed at various points on its outer body. This mechanism and physical aspect of flight is beyond the scope of this discussion and many of you won't even be able grasp the idea.