Unstoppable Propulsion

You alter a pair of boots, shoes, or other footwear and small propulsive clouds form under the soles.  For the duration of the spell, the wearer of the boots has a flying speed of 45 feet, but they cannot descend more than 10 feet in a turn.  At the end of each of their turns, they must roll a d20.  On a 10 or lower, the magic fades and the spell ends.  You cannot have more than one instance of this spell active at a time, if you do both instantly vanish.