How can I make him fall if the up key isn't being held? AS2?

Here is the code of my guy that will jump and (even if you let go of the key) will fall down after a certain period of air time. if(Key.isDown(Key.UP)) { jumping_var = true } if (jumping_var == true) { this._y +=jumpSpeed_var; jumpSpeed_var +=1; if (this._y >=300) { jumping_var = false; jumpSpeed_var = -20; } How can I increment it so that when you let go of the key he starts too fall? I tried by using if(!Key.isDown(Key.UP)) but that didn't work.. Please help :)