Have made a flying character in 3rd person mode but how I decrease height of flying constantly while flying?

[azurelevelblue]

I want to make wingsuit gliding game.I can controll character and fly it in any directions but I want it to just fly while decreasing the height constantly until it lands.How can this happen?
I there is a range of flying and I can fly max height and low height and that range constantly is decreasing until ground level?
How can i do this