You should give vertical power impulse.
And constant downforce if height over earth is bigger then 0.

while(blabla)
{
result=trace(my.pos,vector(my.x,my.y,my.z-2000));
if(result>1){gravity.z-=1*time_step;}
else{gravity.z=0;}
//jump power
if(result=<0&&key_space==1)
{gravity.z=10;}
c_move(my,standart_movement,gravity,blabla);
wait(1);
}

I wrote it very fast so here is main idea.
But it works.
If result dont work well with 0 then use 1.