When I take control on helicopter from gunner position , I'm trying to make it hover in one position , so ill be able to shoot . But as soon as I'm release control ....it starting descending.  
If you playing alone ...you need to be able to command to pilot what to do. If you need move forward..."move forward" ...if you need hover at the same place and don't move ... give him such a command . Or if you release control at list leave it in same position where it was .