I am trying to get the altitude hold working with little success. It seems that no matter what changes I make to the THR_ALT and THR_Rate parameters, the end result is the same as before the changes were made. When the altitude hold is activated, the quad will start settling and eventually touch the ground. I have tried this from 50' down to 10' with no change in behavior. Changing the parameters to the point of oscillation causes the quad to oscillate a little lower each time until ground contact. I am pretty new to this, so I could be missing something simple, but it seems that the altitude hold is just not working, is not sensitive enough or the program is not compensating for battery drain etc. I have the barometer covered per the discussions also. Do I maybe need to add an  external barometer? I know that at lower altitudes I do need to add the sonar.