Hi Guys
In the stock game you have no control of the dive planes.
The problem is probably a stock game bug but it is not one I have come across before.
It maybe a game corruption.
I would recommend uninstalling and re-installing.
Use a registry cleaner before reinstalling
The uncontrolled sinking when you reach a certain depth is a bug that is fixed in TDW's Generic File Patcher.
You can fix that problem here if using Steam.
http://www.subsim.com/radioroom/showthread.php?t=179746
Open Horizons II is a campaign mod
Post #64 Open Horizons II > Campaign Dates > Bases > U-Boats
Peter