What would a diving submarine do to halt its descent?

Answer 1

It depends on the situation. In a controlled descent, Neutral Buoyancy is maintained as sea pressure increases by pumping water out of the trim tanks as/if necessary, and depth control is maintained by the Trim and Dive Planes.

In a casualty situation (such as flooding) the procedure is to go to full speed and head for periscope depth. If the situation is too critical, the Commanding Officer may order an emergency blow of the Main Ballast Tanks. This action dumps compressed air into the full MBT's, forcing water out the openings in the bottom. However, since air is compressed at water depth, it only expands if the boat is ascending - the more it ascends, the more air volume is increased, and the more buoyancy is achieved. If the boat has taken on too much water and cannot maintain a positive ascent, and it is operating in water deeper than the hull limits, implosion will occur at crush depth.

This is just a fact of life working and living on any submarine, and the way to deal with it is that you don't think about it. Fighting the casualty quickly to get to PD or the surface is critical in saving the boat from sinking to a terminal depth, and is the only thing on everyone's mind if a casualty, real or staged, occurs.

Think of it in the same way a plane stalls if it tries to go too high and cannot maintain its lift. At some point, it will lose its ability to maintain flight and fall to the ground. The medium is different, but gravity is the great equalizer in both equations.