A diving plane, also known as a hydroplane, is a control surface found on submarines which allow the vessel to pitch its bow and stern up or down to assist in the process of submerging or surfacing the boat, as well as controlling depth when submerged.
Diving planes function in much the same way as an aircraft's elevator.
Diving planes are usually fitted in pairs, the bow planes at the front of the submarine and the stern planes at the rear.
Diving planes located on the sail (conning tower) are called fairwater planes on US Navy submarines. Fairwater planes do not pitch the ship up or down, they cause the ship to rise or sink on a level plane as they are operated. Newer ships, starting with the third-flight Los Angeles class subs (or 688i's) have eliminated the sail planes, and operate instead with bow planes.