The balance piston system consists of the balance piston disk, the balance piston casing, their associated seals, and the domed shaped cavity, formed by these parts. This cavity is pressurized by the stage 11 high pressure compressor bleed air, controlled by a modulating valve mounted externally on the turbine rear frame. The balance piston casing is attached to the aft inner half of the turbine rear frame. The balance piston disk is attached to the low pressure turbine shaft. Thrust is monitored by a total-pressure probe (P48) and static-pressure probe (PS55).