CODE Supply pressure Return to pump Propeller servo pressure Py Governor spring Hydraulic low pitch adjustment Carbon block Reset post Propeller speed control lever Minimum govenor adjustment Beta valve Pressure relief valve Pilot valve Engine oil CSU pump Beta rod PT6A-67B with fixed propeller speed no movement Reset arm no movement no movement Figure 7-50.  Governing mode.  puts more load on the engine and slows down the propeller. To  test the unit, the speed reset solenoid is activated, and servo  oil pressure pushes against the reset piston to cancel the  effect of the reset spring. With less spring tension acting on  the counterweights, the overspeed governor can be tested at  speeds lower than maximum. On twin installation, a second solenoid valve is mounted on  the overspeed governor and is used in conjunction with the  aircraft autofeather system. The system is switched on for  takeoff and, in the event of an engine malfunction, energizes  the solenoid valve to dump propeller servo oil into the  reduction gearbox sump. The feathering spring and propeller  counterweights move the blade quickly to feather. Many of the hydromatic propellers are used with older type  aircraft involved in cargo operations. A hydromatic propeller  has a double-acting governor that uses oil pressure on both  sides of the propeller piston. Many larger turboprop systems  also use this type of system. The governors are similar in  construction and principle of operation in normal constant- speed systems. The major difference is in the pitch-changing  mechanism. In the hydromatic propeller, no counterweights  are used, and the moving parts of the mechanism are  completely enclosed. Oil pressure and the centrifugal twisting  moment of the blades are used together to turn the blades  to a lower angle. The main advantages of the hydromatic  propeller are the large blade angle range and the feathering  and reversing features.  This propeller system is a double-acting hydraulic propeller  system in which the hydraulic pressure (engine oil pressure)  on one piston dome is used against governor oil pressure on  the other side of the piston. These two opposing hydraulic  forces are used to control and change blade angle or pitch.  Although hydromatic propeller systems are very old, some  are still used on radial engines. Larger new turboprop systems  also use this opposing hydraulic force and double-acting  governor systems.  7-35 Hamilton Standard Hydromatic Propellers