CODE Oil from PGB pump HP and OSG oil Drain oil Pitch change oil Auxiliary feather pump oil Pressure switch P Propeller gearbox HPP/OSG Beta feedback Torque motor Pitch control unit Drain Drain AFP Drain Drain Drain Gearbox oil supply 180-210 psi 1100 psi Governor Drain HP pump Relief valve Drain R1 R2 R3 R4 Feather valve NRV A NRV B Pitch change  press Pilot press FLT fine Ground fine Coarse Coarse Fine P (Energized) (Deenergized) Servo valve Unfeather solenoid Unfeather valve GBEV solenoid Ground beta enable valve Note: AFP(Auxiliary feather pump) SOL SOL Blade Prop Counterweight Figure 7-48.  Propeller control system schematic.  the counterweight force equals the spring tension of the  speeder spring. The governor counterweights are then on  speed. When the engine output power is increased, the power  turbines tend to increase speed. The counterweights in the  CSU sense this acceleration and the counterweights go into  an overspeed condition because of the increase centrifugal  force. This force causes the control valve to move up and  restrict oil flow to the propeller dome.  [Figure 7-50]  The  feathering spring increases the propeller pitch to maintain  the selected speed. Reducing power causes an under-speed  of the counterweights, downward movement of the control  valve, more oil in propeller dome, resulting in a lower pitch  to control propeller speed. The propeller governor houses an  electro-magnetic coil, which is used to match the rpm of both  propellers during cruise. An aircraft supplied synchrophaser  unit controls this function. At low power, the propeller and governor counterweights  do not turn fast enough to compress the speeder spring.  [Figure 7-51]  In this condition, the control valve moves  down, and high pressure oil pushes the dome forward moving  the blades towards low pitch. Any further movement pulls  the beta rod and slip ring forward. The forward motion of  the slip ring is transmitted to the beta valve via the beta lever  and the carbon block. Forward movement of the beta valve  stops the oil supply to the propeller. This prevents the blade  angles from going any lower. This is the primary blade angle  (PBA) and is the minimum blade angle allowed for flight  operation. From this point, the propeller is in the beta mode.  If the engine power is reduced when the propeller is at the  primary blade angle, the propeller speed decreases since the  blade angle does not change. The lock-pitch solenoid valve prevents the propeller from  going into reverse or below the primary blade angle in the  event of a beta system malfunction in flight. The solenoid  is energized by a switch (airframe supplied) mechanically  connected to the propeller slip-ring linkage via a second  carbon block. As oil pressure leaks off around the propeller  shaft oil transfer sleeve, the blade angle slowly drifts back  toward high pitch. This deactivates the low pitch solenoid  valve and restores the oil supply to the propeller servo. The  low pitch solenoid valve cycles (close/open) as backup to the  beta valve function. Moving the power lever backwards causes  the reversing cam and cable to move the beta valve backward,  allowing more oil to flow into the propeller dome, and causing  the blades to go towards reverse pitch.  [Figure 7-52] As the blades move to reverse, the dome pulls the slip ring  forward and moves the beta valve outward, restricting the  oil flow. This stops the blade movement toward reverse. To  obtain more reverse thrust, move the power lever back more  7-33