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Pump Unloader Valve
Part of hydraulic system 3. Commanded by SPDA 2. Reduces the discharge pressure of ACMP 3A during its start-up. This reduces pump torque and therefore the electrical power required to start the pump. Only used in an electrical emergency to avoid overloading the RAT. Closes after the RAT begins supplying power to the AC ESS Bus, and the ACMP 3A starts. System pressure then becomes normal.
Note that in a dual engine failure, ACMP 3A turns off initially when the IDGs go off-line, and the system accumulator provides power to the flight controls on system 3 until ACMP 3A is restarted.