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PTU uses system A pressure to drive a hydraulic motor. The hydraulic motor of the PTU unit is connected through a shaft with a hydraulic pump that can draw fluid from the system B reservoir. The PTU can only transfer power and cannot transfer fluid. The PTU operates automatically when all of the following conditions are met: 

• System B EDP pressure drops below limits. 

• Aircraft airborne. 

• Flaps are less than 15° but not up. 

