... what three Army aircraft? ___ A. UH-1H, AH-1S, and OV-1B. ___ B. UH-1D, AH-1H, and OV-1. ___ C. OV-1A, AH-1G, and UH-1E. ___ D. OV-1, AH-1G, and UH-1C. 4. What type of pump is often used ... valve consists of a casing, inlet and outlet ports, and a ball -and- spring assembly. The ball and spring permit full fluid flow in one direction and block flow completely in the opposite direction. ... extend and retract the cylinder and one OFF position. Hydraulic systems are classified as either open-center or closed-center. Open-center systems have only open-center selector valves and closed-center...