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This is the PA-28R-201 landing gear hydraulic system diagram.

I am wondering how this shuttle valve functions. Research I have done indicates that a shuttle valve blocks one attachment to the valve while allowing another to run through. Only one side or the other, not both or neither.

In the diagram below, everything beneath the shuttle valve is a closed system (other than the thermal relief).

Clearly, the valve cannot be closed on one side or the other or else the pump would be applying pressure through three pistons to a closed valve at the other end of the loop.

Is this shuttle valve different than what I have read them to be? Is it perhaps in actuality that both sides are either open at the same time or both are closed at the same time but never one closed while one is open? If one remains closed, how could the pump move fluid through the complete circuit?

enter image description here

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  • $\begingroup$ Can anyone find a better diagram of a shuttle valve that is representative of this particular one? I think it's really bad. I know there are several types of shuttle valves, so there is not some catch-all, but looking at this diagram, and not having a mechanical background, I wouldn't know which type to even expect this to be. From where I'm sitting it almost looks like both sides would always be blocked off... $\endgroup$
    – Ryan Mortensen
    Jun 5, 2019 at 1:00
  • $\begingroup$ After doing more research, I think the premise of this question is based on and generated by a bad schematic. Based on the service manual, the schematic shown in the second answer below is the same system. In that schematic, it is clear what it is doing. $\endgroup$
    – Ryan Mortensen
    Jun 7, 2019 at 4:17

2 Answers 2

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The shuttle valve is actuated by high or low pressure depending on what you select, gear up or down. It routes the hydraulic fluid to the bottom of the cylinders to raise the landing gear with high pressure, or to the top of the cylinder to lower them with low pressure and to allow the fluid to return to the reservoir.

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Your main problem is that the schematic is a completely useless representation. All the shuttle valve does is connect the extension manifold to the reservoir, when the system isn't pressurized by a selection, to allow the extension side of the actuators to pull fluid directly from the reservoir instead of from the retraction side when the gear is free falling during emergency extension.

It's necessary because there is more actuator fluid volume on the extension side of the actuator than the retraction side, because there is no piston rod taking up space on the extension side. This means you can't just connect the extend and retract manifolds directly together in a closed loop to let the gear free fall.

When the system is pressurized by either an extend or retract selection (which controls pump motor direction), the shuttle valve blocks the direct connection between the extension manifold and the reservoir, and incorporates the extend manifold into the pressure/return loop so fluid can run to or from the pump from both ends of the actuators.

In the diagram below, the box and line in GREEN is the valve position and fluid path with the system unpressurized. The box and line in RED is the fluid path and valve position when the system pressurized by an UP or DOWN selection (the reversible pump running). That's all the shuttle valve does: no selection, extend side of actuators directly routed to reservoir; UP or DN selection, extend side of actuators incorporated into the pressurized hydraulic circuit

enter image description here

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  • $\begingroup$ The OP's schematic is essentially identical to yours... $\endgroup$
    – Juan Jimenez
    Jun 4, 2019 at 18:15
  • $\begingroup$ The part circled in red? Are you serious? $\endgroup$
    – John K
    Jun 4, 2019 at 19:17
  • $\begingroup$ This one looks even more confusion overall, but I think I see a piston that would push the check valve open. So one direction, the check valve opens by virtue of the oil pressure, and on in the other direction a piston pushes it open from the low-pressure side. $\endgroup$
    – Ryan Mortensen
    Jun 4, 2019 at 22:30
  • $\begingroup$ The piston (the box) with the spring is the valve. Whenever the pump is running in either direction, system pressure pushes it to the left against the spring, connecting the pressurized system to the extend manifold directly under it. When the system not pressurized, the spring pushes the valve to the right and connects the extend manifold directly to the reservoir. I've added stuff to the diagram. $\endgroup$
    – John K
    Jun 5, 2019 at 0:47
  • $\begingroup$ @JohnK that last comment was actually referring to the "Gear up check valve". At first, I thought, "why is there a check valve it it opens for both directions of flow anyway?" Then I realized there is a second shuttle or piston-like object that forces that check-vale open, but probably at a higher pressure do to the opposition of the spring and pressure on the check valve from the right side of the diagram. $\endgroup$
    – Ryan Mortensen
    Jun 5, 2019 at 0:56

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