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Gerard,
Failure to extend is actually a very different animal. The hydraulic circuits are not at all the same in the retracted and extended states. When the gear is retracted, the spool valve opens up a low resistance path from the down circuit back to the reservoir. This is to return the rod volume back to the tank. By doing this it makes the low side incapable of retaining any pressure. If a leak occurs from high to low the fluid simply drains into the reservoir. This assumes of course that the spool valve is installed correctly. If it is backwards the low side will bump up against the low side pressure relief valve during retraction.
Even with the spool valve installed correctly and the low side circuit at 0 psi, it is still possible to have the gear fail to extend.
The pump retains pressure in both high and low circuits with check valves at the outputs. Under normal operation, one check valve is closed while the other is held open by the spool valve. When the pump direction is reversed, the spool valve is pushed to the opposite side, allowing the currently open check valve to close and forcing the opposite check valve open. This takes quite a push. It must force open the check valve against the fluid pressure of the hydraulic circuit currently in use. The unique aspect during extension is that the fluid in the high side circuit is subjected to the potential energy stored in the landing gear. As soon as the spool valve cracks the high side check valve, a large volume of hydraulic fluid surges back into the pump pushed along by three gear legs being pulled down by gravity. If this surge is greater than the speed at which the pump can deliver the fluid back to the down side, the spool valve will see a momentary force reversal. I am guessing that the spool valve is pushed back just far enough to allow the high side check valve to close off. Now the fluid entering the down circuit has something to push against and the low side circuit operating pressure is reached very quickly. The pump shuts off even though the gear is not down. All this happens very quickly, in a fraction of a second.
Unlocking the system is then best accomplished by cracking the dump valve. Even though the entire system may now be isolated from the pump with both internal check valves closed, the flow of fluid from the high to low side through the dump valve lowers the pressure on both sides since rod volume is being extracted from the system. The pump fires up again and the gear continues on its way down.
This can all be observed with pressure gauges on both the high and low sides. There is a momentary pressure spike in the down circuit immediately after selecting gear down. When the spike is large enough it will trigger the low side pressure switch and the pump will stop. This is most likely to occur after descending into warmer temps after a long flight at altitude. The cylinders warm up and the pressure in the high side circuit increases prior to gear extension.
Chris
Chris Zavatson
N91CZ
360 std
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