Larry,
All is normal here.
<<On the secret switch note, I am one of those High pressure leaks to Low pressure side and sets low pressure switch guys (after some duration + 1 hr). The opening of the dump valve, after the switch down does nothing, allows pressure off the low pressure switch and fires up the pump. That’s why I keep going back to the shuttle valve test. I find mine (once reversed) now in correctly. The idea that both switches are set and anyone CAN NOT get gear extension is beyond my fathom ability (ok, maybe the dump valve handle could fall off). In other words,
if the fluid is leaking to the low pressure side, then why do it not depressurize through the shuttle valve? The pump would have been in the …… (I don’t know?)??? BdBdBdBdBd (finger lapping against flapping lips)>>
I have looked at this in some detail and the phenomenon is quite reproducible. Our gear hydraulics have a few unique design aspects that allow this gear extension problem to occur. BTW, I am curious if you have pressure gauges so you see what is happening during these
events. It really has nothing to do with leaks from high to low as the low side is vented to the reservoir while in flight. In my plane, I can almost predict when the gear will refuse to come down before I hit the switch. If I descend from very high and cold altitudes into warmer air the hydraulic pressure rises. Sometimes even as high as 1,800 psi while the low side remains at zero. All system pressure is held back by a poppet valve that is pushed off its seat by the spool valve. The spool valve is moved via fluid pressure, most often that generated by the pump motor. Here though we have unique situation. With the gear stowed in the wells we have loaded and cocked three fluid pumps ready to push fluid back to the pump. Here is the scenario. You select gear down. The pump starts to turn building pressure internally. This pushes the spool valve against the poppet valve. Now a
torrent of pressurized fluid rushes in. Remember this fluid is also pressurized by gravity acting much like an acculmulator with gas pressure. It will maintain some level of hydraulic pressure over time while fluid is leaving the system. We therefore do not see an instant loss of pressure. The pump cannot remove fluid as fast as the landing gear is trying to push it in. For an instant, the force on the spool valve reverses. If this reversal was long enough, the poppet valve will close again. Now you have a situation where the low pressure switch can become pressurized, with the gear not deployed. Oftentimes the gear will not get stuck, but a very slight hesitation is noticed after selecting down. What is observed in the system is a pressure spike in the low side an instant after the poppet valve is opened by the spool valve. If the spike is high enough, the pump
stops thinking that the gear is down. Note that all this happens extremely fast such that you may never actually know wether or not the pump came on at all. This spike happens to some degree on every gear extension. This can be observed in the cockpit if pressure gauges are installed (which I highly recommend doing) Whether or not the gear will actually get stuck depends on many variables acting in together.
Chris
Chris Zavatson
N91CZ
360std
www.N91CZ.com