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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
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