REFLECTOR: The Saga of the Gear Circuit

[Al Gietzen]

When the Swings designed the retract system for the Velocity, they put in
the hydraulic pump, some pressure limit switches, and a couple of relays.
The gear went up and the gear went down.

 

Recognizing the importance of knowing the gear was down for landing, they
decided it would be good to have some indicator lights that would confirm
the down and locked position.  Everything was fine.

 

Then somebody said, "Well, suppose that the pilot doesn't do the landing
check list, and doesn't notice that the plane isn't slowing down very well,
and then forgets the "GUMPS" check on downwind that we are all taught when
flying complex airplanes, and, besides that; doesn't confirm that there are
two greens on final."  That would be pretty bad piloting, but they decided
to add a warning horn.  That took a couple more switches, and quite a bit
more wiring.

 

Then somebody said, "Gee, I can't always hear when the gear pump is running,
so how can I tell if the gear is actually going up or going down after I
flip the switch?"  So some more wires and another indicator light were
added.

 

Then somebody said, "You know, sometimes, under certain conditions, when you
switch to 'gear down', nothing happens."  We all learn that this can happen
if the pressure is high on both sides of the hydraulic system, and can be
easily remedied by a quick pull on the gear dump valve, which is already
within handy reach.  But someone decided we should add some more wires and a
'Gear reset' button to, I guess, make it easier.

 

Then someone said, "Sometimes the gear warning horn that was added to save
us from bad piloting goes off when I don't want it to.  How can I shut that
thing up?" So then some more wires, another switch, and another relay were
added for a "horn reset" circuit.

 

Then someone said, "What if we accidentally have the 'Master Switch' on, and
then accidentally switch the gear switch to 'gear-up' when we are on the
ground?  That would be a serious problem."  So then some more wires and an
'airspeed' switch were added so the gear switch (which can't just be bumped
but has to be switched deliberately) can't make the gear go up unless the
plane is flying.

 

Oh, yeah; and while you're at it, if you don't want the Hobbs meter to run
until you are actually flying, we can wire that into the gear indicator
circuit so it doesn't turn on until the gear is up.

 

And then I thought of using the airspeed switch in place of the 'low
throttle' switch for the gear warning horn circuit, which can add a another
DPDT relay and a bit more wire.

 

I think you get my drift. It may be amusing, or it may make one consider the
meaning of 'reliability'. For those still building and haven't yet installed
all this circuitry, you might want to consider it a bit, or at least install
all the gear wiring early in the process; maybe before you put the top on
the fuselage.  I installed the basic operating circuit years ago, but didn't
start installing all the ancillary circuitry until a few days ago when lots
of other wires and things were in the way, and didn't consider the
complexity until I had most of it done yesterday.

 

I now have a better understanding of why some folks choose fixed gear; and
the meaning of P.V.O.T.  It almost sounds like a design done by the
government (well, I suppose by any bureaucracy).

 

Al

 

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