This little article was published in the Lancair Network News several years ago. It describes the installation of landing
micro switches on the over center links rather than (or in addition to) mounted
to main gear bracket.
3 Green and Landing Gear Micro
Switches
When
flying aircraft with retractable gear, one of the greatest moments of
anticipation
occurs while you wait for those three little green lights to
illuminate
upon selecting gear down. I don't know what the GA rate of gear
system
failure is, but I have had five gear incidents in my 300 hours of retract time: One was
in a certified plane, four in experimentals, three of
which were in my 360. Only in the certified plane was there a
true system failure, the others where all indication
problems. The three incidents in my 360 where traced to the
method by which the down and locked position is sensed
by the micro switch on the main gear. To determine if
the links
have actually gone over center, one needs to detect the position of
the two
links relative to each other. On the 360, the micro switches monitor
this only
indirectly. They are positioned such that they measure the relative
position
of the upper over center links to the aircraft structure. Assuming the
relative
position of this link to the fuselage is always the same on all axes
when the
gear is down and locked, this approach would be quite adequate and
simple to
install. In our case, a roll pin in the link depresses the lever on
the micro
switch. The problem with this set-up lies in the fact that both ends
are
supported by bearings that allow rotation about a line drawn between the
bearings.
Rotation about this line, when the gear is down, will raise and lower
the
position of the roll pin relative to the micro switch. A large washer on
the
mounting bolt of the upper over center link is used to prevent this rotation
or at
least most of it. If you adjust your micro switches to trip only when the
links are
truly over center, not much movement is required to indicate an unsafe
condition.
This little bit of movement can occur by forcing the linkage to
rotate as
described above. The quickest remedy is to adjust the switches such
that they
activate a little earlier, possibly just before the links go over
center. By
doing this, any amount of pushing or pulling on the linkage will not
be able to
give a false unsafe condition. The draw back is that you get green
lights
while the gear is not yet locked. From a practical standpoint, if the
gear has
gone down 95% of the way to trip the switch a little early, chances are
it will
travel the remaining 5% and actually lock.
For some time I toyed with the idea of mounting the
switches in such a way that they could only activate
when the links were truly over center, completely
independent
of the above effects. I like solutions that are inexpensive, simple
to build,
and reversible. That way if it doesn't work I haven't lost a lot of
time or
money and I can have my old parts back. The enclosed picture shows the
set-up I
am now testing. It only requires an aluminum plate with four
properly
located holes and some hardware. I removed the levers from some spare
switches,
essentially converting the switches to push button operation, and
mounted
them on the upper links using the aluminum plates. The actuator is an
AN3 bolt,
some washers and a nut installed in an existing hole in the lower
link. This
configuration only shows green when the links are truly over center.
It
is very precise and makes the locked indication completely independent of
any
movement or rotation of the linkage relative to the airframe when the gear
is down. I
have not removed the original switches or wiring, but instead added
an
additional set of lights for testing this new set-up. The entire landing gear system, as designed, is
mechanically simple which helps make an actual system
failure a rare event. During my false indication
incidents,
I trusted the gear more than the indication, but I took the
opportunity
to buzz the control tower anyway.
Chris
Woodland