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 Zavatson
, CA