The last you'll reed on the subject of reed switches

I can understand the perceived need for some kind of electrical interlock system for these wing pins. What I can't understand is why Van's chose this inopportune moment to stray from the KISS Principle. If it was good enough for Kelly Johnson, it's good enough for me. Magnetism is fine for most of the things it's used for, but I think this particular application was too clever by half. This really should have been done using the same type of mechanical switch used to trigger the stall warning.

But as I noted before, one must press on in the face of magnetic adversity; I had to get these things working. I diddled around with the positioning of the little reed switch, I fiddled around with the positioning of the wing pin. I eventually let the wing pin plunger snap back too quickly and the epoxy that was holding the magnet inside the plunger gave way and the magnet came out. This was actually a good thing - the indications that I was seeing on the multimeter made it look as if it was possible that the magnet wasn't all the way down at the bottom of the plunger.

I decided that rather than just gluing the magnet back in, I'd put the pin in the plane and move the magnet around inside the plunger to see if I could find a positioning that worked. I cleaned up the parts (remember that I had wiped a little grease on the plunger to help it move more freely in the wing pin) and carried the parts over to the plane. I put the wing pin in. Nothing on the multimeter. I pulled it back out of the hole to do the simple always-works test to make sure the wires were all still correctly attached to the multimeter.

Nothing.

Hmmm.

I checked the wires and they were all fine. I checked with the other wing pin and it worked too.

Hmmm.

I decided to pull the magnet back out of the plunger and try using it directly against the switch.

No magnet.

Uh-oh! I'd lost the magnet!

It couldn't have gotten too lost though; I'd only gone four or five feet from the work bench to the airplane - that should be a pretty manageable area.

Twenty minutes. It took twenty minutes to find that magnet. I was at the end of my rope, convinced that I had looked everywhere that the magnet could have gone.

Then I turned over my drill.

With that problem solved, I returned my attention to the bigger issue: the switch still wouldn't work. I had been shaving material off of the wing pin plunger tube in tiny little increments up to this point, but this was the last straw. I decided to grind away the full 1/8" allowed to me by law.

That fixed it. I did the same to the left side, and it worked too. I'm not sure why I didn't just do that in the first place.

The next page was the installation of the autopilot disconnect switch. While I may not have held the wing pin reed switches in very high regard, I have the utmost respect for the AP disconnect switch. Should the autopilot ever decide to play the fool and run away with the control of one of the flight controls that I feel it is my right as pilot to always have the final say on, I want this switch to put such an affair right. This switch is so important that the autopilot won't even engage if it isn't working correctly. It is this switch that cost me a few trips to Lancaster to try to debug an autopilot problem on a friend's RV-12. The first few hours of that job were spent trying to prove the switch was actually, well, switching. After that experience, I thought it prudent to test the switch before installing it.

It's good!

There's a tiny pilot hole in the panel that has to be reamed out to half an inch. That's just the job for the Uni-bit. With the morning that I had already had, it came as no surprise when the battery on the drill died on the very last step of the Uni-bit.

Once I got just enough charge on the drill's battery to finish the hole, I installed the switch. The wiring of the switch is a little tricky. It has four wires coming out of it; two of them are pretty short. Those get crimped into a splice with a couple of longer wires. Their limited length is such that they barely make it though the avionics shelf bushing. It's a good thing I left room in the aft bushing!

The other ends of the splices get long white wires crimped in, and those wires get fed all the way back to the wiring duct. I don't need those wires - they're for the optional lighting kit that I won't be buying. They provide power to the cockpit lighting that I won't have.  At least not for $1,300 anyway.

"Say," you're asking yourself, "what does the autopilot have to do with cockpit lights?"

A fair question. As I understand it, the Dynon screen that will provide most of the flight and engine management displays has an auto-dim feature that causes it to reduce its brightness in the dark so as to not ruin the pilot's night vision. The autopilot disconnect switch has a little red light in it that will also dim when the Dynon dims. It knows to do this, apparently, from the two colored wires that are spliced in. The white cockpit light wires are also merged into this circuit so that when the Dynon dims, so do the cockpit lights.

Clever, that, and I'd be enormously impressed, if I was buying the lighting kit. I may change my mind someday, so it makes sense to run the wires now. I'll just coil them up somewhere out of the way until such time as they're needed.

The next step of the autopilot disconnect switch is to plug the other, much longer two wires into the big DB-25 connector that will itself plug into the avionics control box. These wires are what were ultimately found to be the problem on that other RV-12 - they were plugged into the wrong holes. That was unfortunate because it required the use of a pin extraction tool to get them out of there. Not knowing how to use a pin extraction tool, I made myself scarce once the need for said tool was discovered.

I swore that I'd get these wires in the correct holes so as not to ever need to figure out how to use a pin extraction tool. The black and white wires on the outside of the black shrink wrap are the wires in question. I'm nearly 100% certain that I got them in the right holes.

"Wait, did you say outside of the shrink wrap??"

Well, yes. Yes I did.

So, here's what the pin extraction tool looks like in use:

And here are the wires correctly installed and inside the shrink wrap:

And here is the shrink wrap, shrunk:

Shrink wrap is pretty neat stuff. You just apply some heat (I have an electric heat gun, but a blow dryer would work) and it tightens up into a nice protective tube.

Which then gets installed into what's called a clamshell or backshell:

Next comes the installation of the Outside Air Temperature (OAT) probe. This probe will tell the flight computer what the temperature is outside of the airplane. That will be used by the computer to calculate true airspeed. In the world of GPS, where we know to a tenth of a mile per hour how fast we're moving across the ground, true airspeed is actually just shy of useless when it comes to any kind of utility in flying, but knowing the temperature outside of the airplane is useful in other ways.

What's notable about this installation is that this is the first time that I've ever had to drill a hole in the plane with nothing more to guide me than "put it right around here somewhere" directions. There has always been at least a pilot hole before.

Oh, also of note is that it's on the left side of the plane, which is currently another way of saying, "Oh, crap! That's gonna be a royal pain to get at."

It was.

With that done, I needed to take the rest of the day off to rest, by which I mean "mow the yard."

Reed the directions, dummy!

I continued work on the reed switches today. As a quick review, these switches are part of a too-clever-by-half over-engineered solution to the perceived problem of a pilot trying to fly an RV-12 with no wing pins in place to hold the wings on. In other words, this is an overly complex attempt to prove Darwin wrong. Which is not to say that some form of automatic verification isn't needed, mind you, but because of the persnickety nature of the system as designed, Van's was forced to build in an override switch on the panel in case a failure of this temperamental system stranded someone somewhere. And if there is an easy-to-operate override switch, those already fated to a Darwinian end will eventually use it.

Me? I'm way too smart for that.

It is what it is, though, and I have to get these things installed and working. The first order of business was to gather up the mounting hardware. Oh goody, Adel clamps!! Also note that one of the wires on one of the switches has been modified with the addition of a hoop terminal. This hoop will be mounted onto the Adel clamp screw that holds the right-side reed switch in place. That will provide an electrical ground to that switch. The other wire will be attached to a wire that passes through the wiring duct and is itself attached to a wire on the left-side reed switch. The other wire on the left-side reed switch will attach to a wire that goes all the way back up to the avionics bay where it will feed into a computer. Presumably the computer will send a small voltage back down to the switches and monitor the voltage - when it goes to ground, both of the switches are closed (thereby taking the voltage to ground) and therefore (in theory) the wing pins are in place.

Or something like that, anyway. Like I said: overly complex.

Starting on the right side, I squeezed the switch into its new home in the tight embrace of the Adel clamp.

The clamp gets mounted under the passenger arm rest, and getting that clamp in there was a real bugger. There was no room for fancy helpers like safety wire or vise grips; it was just a bare-handed fight to the finish. Once I finally got the clamp secured, I tested the circuit by putting the wing ping plunger right up next to the little hole that allows the reed switch "see" through to the other side of the arm rest. The needled on the multimeter being way over to the right like that means that there is no resistance on the switch, which in turn means that the switch is closed. In other words, it works.

Putting the pin in place didn't work at first, however. It took a lot of wiggling and monkeying around with the switch to finally get it to work.

If I thought getting the Adel clamp in place was an onerous chore, it's only because I hadn't yet tried passing the wires through the two tiny little bushings in the frame and didn't have a good basis of comparison. It was like threading a needle with string before then trying to blindly thread yet another needle three inches away. It didn't take long to see that it wasn't going to work. I went and got a length of safety wire and tried passing it through the bushings, the idea being that I would tape the wires to it and pull them back through the bushings. I couldn't get that to work either, and eventually resorted to using an inspection mirror to get a look behind the frame to aid in getting the safety wire correctly aligned with the second bushing. Success!

Having completed the right side, I went back to the work bench to get the left side switch. You may remember that the left side switch is the one without the hoop terminal on one of the wires.

Yeah.

And I'm the guy that's too smart to ever forget to put in a wing pin!!

Hello, Mr. Darwin.

So, long story short shortened, I removed the incorrect placement of the first switch, replaced it with the correct switch, then installed the first switch on the left side of the plane where it was supposed to be in the first place.

I then tested the switches again.

Neither would work.

Checking the book, it says that I may have to file off some of the wing pin tube that the plunger resides in. In fact, I am allowed to remove up to 1/8". This will expose more of the plunger to the tiny hole that the face of the switch "looks" through.

Having researched this in advance, I knew that this was a likelihood. I also know that there are other approaches. Some have repositioned the switches to make them align in parallel with the plungers, while others have increased the diameter of the "view" holes to improve the magnetic view for the switches.

I'll try the grinding first, but if it doesn't work I will probably enlarge the view holes a little bit.

Taking a Reeding

I had some experience in troubleshooting an RV-12 avionics problem last year and through that I learned a valuable lesson: label the wires!

So...

I finished up page 31-05 by pulling a total of three wires through the wire duct and out to the sides of the plane. One went to the left side where it will carry the signal from the wing lock pin position sensors. The other two went to the right side of the airplane. One will attach to the fuel level sensor in the fuel tank, and the other will attach to.... nothing. It's a GPS Data Out wire that can feed a last known position to the Emergency Locator Transmitter if I buy an optional piece of equipment. I won't.

Pulling the wires through takes a lot of concentration to make sure that I don't end up missing a string when each job is finished. I guess I could label the strings to make it easier now that I have a label maker, but I suppose that could be construed as just showing off.

The next job was one of those deceptively easy sounding jobs: put four snap bushings in. Sounds easy, then you see that they're tiny little bushings destined to be placed into very difficult areas. The holes are hidden behind cross members and other parts of airplane. The left side was easy since that's currently the "bottom," but the right side was trickier because it's now the "top." Those were more challenging because they were harder to reach, harder to see, and the consequences of dropping one of the bushings was almost certain loss of the little critter.

I dropped the same one twice.

Found it both times. I'd go out and buy lottery tickets tonight, but I think that was my quota of good luck for the month.

Just before calling it quits for the night, I got out the multimeter and tested the reed switches that will be mounted at the back of the armrests. For both of you that have been wondering why I spelled 'reading' in the title of this post as 'reeding,' now you know. These little switches do nothing but sense magnetism. If you remember, I put magnets down into the silver pin stops in the wing pins a few pages ago. The idea is that these switches will be positioned so that they will sense the magnet in the wing pin stop pins if, and only if, the pins are in correctly. This will (in theory) prevent someone from inadvertently flying away with the wing pins not secured. I say "in theory" because there's an override switch on the panel; someone will inevitably use the override switch and fly off with unsecured pins someday.

Anyway, I tested the switches because it is apparently difficult to get them positioned so that they work. I wanted to rule out the question of defective switches before beginning the installation.

They worked.