Chapter 22 - Section 4

Switch and Breaker Panel



With the advancement of computers and electronic instrument in the aviation industry, many round gauges, mechanical dial instruments are being replace by electronic displays and accompanying gadgets - that include the traditional switch and breaker panels. I have decided to install the Vertical Power Electronic Breakers for Experimental Aircrafts. I decided on the VPX Pro system. Basically, its a computer controlled power distribution and control system that replaces the many switches and fuses on your Instrument Panel. It's somewhat of a new product and I was not able to find many people that has first hand experience with it. In fact, quite a few people express doubts with the system. However, you can read about it at their web site.


Here's a picture of my VPX unit to be mounted onto one of my shelves.










After countless times of re-laying out my instrument panel, I eventually came to the layout as shown. It also consists of a row of toggle switches that correspond to the VPX Pro unit, in addition, I also has a row of LED indicator lights for quick reference during landings and take-offs. These lights will be tied to various limit switches. If you use Jack Wilhemson's nose lift, a limit switch is already installed. All I have to do is to tie it to my indicator light. I plan to add the same to my canopy lock, speed brake and fuel pump respectively. BTW, the VPX system has a current fault option that I can assign to all electronic instruments for in-flight monitoring as well. 







Limit Switches and Indicator Lights

My IP panel consists of 6 indicator LED lights. The purpose of the LED lights is to provide a quick reference (in additional to the check list) on the status of several crucial components on the airplane. They include:

- Canopy position (lock/unlock);

- Nose gear (down or not);

- Boost pump (on or off); and

- Landing brake (up or down).


I have two (2) more LED light positions that are not assigned at this time. I bought the LED lights from Allelectronics - they are a bit diffused and not too bright - which is perfect for my application. I daisy chained all the indicator (LED) lights to a common ground and power them from one of the power pins from VPX. Once the Master Switch is on, the VPX will be powered up, subsequently, the indicator lights will be activated as well.


Working in concert with the indicator lights are the limit switches (snap action switches). The simplest one is the nose gear because the Wilhemson's nose lift already have a limit switch installed in place. All I have to do is to bring the wires to its dedicated indicator light at the IP (as shown above picture).


Canopy Limit Switch:



Picture #1

The first indicator light is for confirming the closure of the canopy. I decided to mount the limit switch at the opening edge of my side hinged canopy. I bought a couple water tight limit switches from Allelectronics (Picture #1 black switch to the left) - they were inexpensive. To trigger the limit switch, I made a small threaded plunger (aluminum), a threaded tubing (~1/2" long) and a flat spacer (picture left). The tubing is floxed onto the canopy and the plunger is screwed onto it. The reason for the threaded plunger is to provide fine adjustment to the trigger height relative to the limit switch. The spacer is to make sure the trigger button is centered to the plunger.


Picture #2

The limit switch is then mounted directly to the longeron with a couple screws. I position the switch a bit below the top of the longeron to duck below any potential bumping during loading and unloading. Note the thin space behind the limit switch?


Picture #3

The threaded tubing is floxed into the pre-drilled hole on the canopy such that it is flushed to the surface. Once the floxed cured, the plunger is threaded into it. The trigger height is then set by trial and error. It turned out to be 5/32". Later, I just made a 5/32" threaded washer to lock the plunger in place.



Landing Brake Limit Switch



Picture #1

The third indicator light is for confirming the closure of the landing brake. The mounting fixture for this limit switch is quite simple. It consists of a short section (~1" length) of angle aluminum. I carved out a small access hole area at the base of the angle aluminum for the plunger to make contact to the limit switch. I added 3 more small holes of for the mounting screws as shown.


Picture #2

The plunger for closing the limit switch is a small aluminum rod with a threaded hole at one end. I replaced one of the 4 nuts, that hold the landing brake actuator arm bracket, with this aluminum rod. As the landing brake closes, the aluminum rod compresses the limit switch roller arm and lights up indicator light.


Picture #3

This picture shows the mounting location of the limit switch. The entire actuator assembly and limit switch are covered by an enclosure made in prior chapter.  





Together with the dual EFIS and dual AHRS, I also get two (2) magnetometers. Per installation manual, mounting positions of the magnetometers are critical. Briefly, they have to be away from ferrous metals and electronic interference such as strobe lights etc. Some builders mounted their magnetometers at the outboard cavities of their center section spar, while others mount them at the luggage cavity next to the pilot/co-pilot seats. I decided to mount mine at the luggage cavity next to the pilot/co-pilot seat respectively. It is because the locations are away from most electronic instruments, accessible for re-adjustment down the road (compared to the center section spar) and shorter wires harness.


To mount the magnetometers, I made a couple L-shape brackets as shown. The narrower side will be screwed against the side walls inside the luggage cavity (next to the seat back). The magnetometers will be bolted onto the top of the wider surfaces. The slotted holes (on both the surfaces) are for aligning the magnetometers horizontally and vertically (per GRT installation manual).









Here's how I mounted my magnetometers.











I brought the wire harness for both from the AHRS down to the left cable duct (inside the arm rest) past the seat back. From there, it exit the cable duct and up along the back side of the seat back and up to the hole for the canopy lock. The harness for the left magnetometer turns left while the harness for the right routes through the shoulder support cavity and exit out the co-pilot luggage cavity. The harnesses terminates to their respective 9 pin D-sub connectors.