The fuel system in twin engines aircrafts generally include a left and right tank within the corresponding wings. Each tank provides fuel to the engines on that wing. The cross feed valve is used to temporarily connect the two sides of the system. This system allows the shortest route for the fuel and keeps the systems separate, so a leakage on one side cannot drain the entire system. The cross feed valve is generally kept shut-off for this reason. A couple scenarios illustrate, when opening the valve might be necessary.
To keep the plane balanced, both wing tanks should have about the same amount of fuel. However, for various reasons, they could become unbalanced. The cross feed valve allows fuel to flow from the tank with too much fuel to the tank with less fuel. Another scenario would be the loss of an engine. In a twin-engine airplane, the remaining engine would need to be able to draw fuel from both wing tanks.
Another purpose for the cross feed is for the emergency case. If the left fuel pump encounters a problem, the right fuel pump will dump fuel from right tank and supply fuel to both engines through the cross feed valve, which is automatically opened. Vice versa for the right fuel pump.
The implemented design is shown in right picture. It consist of a whilte rigid FOREX foam plate, which is 4 mm thick. On this plate 2 x 2 snap switches are mounted, which are in the ON state at knob position ON and CROSS FEED., i.e. exactly with 90 degree offset. There is no need for a switch at the OFF position, because with the first two ones, the state is exactly defined. This is also true for the positions within the red arcs, which are per definition the OFF state. This is clearly indicated at the bottom with the written red text.
The knob and the lever arm was mounted on a 5 mm round aluminium stock, tightly screwed by M3 grub screws and fed through the holes of the chambers. The lever arms have to be aligned in conjunction with the knobs exactly to the white positions ON and CROSS FEED. The 2 x 2 snap switch were positioned on the back panel of the fuel selector, as shown in the right below picture.
The parts are 3D printed out of PLA and the most important ones are shown below. The knob has been spray with glossy red paint and the chamber, as well as the front panel with dull black paint. The panel text has been created with Power Point and was printed on self-adhesive paper by an inkjet printer. Because this paper has no photo quality, it turned out that the foreseen black colour could not be achieved as originally foreseen. This however is only a minor issue, because the final design looks quite comparable with the original one in the Beech Baron aircraft. Having some spare time in the future, I will print the label once more on glossy photo paper.
Snap switches are typically CLOSED, when the switch is pressed. However, by accident, the wrong fuel selector snap switches were bought, because these switches are OPEN, when the snap switch is pressed. Therefore the software internal logic had to be updated to the one, as shown below.
The fuel selector panel was finally placed into a wooden housing, which also has been painted black. It fits exactly at the bottom front of the pedestal, see picture below.