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Wiring Up the Electrical System




I’m finally breaking our silence on updates. We were planning to wait until our current list of changes was complete, but we’re finding that some of these changes are going to be larger undertakings. One of the main things I’ve been working on recently is the electrical system. The electrical system and its associated controls and data is really the foundation of the rest of the aircraft’s systems, so it was something I was excited to get working.

As of now, it works as it should with the exception of a few edge (non-standard operation) conditions. The way we did it, it’s much more than just a “if(power) do_stuff();” sort of system. I spent some time working with one of our colleagues, who is a 717 crew member, to nail down the 717’s automatic No-Break-Power-Transfer (NBPT) system. I’ll use a picture to illustrate how this works. Note – the area around the display unit is graphically botched in this picture as we are re-texturing it (it had to be redone so we could correct the aspect ratio of the displays). Similarly, we remodelled the overhead switches and are in the process of re-doing the artwork for them, hence their oddness. Without further ado, here’s the electrical synoptic display page with the APU running, the engines off, and all electrical control switches in their standard (automatic) position.

Screenshot 2015-05-12 16.00.44

Each small T-shaped line represents a relay in the electrical system. The hardest part was getting those to open and close properly to allow power to flow through the system – this lets us both determine what buses have power and how. Here’s what I’m talking about. In the next picture, I turned off the left AC bus tie, which means it will open the relay and prevent power from flowing.

Screenshot 2015-05-12 15.59.54

You can see that the left AC bus is no longer powered because there isn’t a way to get power to it anymore (the APU power input is fairly central, it has to flow through the bus tie relays). On the other hand, the left DC bus tie relay and auxiliary DC bus tie relay have closed to supply the left DC bus with power. It is this transfer of power and manipulation of these relays that gives us the simulation of the NBPT system – a loss of regular power to one system, in our 717, will not interrupt service as long as an alternative is available, like the real 717.


We will be doing something to simulate the electrical load values and generator input and frequency, they’re just not present in these pictures. Similarly, the warning message system is on its way – step one was getting power to the rest of the systems.



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