Characteristics and General Definitions — Page 9, Lesson 15

Let's start with where the hardware actually lives, because that shapes everything else about modern instruments. With modern electronic systems, the displays themselves have to be on the flight deck where the crew can see and operate them — that's obvious. But the computing units and the power units are located remotely, in some other part of the aircraft. Usually they sit in a separate compartment called the Avionics Bay, or the Electrics and Electronics Bay — you'll hear it called the E&E Bay. So the glass you look at is just the output; the brain and the muscle are tucked away elsewhere in the airframe. Now, the first real design requirement we care about is readability. A readable instrument should be designed with an eye reference point in mind. That's the anticipated position that the pilot's eye will occupy when viewing the instrument under normal conditions. So the designer assumes where your eye will be, and builds the instrument around that. Here's the subtle part. If the instrument has a design where there's a reference mark or index with a scale behind it — think of a pointer and the numbers behind it — then it's critical that the eye, the index, and the scale are all in line. If they're not, you get an error called parallax. Parallax is simply caused by viewing the instrument from slightly to one side instead of from the front. So if you shift your head off to the side, the pointer appears to line up with a different number than it actually does. That's why you're trained to look at an instrument squarely, from directly in front. Next, let's talk about colour coding. For conventional non-electronic instruments, there's a standardized system of colour coding for operating ranges, and it's widely used. Green means the normal operating range. Yellow, or amber, means the cautionary range. And red means the warning, or unsafe, operating range. So green is where you want to be, yellow is where you need to be careful, and red is where you must not go. Now, for more complex instruments — and this usually means electronic displays — CS-25 sets out a different colour standardization. And this is where the colours take on new meanings. White indicates present status. Blue indicates a temporary situation. Green still means the normal operating range. Yellow or amber still means the cautionary range. And red — well, the excerpt cuts off right there, but you can see the pattern: red is reserved for the warning or unsafe condition, just as it was on the conventional instruments. So the key contrast to hold onto: on conventional instruments, the colours describe operating ranges of a physical quantity. On electronic displays, the same colours are repurposed — white and blue now describe the state of the system itself, present status versus temporary situation, while green, yellow, and red keep their range meanings. That's the shift in thinking as you move from steam gauges to glass.