Characteristics and General Definitions — Page 9, Lesson 9

Let’s start at the very beginning of this chapter, because it sets the foundation for everything else in instrumentation. I’m going to walk you through the general characteristics that apply to every instrument system you’ll ever see in an aircraft, from a simple mechanical dial to a modern glass cockpit. First, the big picture. As a pilot, you get information about the state of your aircraft — its speed, altitude, position, and attitude — through instruments and displays. These can range from the simplest dials with pointers, all the way to modern electronic displays, the so-called ‘glass cockpits’. The appearance and sophistication depend on the vintage and complexity of the aircraft. A simple dial looks very different from a modern cathode ray tube or a liquid crystal screen. But here’s the key point: regardless of that appearance, certain problems are general characteristics of all instrumentation systems. Those problems are range, resolution, accuracy, and reliability. Keep those four words in mind — they’re the theme of this whole section. Now let’s dig into the first big problem: measuring range versus accuracy. This is a fundamental conflict. It’s often necessary to show a large operating range, yet still indicate with accuracy over the whole range. Let me give you the example from the text. An airliner might be limited to a maximum permitted airspeed of 350 knots. So the instrument designer might make the display go up to, say, 380 or 400 knots — a bit of margin above the limit. But here’s the catch: certain speeds are critical to flight safety and need to be read with extreme accuracy — ideally to the nearest knot. If you put the whole range — 0 to 400 knots — on a single revolution of the instrument, then the division representing one knot becomes very small. And a very small division is difficult to read accurately. That’s the core tension: you want a big range, but you also want fine resolution, and the two fight each other on a fixed-size dial. Let me show you how designers deal with this. Look at Figure 1.1, which shows a circular scale that is linear. This is a simple indicator showing the change of value of a parameter over a range of 0 to 30 units. The key idea here is that the accuracy with which these values need to be measured will govern the spacing of the graduation. In other words, if you need to read this instrument very precisely, the graduations — the tick marks — will be spaced further apart so you can see them clearly. If you don’t need that precision, you can pack more range into the same dial by making the graduations closer together. So the graduation spacing is a direct design decision driven by the required accuracy. Now, there’s a second type of circular scale: the non-linear one. Some instruments are required to show changes of parameters more accurately at certain parts of the scale. Look at Figure 1.2, the non-linear example. This shows a rate of climb indicator, where low rates of climb and descent are more easily read than high rates. Think about why that matters. When you’re near level flight, small changes in climb or descent rate are important — you want to see them clearly. But when you’re climbing or descending steeply, you don’t need the same fine resolution; you just need to know you’re going up or down fast. So the scale is compressed at the high end and expanded at the low end. This type of scale is called a logarithmic scale. That’s the technical name — logarithmic — and it means the spacing isn’t uniform; it’s designed so that the low values get more room on the dial, making them easier to read accurately. So let me tie this together. You have two fundamental approaches to the range-versus-accuracy problem. The linear scale spreads the range evenly, and the graduation spacing is set by the accuracy you need. The non-linear, logarithmic scale deliberately gives more space to the part of the range where you need the most accuracy — like low rates of climb — and compresses the rest. Both are solutions to the same underlying problem: how do you show a big range on a small dial while still letting the pilot read the critical values precisely? That’s the heart of this section. The next part of the chapter will build on this idea — how these principles apply to specific instruments and how modern displays handle the same problem in different ways. But for now, make sure you’ve got the four general characteristics — range, resolution, accuracy, reliability — and the two scale types — linear and logarithmic — firmly in mind.