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 want to walk you through the general characteristics that apply to every instrument system you’ll ever see, from the simplest mechanical dial to the most modern glass cockpit display. First, the big picture. As a pilot, you receive information about the state of your aircraft — its speed, altitude, position, and attitude — through instruments and displays. These can range from the simplest of dials and pointers to modern electronic displays, the so-called ‘glass cockpits’, depending on the vintage and the complexity of the aircraft. A simple dial can look very different in appearance and sophistication from a modern cathode ray tube or liquid crystal screen. But here’s the key point: certain problems — of range, resolution, accuracy, and reliability — are general characteristics of all instrumentation systems. No matter how advanced the display, these four issues are always there. Now let’s dig into the first big problem: Measuring Range Versus Accuracy. This is a fundamental conflict. It is often necessary to show a large operating range, yet still indicate with accuracy over the whole range. Let me give you a concrete example. An airliner might be limited to a maximum permitted airspeed of 350 knots. So perhaps the instrument would be designed to display up to, say, 380 or 400 knots — you need a little headroom above the limit. However, certain speeds are critical to flight safety and need to be read with extreme accuracy — ideally to the nearest knot. Here’s the problem: if you put the whole range on a single revolution of the instrument, the division representing one knot will be very small and will be difficult to read accurately. That’s the trade-off — a wide range on one scale means each individual unit gets squeezed, and you lose the ability to read fine detail. So how do we solve this? Let’s look at the Circular Scale (Linear). A simple indicator showing the change of value of the parameter to be measured over a range of 0 to 30 units is shown in Figure 1.1. 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 values very precisely, you space the graduations further apart — but that limits how much range you can fit on the dial. If you don’t need that precision, you can pack more range in. The graduation spacing is driven entirely by the accuracy requirement. Now, some instruments don’t need uniform accuracy across the whole scale. That brings us to the Circular Scale (Non-linear). Some instruments are required to show changes of parameters more accurately at certain parts of the scale. The example in Figure 1.2 shows a rate of climb indicator where low rates of climb and descent are more easily read than high rates. This is a logarithmic scale. So instead of evenly spaced graduations, the scale is compressed at the high end and expanded at the low end — because at low rates of climb, you need fine resolution to detect small changes, while at high rates, you don’t need that same precision. The logarithmic scale gives you accuracy where it matters most. So the core idea here is that instrument design is always a compromise. You have a fixed dial face, and you have to decide how to distribute the range and the accuracy. Linear scales give you even spacing but uniform accuracy. Non-linear, logarithmic scales sacrifice accuracy at one end to gain it at the other — exactly where the flight regime demands it. And this same principle — range versus accuracy — is the thread that runs through every instrument you’ll study in this book.