Figure 1.1 Circular scale - linear — Page 9, Lesson 11

Let’s start with the two figures at the top of the page, because they introduce the whole idea of how a pointer moves over a scale. Figure 1.1 is labelled “Circular scale – linear.” Look at the numbers on it: 0, 5, 10, 15, 20, 25, 30, and the unit is “Kts x 10.” The word linear means the spacing between each number is equal — the same angular distance between 0 and 5 as between 5 and 10, and so on. So if the pointer moves, the change in the reading is directly proportional to the movement of the pointer. That’s a linear scale. Now Figure 1.2 is labelled “Circular scale – non-linear.” This is the face of a rate-of-climb instrument. Notice the numbers: 0, 1, 2, 3, 4, then 4, 3, 2, 1, 5, 5. The spacing is not equal. Near the zero, the divisions are close together, and as you go further out they spread apart. That’s what non-linear means — equal movement of the pointer does not correspond to equal change in the value. On a rate-of-climb indicator, this is deliberate, because the sensitivity you need near zero is different from the sensitivity at higher rates of climb. Now, the core of this section is about high range, long scale displays. Here’s the problem: if an instrument needs to show changes over a high range of values, and those changes need to be read with a fair degree of accuracy, then 360° of pointer movement may not be enough. One full revolution of the pointer simply cannot cover the whole range with the precision you need. The first solution is to let the pointer make more than one revolution. That’s what happens on the airspeed indicator in Figure 1.3. The pointer can go around more than once to cover the required range. But — and this is the key drawback — this type of display may lead to confusion, because you can’t immediately tell which revolution the pointer is on. Figure 1.3 indicates an airspeed of 300 kt. So you have to read the number carefully, because the same pointer position could represent a different value on a different revolution. The second solution is to have a pointer moving over a fixed scale, with a moving scale indicating the larger units. Look at Figure 1.4. The small pointer and dial, inset in the top of the instrument, indicates the hundreds — the 100s. The long pointer and the main dial indicates the tens — the 10s. So this indicator shows an airspeed of 33 kt. The long pointer reads the tens, the small inset dial reads the hundreds. The third solution is a less confusing display: two concentric pointers moving over two separate scales. This is shown on the revolution counter in Figure 1.5. The small needle, with the inner scale, reads tens of units. The large needle, with the outer scale, reads units. So this indicator shows 25½ % rpm for a turbine engine. The large needle gives you the units, the small needle gives you the tens. And then there’s a fourth solution, in Figure 1.6 — the three-pointer airspeed indicator. This displays information in a similar fashion to a clock, with pointers showing hours, minutes and seconds. This system is used on many altimeters. Here’s how it works: the long pointer covers 1000 feet in one revolution, so each division of the scale represents 100 feet. The middle pointer covers 10 000 feet per revolution, so each division marks 1000 feet. And the smallest pointer covers 100 000 feet, so each division represents 10 000 feet. Figure 1.6 indicates a height of 25 950 ft. So to tie it together: when a single 360° pointer isn’t enough, you have four ways to extend the range — multiple revolutions of one pointer, a fixed pointer with a moving scale for larger units, two concentric pointers on separate scales, or three pointers like a clock. Each trades off some clarity for range, and the three-pointer clock-style display is the one that ends up on many altimeters.