The Pressure Altimeter — Page 63, Lesson 68

Let’s start with the reading accuracy problem, because that’s the whole reason these instruments evolved the way they did. The simple altimeter has a single pointer, and that pointer makes one full revolution for roughly every 20,000 feet of altitude change. Think about what that means: the entire usable range of the instrument is squeezed onto one dial revolution, so the scale is coarse. You can read the height, but you can’t read small changes in height with any precision. It’s not sensitive. Then came the three-pointer altimeter. That one gives a much more sensitive indication of height and of change of height, because it uses three pointers of different lengths and speeds — typically one for hundreds, one for thousands, one for ten-thousands of feet. But here’s the severe disadvantage: it can be easily misread. With three pointers all sweeping the same dial, it is not difficult for the pilot to make a reading error of 10,000 feet. And that error is most likely during a rapid descent under difficult conditions, with a high flightdeck workload. Accidents have actually occurred as a result of such misreading. So manufacturers tried various modifications to the pointers and warning systems to prevent this error. One of those was a striped warning sector that appears as the aircraft descends through the 16,000-foot level — a visual cue to catch your attention near that altitude. But the greatest advance was the introduction of the counter-pointer altimeter, which is Figure 5.5. This gives a much more positive indication than the three-pointer dial. The key feature is the digital counters — a row of numbered drums that show the height as an unambiguous number. That removes the misreading problem entirely, because you’re reading digits, not interpreting which of three pointers you’re looking at. However, there’s a trade-off. The digital counters give an unambiguous indication of the aircraft’s height, but they give a relatively poor display of the rate of change of height. A row of digits changing slowly doesn’t tell you quickly whether you’re climbing or descending, or how fast. So the instrument also has a single pointer, and that pointer makes one revolution per 1,000 feet. That gives you the clear indication of change of height, which is extremely important to the pilot — particularly on the final approach in instrument conditions, where you need to sense small altitude changes quickly and precisely. So the counter-pointer altimeter combines the best of both: the digital counters for an exact, unambiguous height, and the single fast pointer for a sensitive rate-of-change display. Now, the excerpt also shows examples of altimeters. There’s a sensitive altimeter reading 265 feet, and an electronic display fitted to a Boeing 737, and an altimeter reading 12,850 feet, which is also given as 3,917 metres. That dual readout — feet and metres — is worth noting, because different phases of flight or different regulations may require altitude in either unit. Let me also point out the figures. Figure 5.4 is the three-pointer altimeter, and it’s shown indicating 24,020 feet. And Figure 5.6 shows the altimeter types together — the sensitive altimeter, the electronic display, and the counter-pointer style. So the core idea of this whole section is: the pressure altimeter’s job is to show you height, but the way it displays that height matters enormously for safety. The simple altimeter is too coarse, the three-pointer is too easy to misread, and the counter-pointer solves the misreading problem with digits while keeping a sensitive pointer for rate of change. That’s the design logic you need to carry forward.