The Pressure Altimeter — Page 63, Lesson 68

Right, let's get into the pressure altimeter. We've covered the basic principle, so now I want to walk you through the critical issue of reading accuracy, because this is where the instrument's design really matters in the cockpit. The simple altimeter we looked at first is not sensitive. Think about it—its single pointer makes one full revolution for roughly every 20,000 feet of altitude change. That's a huge range crammed onto one dial, so a small movement of the needle represents a large change in height. It's just not precise enough for the fine control you need. To solve that, the industry developed the three-pointer instrument. This gives a much more sensitive indication of height and of change of height. But—and this is the severe disadvantage—it suffers from being easily misread. With three pointers of different lengths and speeds, it is not difficult for a pilot to make a reading error of 10,000 feet. That's a catastrophic error, and it's particularly likely during a rapid descent under difficult conditions, when the flightdeck workload is high. I want you to really hold onto that: this misreading has actually caused accidents. So, various modifications were tried to prevent this error. One of those was a striped warning sector—a visual band that appears on the dial as the aircraft descends through the 16,000-foot level. It's a simple alert, but it's not the ultimate fix. The greatest advance was the introduction of the counter-pointer altimeter, which you can see in Figure 5.5. This gives a much more positive indication than the three-pointer dial. Now, here's the key design trade-off. The digital counters—those little windows showing numbers—give an unambiguous indication of the aircraft's height. You read the exact value directly, no interpretation needed. But—and this is crucial—they give a relatively poor display of the rate of change of height. A digital counter that ticks over slowly doesn't tell you how fast you're climbing or descending. That's why the instrument also has a single pointer. This pointer makes one revolution per 1,000 feet. So it gives you that clear, dynamic indication of change of height, which is extremely important to the pilot, particularly on the final approach in instrument conditions. You get the precise altitude from the counters, and the rapid, easy-to-scan trend from the pointer. Now, let me also mention the three-pointer dial in Figure 5.4 for contrast. That's the older design that's so prone to misreading. And finally, look at Figure 5.6 to see the range of altimeter types. You have a sensitive altimeter reading 265 feet, an electronic display fitted to a Boeing 737, and an altimeter reading 12,850 feet, which is 3,917 metres. That last one shows you the dual indication in feet and metres. So the takeaway is this: the counter-pointer design combines the unambiguous precision of digital readout with the dynamic, rate-of-change clarity of a fast pointer. That combination is what makes it the professional standard.