We're starting a brand-new chapter together — Chapter 5, The Pressure… — Page 53, Lesson 61

We're starting a brand-new chapter together — Chapter 5, The Pressure Altimeter. This is one of the most important instruments in your panel, because it tells you your height, and getting that wrong is simply not an option in professional flying. Let me give you the roadmap of what we're going to cover, because this chapter is packed. We'll begin with the principle of operation — how the thing actually works. Then we'll nail down some definitions, because altitude terminology is precise and you'll be tested on it. After that, we'll look at calibration, then the two main types of instrument: the simple altimeter and the sensitive altimeter. We'll talk about reading accuracy, look at some real examples, and then move to the more advanced servo-assisted altimeters. We'll cover tolerances — the acceptable limits of error — then the big one: altimeter errors, including temperature error correction. We'll discuss standard datum settings, what happens with blockages and leaks, density altitude, and finally the preflight altimeter checks you'll actually perform before every flight. The chapter ends with practice questions and answers. So let's start at the very beginning: the principle of operation. The pressure altimeter is essentially an aneroid barometer that's calibrated to read altitude instead of pressure. It works because atmospheric pressure decreases as you climb. The instrument senses the static pressure from the static source — that's the port on the side of the aircraft that samples the undisturbed outside air — and converts that pressure into a height reading on the dial. Now, before we go any further, we need to get our definitions straight, because the word "altitude" gets thrown around loosely, and in aviation it has very specific meanings. Let me walk you through the key ones. True altitude is the actual vertical distance of the aircraft above mean sea level — that's the real, geometric height above the sea. Then we have indicated altitude, which is what you read directly off the altimeter. Absolute altitude is the height above the terrain directly beneath you — so if you're flying over a mountain, your absolute altitude is smaller than your true altitude. Pressure altitude is the height above the standard datum plane, which is the theoretical level where the pressure is 1013.25 hectopascals, or 29.92 inches of mercury. And finally, density altitude is pressure altitude corrected for non-standard temperature — it's the altitude the aircraft "feels" in terms of performance. Now, calibration is how we make the instrument read correctly. The altimeter is calibrated to the International Standard Atmosphere, or ISA. In ISA, at sea level, the pressure is 1013.25 hectopascals, the temperature is 15 degrees Celsius, and the pressure decreases at a specific lapse rate. The instrument is built so that when it senses that standard pressure, it reads the correct altitude. If the actual conditions differ from standard — and they almost always do — then the altimeter will show an error, which is why we have the sub-scale to set the local pressure, and why we have all those error corrections we'll cover later. Let me show you what this looks like. Here's Figure 5.1, which illustrates the terminology — you can see how true altitude, indicated altitude, absolute altitude, pressure altitude, and density altitude all relate to each other and to the standard datum plane. Now let's look at the simple altimeter. Figure 5.2 shows the basic construction. Static pressure is fed into the case of the instrument from the static source. Inside, there's an aneroid capsule — a sealed, flexible metal bellows. As the aircraft climbs, the static pressure outside the capsule decreases, so the capsule expands. As it descends, the pressure increases and the capsule contracts. This expansion and contraction moves a mechanical linkage that drives the pointer across the dial. That's the whole principle — it's a pressure gauge that reads in feet or metres. But the simple altimeter has a limitation: it's not very sensitive. The capsule movement is small, and the linkage can't magnify it enough for precise readings. That's where the sensitive altimeter comes in. Figure 5.3 shows it. The principle of operation is similar to the simple altimeter, but there are important differences. The sensitive altimeter uses a stack of several aneroid capsules instead of just one — this gives more total movement for the same pressure change. It also has a much more precise mechanical linkage with higher gearing, and it includes a sub-scale on the face that lets you set the barometric pressure — the QNH or QFE — so the instrument reads correctly for the local conditions. The sensitive altimeter is the one you'll actually fly with. Now, reading accuracy is critical. The sensitive altimeter typically has a main pointer that makes one revolution per 1,000 feet, and a smaller drum or counter that shows thousands of feet. You read the thousands from the counter and the hundreds from the main pointer. You need to be able to read it to within a small tolerance — we'll get into the exact numbers when we cover tolerances. Let me also mention the servo-assisted altimeter, which is found on larger aircraft. This is a more advanced version where the mechanical movement of the capsules is sensed electronically, and a servo motor drives the display. This gives much higher accuracy and allows the altitude to be fed into other systems, like the autopilot and the altitude alerting system. So that's the structure of the chapter. We're going to go through each of these in detail. The key takeaway right now is that the pressure altimeter is an aneroid barometer calibrated to ISA, reading static pressure as altitude, and the sensitive altimeter with its multiple capsules and sub-scale is the instrument you'll rely on in the cockpit. Let's move on and dig into the details.