We're starting a brand-new chapter together: the Pressure Altimeter — Page 53, Lesson 61

We're starting a brand-new chapter together: the Pressure Altimeter. This is one of the most important instruments on your panel, so let's build it up properly from the ground floor. I want to give you the roadmap of what we're about to cover, because this chapter is dense and every piece connects. We'll start with the principle of operation, then lock in some definitions, then look at calibration. From there we'll build the instrument itself, starting with the simple altimeter, then the sensitive altimeter, and how we read it accurately. We'll look at real examples, then servo-assisted altimeters, and the tolerances we're allowed. Then the big one: altimeter errors, and how we correct for temperature. We'll cover standard datum settings, what happens with blockages and leaks, density altitude, and finally the preflight checks you'll actually do in the aircraft. Let's begin with the core idea. The pressure altimeter works on a beautifully simple principle: as you climb, atmospheric pressure decreases. The instrument measures that static pressure and converts it into a height reading. That's the whole game. Everything else in this chapter is about making that measurement accurate, understanding its limitations, and knowing what happens when it goes wrong. Before we go further, I need to establish some definitions, because we'll use these words constantly. First, altitude. That's the vertical distance of an aircraft above mean sea level, which we abbreviate as MSL. Next, height. That's the vertical distance above a specified datum, which is usually the terrain directly below you. So altitude is above sea level, height is above the ground. Then we have flight level, which is a pressure altitude expressed in hundreds of feet, and it's used above the transition altitude. And finally, we have QFE and QNH. QFE is the pressure setting that makes the altimeter read zero at a particular airfield elevation, so it reads height above that field. QNH is the pressure setting that makes the altimeter read altitude above mean sea level when you're on the ground. These are the building blocks, and I want you to hold onto them because we'll use them throughout. Now, calibration. The altimeter is calibrated against the International Standard Atmosphere, or ISA. This is a theoretical model of the atmosphere, and it assumes specific values: mean sea level pressure of 1013.25 hectopascals, a temperature of 15 degrees Celsius at sea level, and a temperature lapse rate of 1.98 degrees Celsius per 1000 feet up to 36,090 feet, which is the tropopause. Above that, the temperature is assumed constant at minus 56.5 degrees Celsius. The altimeter is built to read correctly only under these standard conditions. That's a critical point, and it's the root of many errors we'll discuss later. Let me show you the terminology visually. Now let's build the instrument. We start with the simple altimeter. The static pressure is fed into the case of the instrument from the static source. Inside, we have an aneroid capsule, which is a sealed, evacuated, corrugated metal capsule. As the aircraft climbs, the static pressure inside the case decreases, so the capsule expands. As it descends, pressure increases and the capsule contracts. This expansion and contraction moves a mechanical linkage, which drives the pointer across the dial. That's the simple altimeter. It's a direct mechanical measurement of pressure, converted to height. But the simple altimeter has a problem: it's not very sensitive. The capsule movement is small, and the reading is coarse. So we move to the sensitive altimeter. The principle of operation is similar, but there are important differences. The sensitive altimeter uses a stack of aneroid capsules, not just one, to increase the movement. It also has a much more complex gear train, with a higher gear ratio, so a small capsule movement produces a large pointer movement. And critically, it has a sub-scale, which is the pressure setting window. This allows you to set the barometric pressure, the QNH or QFE, so the instrument reads correctly for the current conditions. The sensitive altimeter typically has multiple pointers, often a main pointer for hundreds of feet and a secondary pointer for thousands, giving you a much finer reading. Now, reading accuracy. With the sensitive altimeter, you can read the altitude much more precisely, but you have to be careful. The multiple pointers can be misread, and you need to know exactly which pointer indicates what. The accuracy of your reading depends on your ability to interpret the dial correctly, and that's a skill you'll practice. We'll also look at examples of altimeters, and then servo-assisted altimeters. These are used in larger aircraft. In a servo-assisted altimeter, the mechanical capsule movement is converted into an electrical signal, which drives a servo motor to move the pointers. This gives a much more powerful and accurate indication, and it can also feed data to other systems like the autopilot or the flight director. Then we have tolerances. This is the allowable error in the instrument's reading. The altimeter is a precision instrument, but it's not perfect. There are specified tolerances, meaning the reading can be off by a certain amount and still be considered serviceable. These tolerances are checked during maintenance and preflight. Now, the big topic: altimeter errors. The altimeter is calibrated for ISA, but the real atmosphere rarely matches ISA. So we get errors. The main ones are: instrument error, which is mechanical imperfection in the instrument itself; pressure error, which comes from the static source not measuring true ambient pressure, often due to airflow around the aircraft; and temperature error, which is the big one. Because the altimeter assumes a standard temperature lapse rate, if the actual temperature is warmer or colder than standard, the altimeter will read incorrectly. In cold air, the air is denser, so the pressure decreases more rapidly with height, and the altimeter over-reads. That means you're actually lower than the altimeter indicates. This is a serious safety issue, especially when flying near terrain. We have a specific section on temperature error correction. This is where you apply a correction to your indicated altitude based on the difference between the actual temperature and ISA temperature. There are charts and rules of thumb for this, and it's critical for cold weather operations. Then we have standard datum settings. This is the 1013.25 hectopascals setting, which we use above the transition altitude. When you set 1013.25, the altimeter reads pressure altitude, and you express it as a flight level. Below the transition altitude, you use QNH or QFE, depending on the local procedure. We also need to cover blockages and leaks. The static system is critical. If the static source becomes blocked, the pressure inside the case is trapped, and the altimeter will freeze at its last reading. If there's a leak, the pressure inside the case will slowly equalize with the cabin or the outside, giving false readings. You need to know the symptoms and the actions to take. Finally, we have density altitude. This is the pressure altitude corrected for non-standard temperature. It's the altitude the aircraft "feels" in terms of performance. High density altitude means less dense air, which means degraded performance — longer takeoff rolls, reduced climb rates. And we finish with preflight altimeter checks, where you verify the instrument is reading correctly before you fly, by setting the local QNH and checking it reads the airfield elevation within tolerance. That's the full map of where we're going. It's a lot, but we'll take it step by step. Let's start with the principle of operation and the definitions in detail. Are you ready to dive in?