Altimetry — Page 118, Lesson 109

Let’s start with the altimeter itself. An altimeter is an instrument that measures pressure and causes a needle to move across a dial. The dial is calibrated in feet rather than in pressure units, because we know that pressure decreases as altitude increases. So the instrument is really a pressure gauge, but its face is marked in feet of altitude. The altimeter is calibrated in accordance with the ICAO International Standard Atmosphere — the ISA. This ensures that all altimeters, anywhere in the world, will read the same altitude for the same pressure. That’s why we need a standard reference: without it, two altimeters could give different readings for the same pressure, which would be dangerous. Altimeters also have a means of adjusting the needle setting to take changes in the surface atmospheric pressure into account. That adjustment is what we call the subscale setting — you’ll hear terms like QFE, QNH, and SPS later. Now, let’s look at how the altimeter responds to pressure changes. Imagine an airfield at sea level. In one scenario, the pressure at the airfield is 1010 hectopascals, and the altimeter reads zero feet. In another scenario, the pressure at the same airfield has fallen to 1000 hPa. The altimeter, rather than showing a decrease in pressure, now shows an increase in height. So a drop in pressure makes the altimeter indicate a higher altitude, even though you haven’t moved. Here’s the key operational consequence. When you are flying at a constant indicated altitude, the outside air pressure must remain the same. To achieve that, you must fly along a pressure level — a surface of constant pressure. But if you fly into an area of lower pressure, those pressure lines dip downward, so your true altitude decreases. Conversely, when you fly into a region of higher pressure, the pressure lines rise, and your true altitude increases. So the rule is: higher pressure means true altitude is greater than indicated altitude; lower pressure means true altitude is less than indicated altitude. Temperature also affects this. Cold air tends to compact and lower the pressure lines, while warm air expands and raises them. So when you fly into a colder area at a constant indicated altitude, your true altitude decreases. When you fly into a warmer region, your true altitude increases. The rule here is: colder than ISA means true altitude is less than indicated altitude; warmer than ISA means true altitude is greater than indicated altitude. Because pressure changes as you fly, there is a need to be able to reset the altimeter to take account of the fall in pressure. If you reset the altimeter when the pressure changes, the altimeter will read correctly. You do this by altering the altimeter subscale setting. You can set QFE, QNH, or SPS — these are different reference pressure settings — to ensure more accurate readings as you fly. We’ll cover each of those in detail shortly.