The Pressure Altimeter — Page 63, Lesson 72

We're now looking at the servo altimeter, which is the most advanced of the pressure altimeters. Let me start by explaining how it works, because it's quite different from the simple and sensitive types we've covered. The servo altimeter uses a system of two bars, called the 'I' Bar and the 'E' Bar. The capsules inside the instrument move the I Bar. The E Bar is positioned by a servomotor. The key is that these two bars are kept aligned with each other. When they're perfectly aligned, the magnetic fields around them are balanced, and the currents induced in two coils, which we call the 'A' and 'B' coils, are also balanced. Now, when the aircraft changes altitude, the pressure changes, the capsules move, and the I Bar moves. This creates an imbalance between the I Bar and the E Bar. That imbalance disturbs the magnetic fields, which in turn disturbs the currents in the 'A' and 'B' coils. This produces an error signal. That error signal goes to an amplifier, where it's amplified and rectified, and then it's sent to the servomotor. The servomotor does two things at once. It drives the counter-pointer system of the altimeter, which is the display you read. And at the same time, through a cam drive, it re-aligns the E Bar with the I Bar. Once the E Bar is back in line with the I Bar, the error signal stops, and the altimeter shows the correct height. Here's the clever part. In this system, the only work the capsules have to do is move the I Bar. They don't have to drive any gears or pointers directly. This eliminates the effects of friction and manufacturing imperfections that you'd find in the gearing of a conventional altimeter. Because of that, this type of altimeter is sensitive to very small pressure changes, and it's more accurate than the sensitive altimeter. That's especially true at high altitudes, where the pressure change per unit of height is very small. The lag you get with other types of altimeter during rapid height changes is also greatly reduced. Now, let's talk about accuracy and tolerances. The normal instrument error for a servo altimeter is approximately equivalent to the effect of a 1 hPa change of pressure. To give you a sense of what that means in feet: that's about 30 feet at mean sea level, 50 feet at 20,000 feet, and 100 feet at 40,000 feet. There's also a tolerance at mean sea level, under CS-25, of plus or minus 30 feet per 100 knots of calibrated airspeed. Let me walk you through the tolerances for each type, because they're quite different. These values are examples only, you don't need to memorise them, but you should understand the pattern. For a typical simple altimeter, with a range of zero to 35,000 feet, the tolerance at zero feet is plus 100 feet, and at 35,000 feet it's plus 1,000 feet. For a typical sensitive altimeter, with a range of zero to 80,000 feet, the tolerance at zero feet is plus 70 feet, at 40,000 feet it's plus 600 feet, and at 80,000 feet it's plus 1,500 feet. For the typical servo altimeter, with a range of zero to 100,000 feet, the tolerance at zero feet is plus 30 feet, at 40,000 feet it's plus 100 feet, at 60,000 feet it's plus 300 feet, and at 100,000 feet it's plus 4,000 feet. So you can see the pattern clearly. The servo altimeter is much tighter at low and mid altitudes, but even it degrades significantly at the very top of its range. The simple altimeter is the least accurate overall. That's the servo altimeter for you — the most precise of the pressure instruments, but with limits you still need to respect.