The Pressure Altimeter — Page 63, Lesson 72

We're in the middle of the pressure altimeter now, and I want to pick up with the servo altimeter, because that's where this passage is taking us. We've already covered the simple altimeter and the sensitive altimeter, so now let's look at how the servo altimeter gets rid of the weaknesses those two have. The core problem with a conventional altimeter is that the capsules have to physically drive the pointer through a train of gears. That gearing introduces friction, and it introduces manufacturing imperfections, and both of those degrade accuracy. The servo altimeter solves this by separating the sensing job from the indicating job. Here's the principle. Inside the instrument you have two bars — an 'I' Bar and an 'E' Bar. The capsules, the aneroid capsules, are only required to move the I Bar. That's their entire mechanical load. The E Bar is positioned by a servomotor. Now, these two bars are arranged so that they interact with two coils, called the 'A' coil and the 'B' coil. When the E Bar and the I Bar are perfectly aligned, the magnetic fields around those coils are balanced, and the currents induced in the 'A' and 'B' coils are equal. Now, when the aircraft changes height, the static pressure changes, the capsules expand or contract, and the I Bar moves. That creates an imbalance between the E Bar and the I Bar. That imbalance disturbs the magnetic fields, which disturbs the currents induced in the 'A' and 'B' coils. The imbalance produces an error signal. That error signal is passed to an amplifier, where it is amplified and rectified — rectified meaning converted from alternating to direct current — and from there it goes to the servomotor. The servomotor then does two things at once. It drives the counter-pointer system of the altimeter — that's the display you read — and at the same time, through a cam drive, it re-aligns the E Bar back with the I Bar. Once the E Bar is re-aligned, the error signal ceases, and the altimeter indicates the correct height. So it's a closed-loop follow-up system: the motor chases the capsule until the error is zero. Now, why does this matter? Because in this system, the only work the capsules have to do is move the I Bar. They don't drive any gearing. That eliminates the effects of friction and manufacturing imperfections in the gearing of a conventional altimeter. The result is that this type of altimeter is sensitive to very small pressure changes, and therefore more accurate than the sensitive altimeter — particularly at high altitudes, where pressure changes per unit height increment are very small. And the lag you experience in other types of altimeter with rapid changes of height is greatly reduced. Now, let's talk about errors and tolerances, because this passage gives us specific numbers. The normal instrument error is approximately equivalent to the effect of a 1 hPa change of pressure. And that translates to about 30 feet at mean sea level, 50 feet at 20,000 feet, or 100 feet at 40,000 feet. So you can see the same pressure error grows with altitude. There's also a tolerance at mean sea level under CS-25, the airworthiness standard, which is plus or minus 30 feet per 100 knots calibrated airspeed. So that's a speed-dependent tolerance at sea level. Then we have the tolerance tables, and these are given as examples only — you don't have to learn them, but I want you to see the trend. For a typical simple altimeter, range zero to 35,000 feet: at zero feet the tolerance is plus 100 feet, and at 35,000 feet it's plus 1,000 feet. For a typical sensitive altimeter, range zero to 80,000 feet: at zero feet, plus 70 feet; at 40,000 feet, plus 600 feet; at 80,000 feet, plus 1,500 feet. And for a typical servo altimeter, range zero to 100,000 feet: at zero feet, plus 30 feet; at 40,000 feet, plus 100 feet; at 60,000 feet, plus 300 feet; and at 100,000 feet, plus 4,000 feet. Notice the pattern. The servo altimeter starts with the tightest tolerance at sea level — plus 30 feet versus plus 70 for the sensitive and plus 100 for the simple — and it stays much tighter through the mid-range. But at the very top of its range, 100,000 feet, the tolerance blows out to plus 4,000 feet. That's because at extreme altitude the pressure changes per foot of height become so tiny that even a servo system struggles to resolve them. So the servo altimeter is the most accurate where it matters most, but no altimeter is perfect at the extremes. If you look at the figure of the sensitive altimeter, you'll see the principle of operation is similar to the simple altimeter, but with those additional features we discussed. And the counter-pointer display is what the servo altimeter drives. So the key takeaway here is the servo altimeter's closed-loop design: capsules move the I Bar, the error signal drives the motor, the motor drives the display and re-aligns the E Bar, and the error cancels. That's the whole story of why it's the most accurate of the three.