The Pressure Altimeter — Page 69, Lesson 77

We’re now into the pressure altimeter’s errors, and the one I want to walk you through is the temperature error. This is a big one for your exams and for real-world safety, so let’s take it slowly. First, the core idea. Even if the altimeter has no mechanical error at all, it will not show true altitude — that is, height above mean sea level — unless the air column below you has the same surface temperature and the same lapse rate as the standard atmosphere assumed in the instrument’s calibration. The altimeter is calibrated against ISA, the International Standard Atmosphere. If the real air doesn’t match that assumed profile, you get an error. Here’s the key rule: when you fly in colder air — air whose density is greater than ISA at that altitude — the altimeter will over-read. It will show a higher altitude than your true altitude. That’s the single most important fact in this whole section, so hold onto it. Why does that happen? Because pressure decreases more rapidly in cold air than in warm air. Think about it this way: if you’re at a given true altitude in cold air, the pressure at that point is lower than it would be at the same altitude in standard air, assuming the surface pressure is constant. The altimeter is just a pressure gauge — it interprets that lower pressure as if you were higher up. So it over-reads. Cold air, lower pressure at your altitude, altimeter thinks you’re higher than you really are. Now, can we correct for this? Partly. Where the temperature at cruising level deviates from standard, you can make an approximate correction with most navigational computers. But it’s only approximate, because you don’t know the temperatures throughout the rest of the air column below you. And here’s a practical limit: the correction is considered too inaccurate to be worth making at heights above 25,000 feet. Above that, you just don’t bother. Let me walk you through the worked example, because it ties all this together. Say your indicated altitude is 10,000 feet, you have local pressure set, and the COAT — that’s corrected outside air temperature — is minus 25 degrees Celsius. The question is: will your true altitude be more or less than the indicated value? The ISA temperature at 10,000 feet would be about minus 5 degrees Celsius. So you’re flying in colder-than-standard conditions — that’s ISA minus 20 degrees. Now apply the rule: pressure decreases more rapidly in cold air. So at your true altitude in this cold air, the pressure is less than it would be in standard air. The altimeter reads that lower pressure as a higher altitude. So it over-reads — your true altitude is less than what the instrument shows. Using the navigational computer: you set the indicated altitude, 10,000 feet, against a COAT of minus 25 degrees Celsius in the Altitude window. Then you read off the true altitude on the outer scale against 10,000 feet on the inner scale. That gives you about 9,250 feet. So your true altitude is roughly 750 feet lower than indicated. There’s also a rough rule of thumb: approximately 4 feet per 1 degree Celsius away from ISA, per 1,000 feet above sea level. So for ISA minus 20 at 10,000 feet, that’s 4 times 20 times 10, which is 800 feet — close to the 750 from the computer. But for exam purposes, you should use the mathematical method, not the rough guide. Now, the safety takeaway, and I want you to remember this phrase: in flight from HIGH to LOW TEMPERATURE, the altimeter reads HIGH. That’s potentially unsafe, because you think you’re higher than you are. And it’s directly comparable to the barometric error from the earlier worked example: flight from HIGH to LOW PRESSURE also makes the altimeter read HIGH. So both cold temperature and low pressure push your indicated altitude up while your true altitude is lower. That’s the dangerous direction. Finally, there’s a correction table for temperature error. This is used by pilots to add values to published altitudes — like decision heights — to compensate for cold temperatures. The table gives values in feet, based on the aerodrome temperature and the height above the elevation of the altimeter setting source. Let me show you how to read it. The columns run from 200 feet up to 5,000 feet of height above the altimeter setting source. The rows are temperatures from 0 down to minus 50 degrees Celsius. For example, at 0 degrees and 200 feet, the correction is 0. At minus 10 degrees and 200 feet, it’s 20 feet. At minus 40 degrees and 400 feet, it’s 80 feet. And at minus 50 degrees and 5,000 feet, it’s 1,220 feet. The values climb as it gets colder and as you get higher above the source. One important note: the table is based on an aerodrome elevation of 2,000 feet, but it can be used operationally at any aerodrome. So don’t worry that your field isn’t at 2,000 feet — it still works. Here’s the worked example. Your decision height is 400 feet. The aerodrome temperature is minus 40 degrees Celsius. From the table, at minus 40 and 400 feet, the correction is 80 feet. So your revised decision height becomes 480 feet. You add the correction to the published altitude. That way, when the altimeter reads 480 feet, your true altitude is actually 400 feet — because in the cold air the altimeter over-reads by 80 feet. You’ve compensated for the error. So the whole logic of this section is: cold air makes the altimeter over-read, and to stay safe you add correction values to your published altitudes, especially decision heights. That’s the temperature error of the pressure altimeter.