The Pressure Altimeter — Page 69, Lesson 77

Let’s pick this up right where the pressure altimeter’s other errors left off, and look at the temperature error — because this one is both subtle and genuinely dangerous in the cockpit. The core idea is this: even if the altimeter has no mechanical error at all, it will not show true altitude — height above mean sea level — unless the air column below you has the exact surface temperature and lapse rate that the instrument was calibrated against. The calibration assumes the International Standard Atmosphere, ISA. So the altimeter is only exact when the real atmosphere matches ISA. Now here’s the key behaviour: when you fly in colder air — air that is denser than ISA at that altitude — the altimeter will over-read. It will show a higher altitude than you are actually at. That’s the single most important fact in this whole section, so let’s make sure you understand why. In cold air, pressure decreases more rapidly with height than it does in warm air. Think about what that means. 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. The altimeter is just a pressure gauge — it interprets that lower pressure as a higher altitude. So it reads high. You think you’re higher than you really are. That is the classic “high to low, look out below” situation, and it’s potentially unsafe because you could be closer to terrain than your instruments tell you. Now, the book gives you a worked example to cement this. Suppose your indicated altitude is 10,000 feet, you have the local pressure set, and the corrected outside air temperature — COAT — is minus 25°C. The question is: will true altitude be more or less than indicated? Let’s work it. The ISA temperature at 10,000 feet is about minus 5°C. So you’re flying in colder-than-standard conditions — that’s ISA minus 20°. Because pressure decreases more rapidly in cold air, and assuming constant surface pressure, the pressure at your true altitude in the cold air is less than it would be at the same altitude in standard air. The altimeter interprets that lower pressure as a higher altitude, so it over-reads. True altitude is less than indicated. The book then shows you how to compute it. On a navigation computer, you set the indicated altitude — 10,000 feet — against the COAT of minus 25°C in the altitude window, and read off the true altitude on the outer scale against 10,000 feet on the inner scale. You get about 9,250 feet. So you’re roughly 750 feet lower than the instrument says. There’s also a rough rule of thumb: approximately 4 feet per 1°C away from ISA, per 1,000 feet above sea level. But the book is explicit — for exam purposes, use the mathematical method, not the approximation. Now, there’s an important correction table in this section, and I want you to understand how to use it, because it’s a real operational tool. The table gives values, in feet, to be added by the pilot to published altitudes. The columns are the height above the elevation of the altimeter setting source — from 200 feet up to 5,000 feet. The rows are temperatures from 0°C down to minus 50°C. Let me give you a concrete example from the book. Say your decision height is 400 feet, and the aerodrome temperature is minus 40°C. You go to the table, find the column for 400 feet height above the altimeter setting source, and the row for minus 40°C. The correction is 80 feet. So your revised decision height becomes 480 feet. You add the correction to the published altitude. One important note about this table: it’s based on an aerodrome elevation of 2,000 feet, but the book says it can be used operationally at any aerodrome. So don’t worry that your field isn’t exactly 2,000 feet — it’s a general-purpose tool. Now, why do we add the correction? Because in cold conditions the altimeter over-reads — it shows you higher than you are. So to keep your decision height — the height at which you must decide whether to continue an approach or go around — genuinely safe, you raise the indicated altitude by the correction. That way, when the altimeter reads 480 feet, you know you’re actually at your true decision height of 400 feet above the ground. Let me also tie this back to something you’ve already seen. The book explicitly compares this to the barometric error from the previous worked example. In flight from high to low temperature, the altimeter reads high. And in flight from high to low pressure, the altimeter also reads high. Both are the same unsafe direction — the instrument tells you you’re higher than you really are. That’s why the mnemonic “high to low, look out below” applies to both temperature and pressure. One more limitation worth noting. Where the temperature at cruising level deviates from standard, you can make an approximate correction with most navigation computers. But it’s only approximate, because you don’t know the temperatures in the rest of the air column below you. And the book is quite clear: the correction is considered too inaccurate to be worth making at heights above 25,000 feet. So this is really a low-level, approach-phase tool — not something you use at cruise altitude. So, to summarise the whole picture: the pressure altimeter assumes ISA. In cold air it over-reads, so true altitude is less than indicated. You correct for this on approach by adding the table value to published altitudes like decision height. And remember the danger — high to low temperature, just like high to low pressure, makes the altimeter read high, which is the unsafe direction.