Altimetry — Page 135, Lesson 113

I want to walk you through the altimeter errors that matter most from a meteorological standpoint. Apart from any instrument errors inside the altimeter itself, there are two errors we care about here: barometric error and temperature error. Let's start with barometric error. This is the error caused by setting a pressure on the subscale that is not the correct one. In other words, if you dial in the wrong pressure setting, your altimeter will show the wrong altitude. For calculations in the meteorology syllabus, we use a standard figure of 27 feet per hectopascal to work out the difference between indicated altitude and true altitude. So if you set a pressure that's 1 hPa off, your altitude reading will be off by 27 feet. Now, temperature error. Your altimeter is calibrated to match the ICAO International Standard Atmosphere — the ISA. That means it assumes a specific temperature profile as you climb. If the actual temperature is different from the ISA value, the altimeter will be in error. The old rule "HI-LO-HI" still applies here: when you fly from a high-pressure area to a low-pressure area, or from a warm area to a cold area, your true altitude is lower than your indicated altitude. To get a corrected altitude, you'd use a navigational computer or a correction table. Let me be more precise about temperature error correction. Pressure altimeters are calibrated to indicate true altitude under ISA conditions. Any deviation from ISA gives you an erroneous reading — with one important exception: when you're on the ground with QNH set, the altimeter will read the correct elevation of the airfield regardless of temperature. So on the ground, it's fine. But once you're airborne, the error appears. Here's the key relationship: when temperatures are lower than ISA, your aircraft's true altitude will be lower than what the altimeter reads. That's dangerous — you think you're higher than you actually are. The error is proportional to two things: first, the difference between the actual temperature and the ISA temperature, and second, the vertical distance of the aircraft above the altimeter setting datum. The greater that distance, the bigger the error. The standard correction we use is 4 feet per degree Celsius deviation from ISA per 1000 feet. Important note: you must make this calculation over the indicated height difference from the datum for the pressure setting. So you take the indicated height above the datum, multiply by the temperature deviation in degrees Celsius, multiply by 4, and divide by 1000 — or more simply, 4 feet per degree per thousand feet. Let me walk through the example they give. You're making an approach to an aerodrome at mean sea level in Siberia in January. Your decision height is 200 feet indicated. The temperature is -50°C. What's the true height when the altimeter reads 200 feet? First, we need the deviation from ISA. At mean sea level, ISA temperature is 15°C. So the deviation is -50 minus 15, which is -65°C. The indicated height above the datum — mean sea level in this case — is 200 feet. That's 0.2 thousand feet. So the error is 4 times -65 times 0.2, which equals -52 feet. That means the true height is 200 minus 52, which is only 148 feet. That's clearly unacceptable — you'd be 52 feet lower than your altimeter says, which could be catastrophic on approach. So when you're carrying out an aerodrome or runway approach in temperatures colder than standard, you must increase your indicated decision height or minimum descent height according to a correction table. Let me read you that table so you understand the values. For a height above touchdown or above the aerodrome of 200 feet: at -15°C deviation, add 12 feet; at -25°C, add 20; at -35°C, add 28; at -45°C, add 36; at -55°C, add 44; at -65°C, add 52. For 300 feet: at -15°C, add 18; at -25°C, add 30; at -35°C, add 42; at -45°C, add 54; at -55°C, add 66; at -65°C, add 78. For 400 feet: at -15°C, add 24; at -25°C, add 40; at -35°C, add 56; at -45°C, add 72; at -55°C, add 88; at -65°C, add 104. For 500 feet: at -15°C, add 30; at -25°C, add 50; at -35°C, add 70; at -45°C, add 90; at -55°C, add 110; at -65°C, add 130. For 600 feet: at -15°C, add 36; at -25°C, add 60; at -35°C, add 84; at -45°C, add 108; at -55°C, add 132; at -65°C, add 156. For 700 feet: at -15°C, add 42; at -25°C, add 70; at -35°C, add 98; at -45°C, add 126; at -55°C, add 154; at -65°C, add 182. For 800 feet: at -15°C, add 48; at -25°C, add 80; at -35°C, add 112; at -45°C, add 144; at -55°C, add 176; at -65°C, add 208. For 900 feet: at -15°C, add 54; at -25°C, add 90; at -35°C, add 126; at -45°C, add 162; at -55°C, add 198; at -65°C, add 234. For 1000 feet: at -15°C, add 60; at -25°C, add 100; at -35°C, add 140; at -45°C, add 180; at -55°C, add 220; at -65°C, add 260. The key point: with temperatures colder than standard, you must always consider the effect on terrain clearance. Let me give you the example they provide. You're planning a flight at Flight Level 180 over Mont Blanc, which has an elevation of 15,782 feet. The mean sea level pressure is 983 hPa, taken from an aerodrome at mean sea level. The temperature of the air up to the summit is 25°C colder than ISA. You need to determine the true altitude of the aircraft at Mont Blanc and the terrain clearance. That's the kind of calculation you'll need to be able to do — and Figure 9.10 in the book illustrates this scenario.