Altimetry — Page 135, Lesson 115

I want to walk you through a worked example that shows exactly how pressure and temperature corrections combine to give you true altitude — and why getting this right can be the difference between clearing a mountain and hitting it. Let’s set the scene. Your altimeter is set to 1013 hPa — the standard pressure datum. You’re indicating 18 000 feet above that 1013 hPa level. But the actual pressure at sea level below you is different — it’s 30 hPa higher than 1013. That means the air is denser, the pressure levels are compressed closer together, and your altimeter, which assumes standard pressure, is over-reading. You’re actually lower than it says. The height correction for that 30 hPa pressure difference is: 30 × 27 = 810 feet. So your corrected altitude — just for pressure — is 18 000 minus 810, which gives 17 190 feet. That’s your pressure-corrected indicated altitude. But we’re not done. Temperature also matters. If the air is colder than ISA, the column of air is shorter, so you’re even lower than the pressure correction alone suggests. Here the temperature deviation from ISA is minus 25°C. The correction formula is: 4 × (ISA deviation) × (indicated altitude in thousands of feet). So that’s 4 × (-25) × 18 = -1800 feet. Now combine both corrections. Start from your indicated altitude of 18 000 feet, subtract 810 for pressure, subtract 1800 for temperature. That gives a true altitude of 15 390 feet. Now here’s the real-world punchline. Mont Blanc is 15 872 feet high. If you fly at your indicated 18 000 feet without correcting, your true altitude is only 15 390 feet — that puts you 392 feet below the summit. You hit the mountain. To simplify this whole calculation in one go, use the formula: True altitude = indicated altitude + (indicated altitude / 1000 × ISA deviation × 4) + 27 × (actual pressure – pressure setting) Let me read that carefully. You take your indicated altitude. You add the temperature term: indicated altitude divided by 1000, times the ISA deviation in °C, times 4. Then you add the pressure term: 27 times the difference between the actual pressure and the pressure setting you have dialled in. That gives you true altitude directly. Now let’s move to a different example — this time working out a minimum safe indicated pressure altitude. Suppose the temperature is ISA minus 30°C. You calculate that the pressure correction alone gives you a minimum indicated pressure altitude of 9110 feet. Then the temperature correction is 4 × (-30) × 9 = -1080 feet. So to ensure you clear the obstacle in cold conditions, your minimum indicated pressure altitude needs to be 9110 plus 1080 — that’s 10 190 feet. In practice, you round that up to either 10 500 feet, which is flight level 105, or 11 000 feet, flight level 110, depending on the status of your flight and the type of airspace you’re operating in. Now I want to introduce three critical definitions that govern how we transition between altitudes and flight levels. First, transition altitude. That’s the altitude at or below which the vertical position of an aircraft is controlled by reference to altitude — meaning you use QNH, so your altimeter reads true altitude above mean sea level. Second, transition level. That’s the lowest flight level — set to 1013 hPa — that is available for use above the transition altitude. So once you climb through the transition altitude, you switch to standard pressure 1013 and fly at flight levels. Third, transition layer. That’s the airspace between the transition altitude and the transition level. It’s the buffer zone where you make the changeover. Finally, I have a table of practice examples for you to fill in. Assume 1 hPa equals 27 feet. The table gives you QNH, altimeter setting, true altitude, or altimeter reading — and you work out the missing value. The answers are provided so you can check your work.