The Pressure Altimeter — Page 63, Lesson 73

Let’s pick up right where the altimeter’s construction leaves off and move into the part that really matters in the cockpit: the errors that affect it. I want to walk you through the main altimeter errors. The book lists them clearly, and you need to know each one by name, what causes it, and how it shows up in the reading. The first thing to understand is that these errors are many, and the extent of some of them varies with altimeter type. A lot of effort goes into improving instrument accuracy, and the permissible tolerances of modern altimeters are smaller than with earlier types. So the instrument itself is better than it used to be, but errors still exist. There are also errors caused by deviation of the actual atmosphere from standard conditions, and by the difficulty in sensing correctly the outside air pressure. So we have two broad sources: the instrument and the atmosphere around it. Let’s go through them one by one. First, Position (or Pressure) Error. This is largely due to the inability to sense the true static pressure outside the aircraft. The static vent isn’t perfect — it can’t capture the exact ambient pressure because of airflow around the aircraft. This is described in the chapter on Pressure Heads. The error is usually small, but it increases at high Mach numbers, and consequently at high altitudes, which are usually associated with high Mach numbers. So the faster and higher you go, the more this error grows. Second, Instrument Error. This comes from manufacturing imperfections, including friction in the linkage, and it causes errors throughout the operating range. The errors are kept as small as possible by adjustments within the instrument, and the calibration procedure ensures that they are within permitted tolerances. Residual errors may be listed on a correction card. So even after calibration, whatever is left over gets written down on a card so you know the remaining error. There’s an important note here: with the sensitive altimeter, the error increases with altitude. That also explains why the decrease of accuracy with altitude is less serious with the servo altimeter. So the servo altimeter handles high altitude better. Third, Manoeuvre-induced Error. This is caused by transient fluctuations of pressure at the static vent during change of, mainly, pitch attitude, and delays in the transmission of pressure changes due to viscous and acoustic effects in the static pipeline. So when you pitch the aircraft, the pressure at the static vent fluctuates briefly, and the pressure changes are delayed as they travel through the static line. This is discussed more fully in Chapter 2 on pressure heads. Fourth, Barometric Error. This one is tied to the pressure subscale. Providing the altimeter has a pressure subscale, and the local pressure is set on it, the altimeter will indicate height AMSL — that is, height above mean sea level — though still subject to the other errors. If the local surface pressure has changed since the pressure value was set, a barometric error of roughly 30 feet per hectopascal will result. And here’s the key direction: if pressure has fallen, the altimeter over-reads. So a drop in pressure makes the altimeter read higher than the true altitude. Now, the book gives you an example problem, and I want to work through it with you because exam questions sometimes include the term ‘height involved’, which complicates matters. Think carefully when answering. Here’s the scenario. An aircraft flies from ‘A’ to ‘B’ at a constant indicated altitude of 10,000 feet, with the pressure at ‘A’ of 1025 hPa set on the subscale THROUGHOUT THE FLIGHT. On arrival overhead ‘B’, where the pressure is 995 hPa, what will be the true altitude, assuming that there are no other errors, and assuming that 1 hPa corresponds to 30 feet? Let’s work it out. The pressure at ‘A’ is 1025 hPa, and at ‘B’ it’s 995 hPa. The difference is 1025 minus 995, which is 30 hPa. Since 1 hPa corresponds to 30 feet, a 30 hPa difference corresponds to 900 feet. Now, the pressure has fallen from ‘A’ to ‘B’ — it went from 1025 down to 995. And we just learned that if pressure has fallen, the altimeter over-reads. So the altimeter is reading 10,000 feet, but it’s over-reading by 900 feet. That means the true altitude is 10,000 minus 900, which is 9,100 feet. So the true altitude overhead ‘B’ is 9,100 feet. The altimeter says 10,000, but because the pressure fell, it’s reading too high, and the true altitude is lower. That’s the core of barometric error in practice. You set the pressure at departure, and if the pressure changes along the way, your altimeter reading drifts from true altitude. And the direction is critical: pressure falls, altimeter over-reads, true altitude is lower than indicated. Let me also make sure you have the visual sense of the instrument. shows the sensitive altimeter, and the principle of operation is similar to the simple altimeter but with the refinements we discussed. shows the counter/pointer altimeter, which is the modern digital-style readout. And shows a sensitive altimeter reading 265 feet, so you can see how the subscale and the pointers work together. So to sum up the errors: position error from imperfect static sensing, instrument error from manufacturing and friction, manoeuvre-induced error from pitch changes and static line delays, and barometric error from pressure changes after you set the subscale. Each one has its own cause and its own behaviour, and you need to know them cold for the exam.