The Pressure Altimeter — Page 63, Lesson 73

We're now into the errors that affect the pressure altimeter. Let me walk you through them one by one, because each one has a distinct cause and a distinct character. First, understand the big picture. The altimeter is a mechanical instrument, and it reads pressure. So its errors fall into two broad families: errors from the instrument itself, and errors from the atmosphere not behaving as we assume. The book lists them as Position Error, Instrument Error, Manoeuvre-induced Error, and Barometric Error. Let's start with Position Error, which is also called Pressure Error. This comes from the fact that the altimeter can't sense the true static pressure outside the aircraft. The static vent isn't perfect — airflow over the fuselage disturbs the pressure at the vent. This error is usually small, but it increases at high Mach numbers. And because high Mach numbers usually go with high altitudes, it tends to be worse up there. Next, Instrument Error. This is purely mechanical. Manufacturing imperfections, including friction in the linkage, cause errors throughout the operating range. The instrument is adjusted internally to keep these as small as possible, and the calibration procedure ensures they stay within permitted tolerances. Any residual errors may be listed on a correction card. Now here's a key note from the book: with the sensitive altimeter, this error increases with altitude. That's why the decrease of accuracy with altitude is less serious with the servo altimeter — the servo type doesn't suffer that growing error the same way. Then we have Manoeuvre-induced Error. This is caused by transient fluctuations of pressure at the static vent during changes of, mainly, pitch attitude. And there's a second cause: delays in the transmission of pressure changes due to viscous and acoustic effects in the static pipeline. So when you pitch, the pressure at the vent wavers, and the pipe itself slows down the pressure change reaching the instrument. This is discussed more fully in the chapter on pressure heads. Finally, the big one for everyday flying: Barometric Error. Providing the altimeter has a pressure subscale, and you set the local pressure on it, the altimeter will indicate height AMSL — that's height above mean sea level — though still subject to the other errors. But if the local surface pressure has changed since you set the value, you get a barometric error of roughly 30 feet per hectopascal. And here's the direction: if pressure has fallen, the altimeter over-reads. Let me give you the example problem from the book, because it shows exactly how to apply this. An aircraft flies from 'A' to 'B' at a constant indicated altitude of 10,000 feet. The pressure at 'A' is 1025 hPa, set on the subscale throughout the flight. On arrival overhead 'B', the pressure there is 995 hPa. What is the true altitude, assuming no other errors and that 1 hPa corresponds to 30 feet? Work it out with me. The pressure has fallen from 1025 to 995 — that's a drop of 30 hPa. At 30 feet per hPa, that's 30 times 30, which is 900 feet. Since pressure fell, the altimeter over-reads. So it's reading 10,000 feet, but it's actually 900 feet lower than that. The true altitude is 10,000 minus 900, which is 9,100 feet. Now, the book adds a caution about the term 'height involved' — exam questions sometimes use it, and it complicates matters. Think carefully. In this problem, the 'height involved' is the 900 feet of error, not the 10,000 feet of indicated altitude. The true altitude is what the aircraft is actually at, which is 9,100 feet. Let me show you the sensitive altimeter and the counter-pointer altimeter so you can see the hardware behind these errors. So to summarise the four errors: Position Error from imperfect static sensing, Instrument Error from mechanical imperfections, Manoeuvre-induced Error from pitch changes and pipeline delays, and Barometric Error from a stale pressure setting — 30 feet per hPa, over-reading when pressure falls. That last one is the one you'll correct in the cockpit every flight.