The Pressure Altimeter — Page 69, Lesson 81

Let’s pick this up right where the static system leaves off, because the pressure altimeter is the instrument that turns that static pressure into a height reading. I want to walk you through what happens when that static feed gets blocked or leaks, then we’ll cover density altitude, and finally the preflight altimeter check with a worked example. First, blockages. If the static source becomes blocked, the altimeter will not register any change in height. The height at which the blockage occurred will still be indicated, regardless of any climb or descent. Think about it — the altimeter compares static pressure to the pressure inside its sealed aneroid capsule. If the static line is sealed off, the pressure inside the instrument is frozen at whatever it was when the blockage happened. So you climb, the outside pressure drops, but the instrument still sees the old pressure, and it keeps showing the height where the blockage occurred. On many aircraft, an alternative source of static pressure will be available — that’s your backup, and it’s covered in Chapter 2 under the Emergency Static Source. Now leaks. If the static line fractures in a pressurized aircraft, the altimeter will show the lower cabin altitude rather than aircraft altitude. Why lower? Because in a pressurized cabin, the cabin pressure is held higher than the ambient outside pressure. A fracture lets that higher cabin pressure into the static line, so the instrument senses a higher pressure than the true ambient, and it interprets that as a lower altitude. So you get cabin altitude displayed, not your true aircraft altitude. In an unpressurized aircraft, a fracture in the static line will normally result in the altimeter over-reading. The reason is aerodynamic suction — the airflow over the fuselage creates a lower pressure in the cabin than the ambient outside pressure. That lower pressure leaks into the static line, the instrument senses a lower pressure than true ambient, and it over-reads. So the rule of thumb: if the aircraft is climbing, the altimeter will under-read; if the aircraft is descending, the altimeter will over-read. And the amount of the error will increase as the aircraft moves away from the height at which the blockage occurred. The further you get from that frozen height, the bigger the discrepancy between the true pressure and what the instrument is seeing. Let me show you that with a diagram. That figure shows the static feed blocked — if the aircraft is descending, the altimeter will over-read, exactly as I described. Now, density altitude. This is a definition you need to hold onto. Density altitude can be defined as the altitude in the standard atmosphere at which the prevailing density would occur. Alternatively, it’s the altitude in the standard atmosphere corresponding to the prevailing pressure and temperature. So it’s a way of expressing the actual air density you’re flying through, referenced back to the standard atmosphere. It’s a convenient parameter in respect of engine performance figures — because engine power depends on air density, not just pressure altitude. On a hot day, the air is less dense, so the density altitude is higher than your pressure altitude, and your engine performs as if you were at that higher altitude. Now the preflight altimeter check. In the UK, the apron is the designated location for pre-flight altimeter checks. The apron is the loading and unloading and/or parking area. The apron elevation is displayed in the flight clearance office of the aerodrome concerned, and it’s also published in the AGA section of the UK Air Pilot. AGA stands for Aerodromes, Air Traffic Services and Airspace — that’s the section of the UK Air Pilot that carries aerodrome information. Let’s work through the example problem, because this is where it all comes together. We have an aerodrome elevation of 235 feet, an apron elevation of 225 feet, the height of the altimeter above the apron is 20 feet, and the altimeter reading with QFE set is 40 feet. First, the apron is 10 feet below the stated aerodrome elevation — 235 minus 225 is 10. So assuming the QFE is set for the aerodrome level, an altimeter on the apron should read minus 10 feet. QFE is the pressure setting that makes the altimeter read zero at the aerodrome elevation, so at the apron, which is 10 feet below that, it should read minus 10. But the instrument is positioned in the aircraft, 20 feet above the apron. So it should show minus 10 plus 20, which equals plus 10 feet. That’s what the altimeter ought to read. Its actual reading is plus 40 feet. So it is over-reading by 30 feet — 40 minus 10 is 30. That gives us an instrument error of plus 30 feet. So the whole check is about comparing what the altimeter should read, given the geometry of the apron and the aircraft position, against what it actually reads, and the difference is your instrument error. That’s the preflight altimeter check in practice. Let me show you the datum diagram that ties this together. That figure shows the datum relationships — aerodrome elevation, apron elevation, and where the altimeter sits relative to them. So to summarise what we’ve covered: a blocked static source freezes the altimeter at the height of the blockage; a fracture in a pressurized aircraft shows cabin altitude; a fracture in an unpressurized aircraft over-reads due to aerodynamic suction; climbing under-reads, descending over-reads, and the error grows with distance from the blockage height. Density altitude is the standard-atmosphere altitude matching the prevailing density, or equivalently the prevailing pressure and temperature. And the preflight check uses the apron elevation, the aircraft’s height above it, and a QFE reading to quantify instrument error.