The Pressure Altimeter — Page 69, Lesson 81

Let’s pick up with the pressure altimeter and look at what happens when things go wrong — blockages and leaks in the static system. First, the static source. That’s the little vent or port on the side of the aircraft that samples the outside air pressure. The altimeter uses that static pressure to work out your height. Now, if that static source becomes blocked, the altimeter will not register any change in height. Whatever height you were at when the blockage occurred — that’s the value it will keep showing, no matter whether you climb or descend. The instrument is frozen at that reading. On many aircraft, though, there’s an alternative source of static pressure available. That’s a backup port you can select if the primary one blocks. Good to know it exists, because a blocked static source is a serious failure. Now, what about a fracture — a break — in the static line itself? That’s the pipe running from the static port to the instruments. In a pressurized aircraft, the cabin is kept at a higher pressure than the outside. So if the static line fractures, the altimeter will show the lower cabin altitude rather than the true aircraft altitude. The cabin pressure leaks into the line, and the instrument reads that instead. In an unpressurized aircraft, a fracture in the static line normally results in the altimeter over-reading. Why? Because the pressure in the cabin is lower than the ambient — the outside — pressure, due to aerodynamic suction. The airflow over the fuselage sucks air out, creating a slight vacuum inside. So the altimeter sees a lower pressure than it should, and it over-reads. Now, here’s the key relationship for a blocked static source. 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. So the further you climb or descend from that frozen height, the bigger the error gets. Let me show you that with a diagram. That’s the static feed blocked case — descending, over-reading. Now, let’s move on to density altitude. This is a term you’ll hear a lot. 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 in terms of a standard atmosphere altitude. It’s a convenient parameter when it comes to engine performance figures — because engines care about air density, not just pressure. Next, preflight altimeter checks. In the UK, the apron is the designated location for these checks. The apron is the loading and unloading and/or parking area of the aerodrome. 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 contains aerodrome information. Now, let’s work through an example problem to see how a preflight altimeter check is done. Here are the details: Aerodrome elevation: 235 feet. Apron elevation: 225 feet. Height of the altimeter above the apron: 20 feet. Altimeter reading with QFE set: 40 feet. QFE, by the way, is the pressure setting that makes the altimeter read zero at the aerodrome elevation — or in this case, at the apron. Let’s work through the solution. The apron is 10 feet below the stated aerodrome elevation. So, assuming the QFE is for the aerodrome level, an altimeter sitting on the apron should read minus 10 feet. 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 it should read. Its actual reading is plus 40 feet. So it is over-reading by 30 feet. That gives us an instrument error of plus 30 feet. So the altimeter is showing 30 feet higher than it should. That’s the instrument error you’d record from this preflight check. That’s the core of blockages, leaks, density altitude, and the preflight check. Let’s keep going when you’re ready.