The Pressure Altimeter — Page 53, Lesson 65

Let's start with the simple altimeter, because everything else builds on it. The instrument's case is sealed, and static pressure is fed into it from the static source — that's the port on the aircraft that samples the outside air pressure. As the aircraft climbs, height increases, so static pressure decreases. Inside the case, there's a capsule, and as the outside pressure drops, the capsule expands. That expansion is controlled by a leaf spring, which resists it and keeps the movement smooth and predictable. Now, that expansion is tiny, so a mechanical linkage magnifies it and converts it into a rotational movement of a single pointer over the height scale. The linkage also incorporates a temperature-compensating device. That's there to minimize errors caused by expansion and contraction of the linkage itself, and changes in spring tension, both of which happen when the temperature of the mechanism fluctuates. So the instrument is designed to stay accurate despite temperature changes. The simple altimeter has a setting knob, which is geared to the pointer. With this knob, you can set the pointer to read zero with the aircraft on the ground, so that when you're airborne, the altimeter indicates approximate height above aerodrome level. Alternatively, you can set the pointer, before flight, to the aerodrome elevation, so that when airborne, the instrument shows approximate height above mean sea level. So the same knob lets you choose your reference — either the field you're at, or sea level. Now, the simple altimeter wasn't accurate enough, so it was developed into the Sensitive Altimeter. The principle of operation is similar, but there are refinements. First, a bank of two or three capsules gives the increased movement necessary to drive three pointers. These are geared 100:10:1. The smallest pointer indicates 100,000 feet per revolution, the next 10,000 feet per revolution, and the largest 1,000 feet per revolution. So you have a fine pointer for hundreds, a middle one for thousands, and a coarse one for tens of thousands. Second, jewelled bearings are fitted. These reduce friction and the associated lag in indications — meaning the pointers respond more quickly and accurately. Some altimeter systems also employ "knocking" or "vibrating" devices to help overcome the initial inertia of the internal gear train when transmitting movement from the capsules to the pointers. That's a mechanical nudge to get things moving. Third, and most important, a variable datum mechanism is built in. With the aid of a setting knob, this enables the instrument to be set to indicate height above any desired pressure datum. Here's how it works: the pilot turns the knob until the desired pressure level — say, 1005 hPa — appears on a pressure subscale on the face of the instrument. As the knob is turned, the height pointers rotate. When the procedure is completed, with the subscale showing the desired 1005, the altimeter indicates the aircraft's height above that pressure level. So if, for instance, the aerodrome level pressure happened to be 1005 hPa, the altimeter would be reading height above the aerodrome. The subscale setting only changes when the pilot turns the knob. A change in altitude or surface pressure has no direct effect on the reading of the subscale — it stays where you set it until you change it. That's the core of the pressure altimeter — the simple version and the sensitive version with its three pointers, jewelled bearings, and the variable datum mechanism that lets you set your reference pressure.