The Pressure Altimeter — Page 53, Lesson 65

Let's get into the pressure altimeter. I want to start with the simple altimeter, because it's the foundation everything else builds on. The core idea is beautifully simple. Static pressure — that's the ambient air pressure around the aircraft, taken from the static source — is fed into the case of the instrument. As the aircraft climbs, height increases, and static pressure decreases. That's the fundamental relationship we're exploiting: lower pressure at higher altitude. Inside the instrument, there's a capsule — a sealed, flexible container. As the outside static pressure drops, the capsule expands. But it doesn't expand freely; it's controlled by a leaf spring, which resists that expansion and gives the capsule a predictable, repeatable response to pressure. Now, the capsule's expansion is tiny. So a mechanical linkage magnifies that expansion and converts it into a rotational movement of a single pointer over the height scale. That's the whole instrument: pressure in, pointer movement out. But there's a subtlety I want you to appreciate. The linkage itself is made of metal, and metal expands and contracts with temperature. If the linkage expands, it could shift the pointer even though the pressure hasn't changed. So the linkage incorporates a temperature-compensating device. Its job is to minimize errors caused by expansion and contraction of the linkage, and also changes in spring tension due to fluctuations in the temperature of the mechanism. So the instrument is designed to be stable across temperature changes. Now, the simple altimeter has a setting knob, and this knob is geared to the pointer. This is important. By turning the knob, the pilot can physically rotate the pointer relative to the mechanism. There are two standard ways to use this. First, with the aircraft on the ground, you can set the pointer to read zero. Then, when airborne, the altimeter indicates approximate height above aerodrome level — that is, height above the airfield. Alternatively, before flight, you can set the pointer to the aerodrome elevation — the published height of the airfield above mean sea level. Then, when airborne, the instrument shows approximate height above mean sea level. So the same knob lets you choose your reference: aerodrome level or mean sea level. That's the simple altimeter. Now, the problem. 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, and I want to walk you through each one. First, instead of a single capsule, there's a bank of two or three capsules. This gives the increased movement necessary to drive three pointers. And here's the gearing: the three pointers are geared 100:10:1. Let me make sure you understand what that means. The smallest pointer indicates 100,000 feet per revolution. The next one indicates 10,000 feet per revolution. And the largest indicates 1,000 feet per revolution. So you read the three pointers together to get the full altitude — the largest pointer gives you thousands of feet, the next gives ten-thousands, and the smallest gives hundred-thousands. Second refinement: jewelled bearings are fitted. These reduce friction, and friction is what causes lag in the indications — the pointer lagging behind the actual pressure change. Jewelled bearings cut that lag down. There's a note here worth mentioning. Some altimeter systems employ "knocking" or "vibrating" devices. These help overcome the initial inertia of the internal gear train when transmitting movement from the capsules to the pointers. So if the gear train is sluggish at the start of a movement, a knock or vibration frees it up. Third refinement: a variable datum mechanism is built in. This, with the aid of a setting knob, enables the instrument to be set to indicate height above any desired pressure datum. This is the big upgrade over the simple altimeter. Instead of just zero or aerodrome elevation, you can set any pressure level you want. Here's how the variable datum mechanism is used. The pilot turns the knob until the desired pressure level — say, 1005 hPa — appears on a pressure subscale on the face of the instrument. That's the little window showing the pressure setting. 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. That's the connection: set the pressure, get the height above that pressure level. One more critical point, and I want you to hold onto this. 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. The subscale is purely a pilot-set reference. The altitude reading changes with pressure, but the subscale stays where you put it until you turn the knob again. Let me show you the basic layout. Here's the simple altimeter with static pressure fed into the case. And here's the sensitive altimeter with the bank of capsules and the three pointers. So to tie it together: the simple altimeter gives you a single pointer and a knob for zero or aerodrome elevation. The sensitive altimeter gives you three pointers geared 100:10:1, jewelled bearings to cut lag, and a variable datum mechanism so you can set any pressure level and read height above it. The subscale only changes when you turn the knob — altitude and surface pressure don't touch it.