The Pressure Altimeter — Page 53, Lesson 63

Let’s start with the big picture. The pressure altimeter is, at its core, a simple, reliable pressure gauge that is calibrated to indicate height. That’s the whole principle in one sentence. It doesn’t measure height directly; it measures pressure, and then its scale is marked so that the pressure reading is shown as a height. Why does pressure tell us about height? Because the pressure at any point depends on the weight of the column of air that extends vertically upwards from that point all the way to the outer limit of the atmosphere. Think of the air above you as a tall, heavy column. The higher an aircraft flies, the shorter the column of air above it, and consequently the lower the atmospheric pressure at the aircraft. So the greater the height, the lower the pressure. By measuring pressure, the altimeter measures height. But here’s the catch: the relationship between pressure and height is not a linear one. That means if you double the height, you don’t simply halve the pressure. So calibrating the altimeter scale is not a simple matter. And it gets even more complicated. High and low pressure weather systems produce pressure differences in the horizontal plane, and the temperature of the air at the surface, as well as the temperature lapse rate in the air above, vary considerably. All of this affects pressure. So the instrument has to be built to handle a non-linear, variable atmosphere. Now, before we go further, I want to make sure you have the definitions straight, because pilots must be familiar with these. There are four key terms. Height is the vertical distance of a level, point, or object considered as a point, measured from a specified datum. The datum can be anything you choose—it’s the reference you measure from. Elevation is the vertical distance of a fixed, non-moving point or object measured from MSL. MSL is mean sea level. So a mountain’s elevation is its height above mean sea level, and it doesn’t move. Altitude is the vertical distance of a moveable object measured from MSL. So when an aircraft is flying, its altitude is its vertical distance above mean sea level. Pressure Altitude is the altitude of the aircraft with reference to the pressure level of 1013.25 hPa. That’s the standard pressure setting. So pressure altitude is what the altimeter reads when you set the sub-scale to 1013.25 hectopascals. There’s one more term: True Altitude. True height means the height of the aircraft vertically above the surface immediately below. It’s used more often in connection with radio or radar altimeters than with pressure altimeters, because those measure actual distance to the ground below. Now, calibration. The altimeter is calibrated in accordance with the Standard Atmosphere, ISA, over its entire operating range. That range is usually from 5000 feet below sea level up to 80,000 feet. So the instrument is built to match the ISA model of the atmosphere across that whole span. There are three important notes about how this calibration is achieved. First, the pressure altimeter is calibrated to give a linear presentation of the non-linear atmospheric distribution. That’s a clever trick. Since pressure doesn’t change linearly with height, the instrument uses a variable magnification lever system and the dynamic design of the capsules to make the needle move linearly with height anyway. So the pilot sees a nice, even scale even though the physics underneath is non-linear. Second, temperature compensation is achieved by the use of a bimetal compensator connected in the lever and linkage system. Temperature affects the mechanical parts, so a bimetal strip—two metals that expand at different rates—bends with temperature and corrects the reading. Third, and this is a conversion you’ll use constantly: 1013.25 hPa equals 29.92 inches of mercury, which equals 14.7 pounds per square inch. Those are three different units for the same standard sea-level pressure. So, to tie it together: the altimeter is a pressure gauge, calibrated to ISA, with a lever system to linearize the non-linear pressure-height relationship, a bimetal compensator for temperature, and a scale that reads in height. The definitions of height, elevation, altitude, and pressure altitude give you the precise language for what the instrument is telling you.