The Pressure Altimeter — Page 53, Lesson 63

Let’s start with the heart of the instrument. The pressure altimeter is, at its core, a simple, reliable pressure gauge that is calibrated to indicate height. That’s the whole idea in one sentence. It doesn’t measure height directly; it measures pressure, and then its scale is marked so that the pressure reading tells you a height. Why does that work? Because of how the atmosphere behaves. 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 it as a stack of air sitting on top of you. The higher an aircraft flies, the shorter that column of air above it becomes, and consequently the lower the atmospheric pressure at the aircraft. So the greater the height, the lower the pressure. By measuring the pressure, the altimeter measures height. That’s the fundamental principle. Now, there’s a catch. The relationship between pressure and height is not a linear one. It’s not a straight-line relationship. That means calibrating the altimeter scale is not a simple matter. The pressure doesn’t drop evenly as you climb; it drops more rapidly at lower altitudes and more slowly higher up. And it gets even more complicated. High and low pressure weather systems produce pressure differences in the horizontal plane. So at the same height, the pressure can be different depending on where you are horizontally. Furthermore, the temperature of the air at the surface and the temperature lapse rate in the air above vary considerably, and this affects pressure. So temperature changes the pressure, which changes what the altimeter reads. Let me give you some definitions you need to be familiar with, because these terms get used very precisely in aviation. Height is the vertical distance of a level, point, or object considered as a point, measured from a specified datum. So height is measured from some chosen reference, whatever that reference is. Elevation is the vertical distance of a fixed, non-moving point or object measured from MSL. MSL is mean sea level. So a mountain top has an elevation, because it doesn’t move. Altitude is the vertical distance of a moveable object measured from MSL. So an aircraft in flight has an altitude, because it moves. Pressure Altitude is the altitude of the aircraft with reference to the pressure level of 1013.25 hPa. That’s a specific standard pressure setting. We’ll come back to that number. There’s one more term: True Altitude. True Height means the height of the aircraft vertically above the surface immediately below. So it’s the actual distance from the aircraft straight down to the ground beneath it. This is used more often in connection with radio or radar altimeters than with pressure altimeters, because a pressure altimeter doesn’t directly measure that true distance to the ground. Now, calibration. The altimeter is calibrated in accordance with the Standard Atmosphere, which is abbreviated 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 here. Note 1: The pressure altimeter is calibrated to give a linear presentation of the non-linear atmospheric distribution. In other words, even though the real atmosphere is non-linear, the instrument is designed so that its scale reads linearly. This is achieved by the use of a variable magnification lever system and the dynamic design of the capsules. The capsules are the aneroid capsules inside the instrument that expand and contract with pressure, and the lever system magnifies their movement in a way that compensates for the non-linearity. Note 2: Temperature compensation is achieved by the use of a bimetal compensator connected in the lever and linkage system. A bimetal compensator is a piece made of two different metals bonded together that bend with temperature changes, and it’s connected into the mechanism to correct for temperature effects on the instrument. Note 3: This is a conversion you should remember. 1013.25 hPa equals 29.92 inHg, which equals 14.7 psi. Those are three different units for the same standard pressure. hPa is hectopascals, inHg is inches of mercury, and psi is pounds per square inch. So to tie it all together: the altimeter measures pressure, and because pressure decreases with height, it can be calibrated to show height. But because the pressure-height relationship is non-linear and affected by weather systems and temperature, the instrument needs careful mechanical design — the variable magnification lever system, the dynamic capsule design, and the bimetal compensator — to give you a reliable reading. And all of that is calibrated against the Standard Atmosphere, ISA, with 1013.25 hPa as the reference pressure level.