Altimeters are a very useful tool for a variety of applications including flying aircraft, hiking, climbing, skiing, and more. Altitude measurement is made by determining atmospheric pressure, otherwise known as barometric pressure. Atmospheric pressure refers to the pressure that is caused by gravity pulling in the air from the atmosphere towards Earth. As pressure is caused by the accumulated air weight, atmospheric pressure is reduced while travelling upwards in elevation. With an understanding of atmospheric pressure and how it differs depending on elevation, we can now discuss how the altimeter aircraft feature provides pilots with altitude readings.
While there are both barometric altimeters and radio altimeters, the altimeter aircraft most commonly rely on for most of a flight is the barometric altimeter, which uses atmospheric pressure to obtain results. The radio altimeter, on the other hand, measures altitude by sending out a radio signal below the aircraft then timing how long it takes to bounce back. With the barometric altimeter, a preset pressure value is made, usually calibrated to sea level. By making this preset, mercury inside aneroid wafers within the instrument is measured from 29.92” Hg. Outside of the wafer, static pressure adjusts with the climb and descent of the aircraft. As static pressure moves away from 29.92” Hg, the imbalance in pressure causes the wafers to begin expanding or contracting, causing a needle on the altimeter gauge to adjust to display indicated altitude in feet.
Static pressure for the altimeter is obtained through the use of a static port. The static port is a part of the pitot-static system and is a hole located in a relatively undisturbed area on the fuselage. Here, the static port can obtain atmospheric pressure readings with good flow of air. While many aircraft may only have one static port, others may feature multiple as a safety measure in the case that one becomes blocked. For measurement, static pressure decreases as the aircraft climbs, and this rate is typically one inch of mercury per 1,000 feet ascended or descended.
By themselves, aircraft altimeters are fairly accurate due to the advanced sensor barometric NSN parts and other internal components that work to provide precise readings. Inaccurate readings and functionality could be caused by a pilot who calibrates the system incorrectly. With a wrong calibration, incorrect altitude readings may be provided which can be very dangerous. Because of this, the general rule is to always set the altimeter with the sea level pressure of 29.92” Hg. Atmospheric conditions such as pressure and temperature can affect readings, however, and pilots should take these into consideration when travelling to areas that are far away from where the altimeter was first calibrated. Due to these issues, aircraft such as those within civil aviation also utilize the radio altimeter in order to ensure that aircraft do not fly in hazardous altitudes during take-off, landing, and cruising. As radio altimeters only measure up to 2,500 feet off of the ground, they cannot fully be utilized in place of the standard barometric altimeter.
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