Piloting an aircraft is quite a complex endeavor, requiring one to govern the trajectory of the vehicle, manage various systems, maintain communication with ATC, keep awareness of their surroundings, and so much more at once. One of the most important things that pilots must do to ensure a safe flight is to be constantly aware of their position in the atmosphere, as many things are dependent on altitude, ranging from engine performance to collision avoidance. In order to monitor the aircraft’s exact altitude at all times, one must rely on a standard cockpit instrument known as the altimeter. This instrument is always present in some shape or form within the aircraft, regardless of the type of aircraft you are flying or the type of cockpit your model features. As such, it is important that any current or prospective pilot has a good understanding of such instruments, and how their readings are obtained.
While there are many other activities that make use of the barometric altimeter, pretty much all work in a similar way to get the same type of reading, generally just varying in their parts and design. The altimeters found within aircraft are of the aneroid barometer variety, and they rely on an aviation system known as a static port to conduct readings. The static port is a part of the wider pitot-static system, which consists of pressure-sensitive instruments that allow for flight characteristics like airspeed, Mach number, altitude, and altitude trend to be determined.
The static port itself is situated outside of the aircraft, coming in the form of a flush-mounted hole that is located in a relatively undisturbed area. With this placement, the static port can access airflow to obtain what is known as static pressure. Depending on the aircraft, there may be more than one static port, but they will always be placed in an area where they are not exposed to relative wind.
In general, air pressure decreases as the altitude increases, often following a rule of one inch of mercury per 1,000 feet climbed. In order to set a usable standard across the board, aneroid altimeters used in aircraft are specifically calibrated to read pressure in terms of altitude above mean sea level, and this model was established by the International Standard Atmosphere (ISA). So, as an aircraft ascends and descends, changes in atmospheric pressure will be reflected in the static pressure obtained by the static port. That being said, how does the instrument itself use this pressure to make a reading?
For conventional altimeters, such instruments will have an internal compartment where static pressure is directed from the static port into the camber, surrounding a sealed disc known as an aneroid or bellows. As the aircraft climbs upward, air pressure within the case will decrease, causing the bellows to expand. During a descent, the opposite occurs as increased pressure compresses the bellows. As the bellows adjust in any way, their mechanical connection to the instrument face with gears will cause a needle to adjust across a dial. This makes it very easy to determine current altitude through the movement of this needle pointer as the dial is marked with various altitude measurements. While many modern aircraft now have digital instruments, the measurements are still conducted in the same way, the reading simply being displayed on a digital display instead of a mechanical gauge.
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