How Does an Altimeter Work?

When piloting an aircraft, the altimeter is an indispensable instrument for safe and efficient operations. Altimeters are used to determine the altitude of an aircraft in comparison to sea level. In order to garner such readings, the instrument relies on various parts and components that allow it to measure pressure. As altimeter readings are consistently used for flight planning, flight operations, and may require fixes for errors, understanding how the altimeter functions is crucial for any pilot.

Within a standard altimeter instrument, a stack of sealed aneroid wafers are present and contain an internal pressure 29.92” Hg which is the value normally found at sea level. As the altimeter is connected to the static port, static pressure will enter the system and surround the wafer within the case. Depending on the pressure of air from the static port, the wafer will either expand or contract. Higher static pressures will cause the wafer to contract, while lower values will allow the android wafers to expand. Generally, compression of the wafer denotes a decrease in altitude while expansion is an increase. Furthermore, every 0.1” Hg is equivalent to 100 feet in altitude, and this value can be useful when monitoring altitude and flight plans.

For more conventional gauges, mechanical linkages will attach the wafer to a needle placed on the instrument face, and the movement of the aneroid wafer will cause the needle to move in relation to changes. On the face of the instrument, the most common configuration is to have 3-hands. For the three hands, the longest hand denotes increments of 10,000 feet, the shortest hand is increments of 1,000 feet, and the medium hand is increments of 100 feet. As such, it can be fairly straightforward to take readings from the instrument.

While readings from the altimeter are crucial for piloting, it is important to be aware that the indicated airspeed (IAS) values given by the instrument do not account for any instrument errors, atmospheric conditions, etc. When the indicated airspeed is corrected for any instrument errors, installation errors, or position errors, the calibrated airspeed (CAS) is found. This value can still be further refined with charts or a flight computer to receive true airspeed (TAS). True airspeed is the most important value to find out of the various airspeed types as it is what is used for flight planning.

In many modern aircraft, the conventional dial and gauge instruments have been replaced with more advanced Electronic Flight Displays (EFDs). With such assemblies, the readings of an altimeter are obtained with the use of an Air Data Computer and the static port is still relied upon for pressure. Despite this, the diaphragm is not present as barometric air pressure is measured and relayed to the primary flight display for a reading to be displayed. As compared to the more traditional altimeter systems, the EFD utilizes less moving parts which can be beneficial in some aspects.

With the proper instruments and systems in place, measuring and monitoring altitude can be done with ease for the benefit of flight planning and flight safety. At NSN Sourcing, we serve as an online distributor of Aircraft parts with over 2 billion new, used, and obsolete items readily available for purchase. To ensure that customers only receive what is best, we subject all items to rigorous quality assurance testing and inspections as well as ship every item alongside its qualifying certifications or manufacturing trace documentation as applicable. If you are facing an AOG situation and need parts fast, we also provide expedited shipping on a number of items that we carry. Get started today and see why customers choose to rely on NSN Sourcing for all their operational needs.


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