An Overview of the Aircraft Altimeter

Flight instrument parts are a crucial aspect of aircraft, allowing pilots to determine altitudes, airspeed, heading, and other flight characteristics with high amounts of precision and accuracy. This is highly beneficial for numerous reasons, permitting exact readings for carrying out safe operations and allowing for pilots to conduct instrument flight rules (IFR) flight for when meteorological conditions are too poor for visual flight. The aircraft altimeter in particular is a staple instrument that permits the measurement of altitude above a fixed level. As an instrument that is paramount to safe flight, having a general understanding of the functionality and use of altimeter cockpit instruments is crucial for anyone invested in flight.

For the means of aircraft piloting, the aircraft altimeter is set to determine the position of the vehicle in relation to sea level. While the most common calibration for such instruments is hectopascals (hPa), Japan and North America utilize inches of mercury (inHg). In general, the altimeter indicator instrument functions like a barometer, utilizing pressure differences from a standard to detect the height of the aircraft. As such, the altimeter relies on the static system which is capable of sampling ambient, still air for atmospheric pressure readings. When comparing readings to absolute pressure (29.92 in-Hg) with the use of an aneroid capsule, the subsequent expansion or contraction of the aneroid will actuate linkages and gears to move a needle across a dial to indicate a measurement.

It is important to note that many flight instrument parts are set to standard baselines such as sea level for altimeters, meaning that the distance to sea level as read on the instrument does not account for any hills, mountains, or changes in elevation. As such, it is paramount that pilots take such factors into consideration when navigating without visuals, utilizing other equipment such as GPS navigation systems to remain safe. For other aircraft types such as gliders, the altimeter will often be set to the height above ground rather than sea level.

With the constant advancements of technologies and the requirements of varying aircraft, there are a few common altimeter types that rely on radio, radar, laser, or capacitive technologies for measurements. The drum type altimeter cockpit instrument is one that only has one pointer for a single revolution that accounts for 1,000’, utilizing numbers for 100’ intervals and markings for 20’ intervals. The drum is geared to a pointer mechanism, and readings may be obtained by looking at the drum and then the pointer. Sensitive altimeter indicator instruments have an adjustable barometric scale, permitting a reference pressure to be set for measurements. Generally, the range of the scale sits between 28.00” to 31.00” Hg. The absolute altimeter is the final common type which is also known as a radio altimeter, and such instruments measure altitude through the use of radio signal pulses. In order for measurements such as altitude to be given to air traffic controllers, aircraft will often have altimeter encoder equipment which will send a series of pulses to the ground which will identify the flight level that the aircraft is traveling at for the benefit of ATC. Additionally, altimeters often follow a number of regulations which require testing, inspections, and general rules for use and training.



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September 15, 2021

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