As pilots conduct countless flight operations across the globe, flight instruments serve to provide critical data concerning the aircraft’s position, speed, heading, and more. With the altimeter in particular, the pilot can always be aware of the aircraft’s height in relation to sea-level. As such readings are critical to the safe operation of the aircraft, it is extremely important that pilots are familiar with altimeters and how to utilize their information to make well-informed flight decisions.
While there are various types of altimeters that one may procure for their needs, most operate similarly to a more robust barometer. As such, many altimeters gather their data by measuring atmospheric pressure. To do this, the altimeter consists of a sealed housing that contains a stack of aneroid wafers. As the altimeter relies on pressure readings from the static port, a tube will connect the altimeter to a relatively undisturbed area of the fuselage. As the chamber begins to equalize in its pressure with the outside atmosphere, the aneroid wafers will begin to expand or contract proportionally. If the static pressure obtained from the static port is higher than the chamber pressure at its base, the aneroid wafers will begin to condense and retract. If pressure is lower outside of the aircraft, then the wafers will expand. As atmospheric pressure drops as an aircraft ascends into the sky, the wafer will often expand during takeoff maneuvers and contract during descents.
Regardless of whether the altimeter expands or contracts due to static pressure, the movement of the wafers cause needles within the altimeter gauge to move in response. With the altimeter placed on the flight deck of the cockpit, pilots can quickly and easily see changes of altitude in real time. As readings are provided in measurements of altitude, rather than of pressure, the pilot is not required to carry out any calculation or conversion. With a more standard and conventional altimeter instrument, the gauge is presented with three needles that can display readings in increments. With the longest length needle, altitude is measured in increments of 10,000’. With the medium needle, measurements are made in 100’ increments. For the last and shortest needle, the measurements provided are increments of 1000’.
If a particular aircraft is operating under International Civil Aviation Organization (ICAO) guidelines, the default calibration of the instrument should be at 29.92” Hg. This pressure measurement is equal to the atmospheric pressure at sea-level, and thus is often considered to be the most optimal baseline for altimeter operation. Furthermore, altitude pressure generally follows a decrease rate of 1” Hg for every 1,000’ ascended, making measurements fairly straightforward.
While the altimeter is a tool that is paramount to the safe operation of an aircraft, it should not solely be relied on for directing an aircraft. For one, altimeters only measure altitude in relation to sea-level, and thus cannot warn the pilot of any upcoming obstructions or mountains. Furthermore, temperature variations can cause readings to adjust, and thus pilots should use other observations and data to make more informative decisions. Despite these shortcomings, altimeters are still extremely useful for improving safety and should always be used correctly.
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