While the flight instruments used on different aircraft vary greatly, there is one device that every plane is equipped with—the altimeter. First implemented on aircraft in 1928, the altimeter ushered in a new era of aviation in which the pilot could gauge their altitude accurately and not purely based on visual information. The rest of the 20th century brought about huge advancements in avionics technology, including the precision of the altimeter. Today, several devices are used concurrently to accurately represent altitude in real-time.
The pressure altimeter is the oldest and most simple form of an altimeter still used on aircraft today. It works on the principle that atmospheric pressure drops linearly as altitude increases. The static port, which is the inlet for the instrument, continuously takes ambient air from outside the aircraft and delivers it to a component called an aneroid disc. As the plane increases in altitude above sea level, the aneroid disc expands; thus, a mechanical signal is sent to turn the pointer on the face of the altimeter in the cockpit.
While the number displayed on a pressure altimeter gives a good idea of general height, there are several types of altitude, and some adjustments must be made to translate between them. The indicated altitude is the pressure that is read directly off of the altimeter. For this to be accurate, the pilot must adjust the altimeter to match the airport's altitude before taking off. Additional variables that make the indicated altitude inaccurate are extreme fluctuations in temperature and a temporary false drop in pressure created by the Venturi effect.
Pressure altitude refers to the aircraft's height above the datum plane, the theoretical level at which the atmospheric pressure is exactly 29.92 inHg, and the temperature is exactly 15 °C. These numbers were chosen because the atmospheric pressure is 29.92 inHg at sea level, and the ISA's standard temperature at sea level is 15 °C. Thus, it is equal to true altitude when standard atmospheric conditions apply. Density altitude is calculated from pressure altitude but is corrected for nonstandard temperature. This number does not actually indicate height, but rather, it estimates the aircraft's performance at the given altitude.
The true altitude gives pilots the true height above sea level, assuming that all the aforementioned variables are accounted for. All charts that include the height of various airports and obstacles are expressed in true altitude. Finally, absolute altitude is what comes to most people's minds when thinking of altitude. Absolute altitude represents the true height of the airplane over the ground. This value, also called height above ground level (AGL), is not measured by a standard pressure altimeter.
Because absolute altitude is an important metric in helping pilots land the aircraft and avoid a collision, airplanes are also equipped with radar altimeters. These devices are used primarily during the landing sequence and work by sending radio waves to the ground and measuring the time it takes to return to the aircraft. Radar altimeters generally only work up to 2,500 ft above ground level but are extremely accurate in reflecting the current absolute altitude. When both radar and pressure altimeters are used together, pilots are equipped with a precise height during the entire fight.
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