Altimeter
An altimeter is an instrument that measures altitude (the distance above sea level or some other chosen point). Aircraft use two different types: the aneroid (barometric) altimeter and the radio altimeter.
The aneroid altimeter derives a quantity for altitude by measuring the decreasing air pressure that occurs with increase in altitude, while a radio altimeter bounces a radio signal from an object aloft to the ground and determines the distance aloft as a function of the time it takes for the signal to return.
The history of the altimeter begins with the invention of the mercury barometer, the first device to measure air pressure. In 1643, Italian physicist Evangelista Torricelli, a pupil of Galileo, filled a tube with mercury. One end of the tube was closed; the other open end was turned upside down and inserted in a cup of mercury. Because the air exerted pressure on the mercury in the cup, about thirty inches of mercury remained in the tube.
After Torricelli performed this experiment to prove that air exerted an intrinsic pressure, others wondered if there was a way to use it to show the difference in pressure at sea level and the tops of mountains. Since air thins as altitude increases, the mercury in a barometer should fall as less pressure is exerted on the cup's contents. Once Blaise Pascal (1623-1662), the French mathematician and physicist, proved this was the case in the 1648, the barometer came to be used to measure altitude. The device's earliest use in flight was by French balloonist Jacques Charles, who took one aloft in 1783.
Airplanes use a variation of the barometer called the aneroid altimeter, which does not depend on any liquid to determine the altitude. Virtually the same as the aneroid barometer, invented by Lucius Vidi in 1843, it records the pressure exerted on an sealed compartment from which some of the air has been removed. The surface of the compartment bulges or contracts slightly as the outside air pressure changes, and the movements of the compartment are indicated on a needle which is connected to the box by a chain, lever, and springs. Aneroid altimeters are surprisingly accurate (giving readings within 1.5 ft [.5 m] at 11,000 ft [3,353 m]) and are more sensitive to changes than mercury-filled altimeters. An additional benefit of the aneroid altimeter is its ease of operation since it does not need liquids and stands up well to jarring.
A drawback of the aneroid altimeter is its sensitivity to weather conditions. If the barometric pressure changes during the course of a flight, the altimeter will not indicate the correct altitude at the arrival airport. For example, if the pilot flies into an area of lower barometric pressure, the altimeter will read slightly too high. For this reason, pilots must constantly monitor local weather reports. Additionally, at or above 18,000 feet, all aircraft must set their aneroid altimeters to 29.91 inches of mercury; this ensures that everyone using the high-altitude flight corridors have altimeters that read identically, reducing the chance of mid-air collisions.
The second type of altimeter uses radar, developed by Sir Robert Watson-Watt in the 1930s. It sends out a radio wave which bounces off the surface of the Earth. The reflected signal returns to the source, and a device measures how long the waves took to make the trip back and forth. Since electromagnetic waves always travel at the speed of light (186,000 miles per second), an absolute measurement of altitude is quickly computed by the device. Unlike aneroid altimeters, radar altimeters do not require adjustment to weather conditions or frequent calibrations. Additionally, such radar capability can be used to warn of terrain proximity, aiding pilots in avoiding instances of controlled flight into terrain (CFIT), a common cause of aviation fatalities.
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