How does altimeter measure altitude

A standard altimeter contains a stack of sealed aneroid wafers with an internal pressure of These wafers expand and contract based on the static pressure inside the casing of the altimeter. This static air enters the casing through a tube attached to the static ports on your airplane. The chamber is otherwise sealed, so only static air from directly outside the airplane enters the chamber.

A lower static pressure less than Mechanical linkages connect the movement of these wafers to needles on the interior face of the altimeter. Compression of the wafers translates to a decrease in altitude, while expansion translates to an increase in altitude. Reading a standard 3-hand altimeter is easy. Across the globe, or even across a few miles, different air pressure can have a dramatic effect on altimeter settings.

As you fly from high-pressure weather systems to low-pressure systems or vice versa , you need to adjust your altimeter to get an accurate Mean Sea Level MSL altitude reading on your altimeter. You reset your altimeter to match local, nonstandard station pressure readings, using the Kollsman window on your altimeter.

This is usually done every NM for aircraft flying below FL Where do you get the altimeter settings from? So, let's say you took off with an altimeter setting of The altimeter would be off by approximately feet in altitude if you didn't make any adjustments.

With all airports and all pilots using sea level as a reference, all altimeters should be indicating the height above sea level or MSL. Altimeters usually have three hands, much like a clock. There is also a smaller outer pointer that points to tens of thousands of feet. The number read on the instrument face is called the indicated altitude. This may or may not represent height above MSL. It depends on whether or not the pilot has set the correct altimeter setting. As noted, the temperature can change the distance between pressure levels.

The altimeter has no way to compensate for temperature, so pilots must be aware that indicated altitude may not equal true altitude, especially when it is very hot or very cold. Since warm temperatures expand the pressure levels, indicated altitude would be lower than true altitude on a very hot day. While not ideal, this does not pose a danger since this would give a plane more distance between the land and obstacles. On very cold days , however, the pressure levels are compressed together, and the pilot will need to take action to prevent problems.

A calibration chart provides the appropriate amount to add to the planes indicated altitude to ensure it remains as close as possible to true altitude. Large aircraft are often fitted with radar or radio altimeters that measure height AGL when near the ground. This is because the density of air is lower thinner at high altitudes.

It exerts less pressure on the Earth below. The atmospheric pressure on Denali, Alaska, is about half that of Honolulu, Hawai'i. Honolulu is a city at sea level. Altitude readings can also change due to weather , as air pressure decreases during storm s.

A simple barometric altimeter includes a sealed metal chamber , a spring, and a pointer that shows altitude in meters or feet. The chamber expand s as air pressure decreases and contract s as it increases, bending the spring and moving the pointer. Not all altimeters depend on air pressure. Radar and laser altimeters, found on some aircraft and spacecraft, work similarly to sonar measurement s of the seafloor. These altimeters send a radio or laser signal toward the surface and measure the time it takes for the signal to bounce back.

The time it takes for the signal to bounce back or echo to the aircraft is then translate d to an elevation. When used in satellites, radar and laser altimeters are able to combine altitude measurements to create accurate topographic map s of both land and ocean surfaces. An altimeter measures altitude. Photograph by Luis Marden, National Geographic. Satellites can be natural, like moons, or artificial. Media Credits The audio, illustrations, photos, and videos are credited beneath the media asset, except for promotional images, which generally link to another page that contains the media credit.

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