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# Aerospace/Aviation/Altimeter Flight instrument.

Question
Dear Laura

Do the altimeter Flight instrument indicates with accuracy the plane Altitude from the ground level to the pilot?.

How does the Altimeter Flight instrument measure the Altitude?.

Thanks
Prashant

Hello,
The altimeter is typically used to indicate the airplane's altitude above sea level, not necessarily above ground level, for aircraft flying below 18,000' in the USA. Above 18,000' in the USA, airplanes all use the altimeter to set one standard barometric pressure, and then that is what the altimeter indicates the height by reference to.
A good article explaining how the altimeter actually works is found here: https://en.wikipedia.org/wiki/Altimeter
Here is an excerpt from the article:
"In aircraft, an aneroid barometer measures the atmospheric pressure from a static port outside the aircraft. Air pressure decreases with an increase of altitude—approximately 100 hectopascals per 800 meters or one inch of mercury per 1000 feet near sea level.

The aneroid altimeter is calibrated to show the pressure directly as an altitude above mean sea level, in accordance with a mathematical model defined by the International Standard Atmosphere (ISA). Older aircraft used a simple aneroid barometer where the needle made less than one revolution around the face from zero to full scale. This design evolved to altimeters with a primary needle and one or more secondary needles that show the number of revolutions, similar to a clock face. In other words, each needle points to a different digit of the current altitude measurement.

Modern aircraft use a "sensitive altimeter,". On a sensitive altimeter, the sea-level reference pressure can be adjusted with a setting knob. The reference pressure, in inches of mercury in Canada and the US, and hectopascals (previously millibars) elsewhere, is displayed in the small Kollsman window,[5] on the face of the aircraft altimeter. This is necessary, since sea level reference atmospheric pressure at a given location varies over time with temperature and the movement of pressure systems in the atmosphere."
Questioner's Rating
 Rating(1-10) Knowledgeability = 10 Clarity of Response = 10 Politeness = 10 Comment Dear Laura Thanks. Thanks & Regards, Prashant S Akerkar

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