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Question
Dear Marc

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

How does the Altimeter instrument measure the Altitude?.

Thanks
Prashant

Answer
Altimeters do not measure accurately the height above the ground. It is only a relative number.
The altimeter has a very sensitive pressure reading device called an aneroid. It is a series of hollow disk made of very thin material The number of disk depends on the sensitivity of the altimeter. They expand as the ambient pressure is reduced. The linear movement is used to drive a high gain transmission of gears to drive the indicator. To try to establish an altitude the altimeter is set to the local barometric pressure using the set knob on it. From they establish the relative altitude. The problem is that the pressure at a given altitude varies with temperature and is referred too as density altitude. This means that the pressure at a given height will vary with temperature. So even though you have set your altimeter to the local barometric setting your height above a given point will vary from day to day depending on the temperature. However what it does do is have the airplanes flying at the correct height with relation to that barometric setting So now the controller has the airplanes flying at the heights they are assigned. So now we have airplanes flying at height assigned and not accurately at height above the ground. This appears to have things under control. This now assumes that the pilot has his altimeter at the correct barometric setting and that the aircraft static port is giving an accurate static pressure. The cause can be a leaky static line introducing cabin pressure or the static pickup blocked. For example waxing the airplane and getting some in the port or using an object to clean it that changes it's shape. IFR aircraft require periodic altimeter checkups that includes checking the static line for a leak.  Hope this gives you what you need. BTW the standard practice is to set your altimeter to 29.92 if the local pressure is not known. When landing, if someone on the ground gives you the number his altimeter has that is set at zero then yours should also read zero when on the ground. If not someone has a bad altimeter.

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Marc Ebelini

Expertise

Basis aircraft and engine maintenance. Avionics and other electronic questions related to computers and radio communications.

Experience

FAA licensed comercial pilot, A&P mechanic, former authorized inspector, ground instructor certified in aircraft, powerplant and radio navigation. FCC commercial license.

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