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Altimeter readings from ATC

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Brook
Trainee
Trainee


Joined: Mar 12, 2004
Posts: 91
Location: San Francisco, CA
Posted: Fri May 29, 2009 4:56 am 
When an ATC Center gives you an altimeter reading, WHERE is the reading being taken and how is applicable to you at your position and altitude?

Are they reading the barometer where they sit, or can they give you an accurate reading for your altitude?
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Traches
First Officer
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Joined: Jan 31, 2008
Posts: 491
Location: Louisville, KY, USA
Posted: Fri May 29, 2009 10:00 am 
My understanding is that they are giving you the barometric pressure at sea level at your given location. They measure it at the airfield and adjust it based on the field elevation. All it does is gives a base point to ensure that everyone's altimeter is accurate!
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99jolegg
Chief Captain
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Joined: Jun 26, 2004
Posts: 5430
Location: UK
Posted: Fri May 29, 2009 12:21 pm 
Brook wrote:When an ATC Center gives you an altimeter reading, WHERE is the reading being taken and how is applicable to you at your position and altitude?

Are they reading the barometer where they sit, or can they give you an accurate reading for your altitude?


They give you either QNH or QFE.

QNH is the airfield pressure reduced to sea level via ISA (International Standard Atmosphere) figures i.e. 1mb per 27ft. This gives the pilot their altitude above mean sea level

QFE is the airfield pressure. Therefore, when you select the QFE and start flying (from the same aerodrome) you will have a height above the airfield

As an aside, QFF is similar to QNH but it is aerodrome pressure reduced to sea level via the actual conditions rather than ISA conditions

QNE is (strictly speaking) a height that is read on the altimeter when landing with 1013.25mb set on the altimeter. A common misconception is that QNE = 1013.25mb = standard pressure but it's actually a height.

Does that make sense?
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Tailhook
Chief Captain
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Joined: Oct 12, 2005
Posts: 8472
Location: El Dorado
Posted: Sat May 30, 2009 3:04 am 
Rolling Eyes
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Westernstyle
Captain
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Joined: Feb 21, 2007
Posts: 748
Location: KMLB
Posted: Sat May 30, 2009 3:08 am 
Ok then Question
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99jolegg
Chief Captain
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Joined: Jun 26, 2004
Posts: 5430
Location: UK
Posted: Sat May 30, 2009 1:47 pm 
Tailhook wrote:Rolling Eyes


There's some more info here: http://en.wikipedia.org/wiki/Atmospheric_pressure
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Tailhook
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Joined: Oct 12, 2005
Posts: 8472
Location: El Dorado
Posted: Sun May 31, 2009 12:50 am 
99jolegg wrote:
Tailhook wrote:Rolling Eyes


There's some more info here: http://en.wikipedia.org/wiki/Atmospheric_pressure


...and here: http://flyawaysimulation.com/postt35184.html
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