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Barometric error

Old 7th Nov 2012, 20:58
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Barometric error

Can someone please explain to me what Barometric error, as regards to altimeter instruments errors is?

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Old 7th Nov 2012, 22:34
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Pressure altimeters have 3 main errors:

1. No machine is perfect, so instrument error, lag and hysterisis will affect any such altimeter to a degree.

2. Installing an altimeter in an aircraft will induce certain errors, such as position error and cockpit temperature.

3. Even if both altimeter and installation are 100% perfect (impossible), the atmosphere in which the aircraft is flying probably isn't 100% International Standard Atmosphere, to which the altimeter is calibrated. The resulting error is termed 'barometric'.

These errors may be additive or subtractive - but they will always be there.

A good pressure altimeter is probably accurate to 30 ft - but that also assumes that the pressure setting datum is accurate.
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Old 9th Nov 2012, 09:06
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Here you go...

The sea level presure in column A is 1000mb and the aircraft is flying at the 700mb pressure level.

The sea level pressure in column B is 900mb, so the 700mb pressure level is lower.

The aircraft will therefore descend with a constant altimeter reading because it is merely following the 700mb pressure level. Remember that the altimeter is simply a barometer.

All that has to be done in column B is to reset the subscale to 900mb.

So barometric error is not an error of the instrument. It may more aptly be described as "finger trouble" by flying with a mis-set subscale setting.


Last edited by Lightning Mate; 9th Nov 2012 at 10:02.
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Old 9th Nov 2012, 09:41
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Dick Whittingham is offline  
Old 9th Nov 2012, 10:03
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Thanks Dick.

Diagram amended.
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Old 9th Nov 2012, 20:49
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It pains me to say it, but shouldn't those be hectopascals?
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Old 9th Nov 2012, 21:47
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EASA LOs in instrumentation DO NOT consider barometric 'error' as an error at all - which is isn't. The altimeter is working correctly indicating the vertical distance from the sub-scale setting. Just because the setting is not referenced to MSL or airfield QNH doesn't mean that it isn't working correctly - it just means the indication have little or no useful value.
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Old 10th Nov 2012, 11:46
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It pains me to say it, but shouldn't those be hectopascals?
....and it pains me to ask if you have ever seen an altimeter subscale graduated thus??
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Old 10th Nov 2012, 12:58
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Milibars and hectopascal are the same thing, I don't see where the issue is.

Last edited by AirGek; 10th Nov 2012 at 12:58.
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