Barometric error
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.
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.
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.
LM
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.
LM
Last edited by Lightning Mate; 9th Nov 2012 at 09:02.
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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.
