Barometric error
 

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

Cheers

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.

Here you go...

http://i636.photobucket.com/albums/u...etricError.jpg


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

900

Dick

Thanks Dick.

Diagram amended. :ok:

It pains me to say it, but shouldn't those be hectopascals?

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.

....and it pains me to ask if you have ever seen an altimeter subscale graduated thus??

Milibars and hectopascal are the same thing, I don't see where the issue is.