Barometric error

[akafrank07]

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

Cheers

[BEagle]

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.

[Lightning Mate]

Here you go...

http://i636.photobucket.com/albums/uu82/Lightning_29/BarometricError.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

[Dick Whittingham]

900

Dick

[Lightning Mate]

Thanks Dick.

Diagram amended. :ok:

[fwjc]

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

[Jockster]

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.

[Lightning Mate]

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??

[AirGek]

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