Altimeter Errors

[lde1988]

I thought that i understood altimeter pressure errors, but i've just come across two different questions on the subject and the answers seem to contradict each other - so basically now i'm confused.

1 - It was my understanding that the ALT was calibrated with respect to ISA pressure lapse rate. When density or pressure are higher the ALT will over read.

However, i've just come across a question. - On leaving an airfield (75ft above MSL) the QNH is set at 1027.5. On approach to the destination airfield (3000ft above MSL) the starting QNH is reset, however the MSL pressure at the destination is 998mb. What will be the error? the answer given is that the ALT will over read by 885ft - hence as pressure decreases ALT will over read.

Can anyone shed any light on this???

Many Thanks

[le Pingouin]

1mb (or 1hPa as we now call it in Oz) change in pressure = 30ft change in altitude

1027.5 - 998mb = 29.5mb pressure difference

29.5 x 30ft/mb = 885ft


It helps to think of it as pressure surfaces & not altitudes. You can ignore aerodrome elevation for this as it's all about differences, so I'll say you're at MSL. What's the pressure on the ground at departure? 1027.5mb.

Given that pressure decreases at 1mb per 30ft increase in height, where is the pressure 998mb? Way above your head. 885ft above to be precise.

Climb to 885ft & measured the pressure - it will be 998mb. So if you flew at a constant 998mb pressure to a location where the QNH was 998mb what would be your altitude? Zero.

The pressure surfaces slope downwards to the lower pressure, so as you fly towards an area of lower pressure your height above the ground decreases. Remeber you're flying at a constant pressure, not constant height AMSL. Which means your altimeter is reading high as you fly towards the lower pressure.

I think of it as the low pressure is above you it comes down to meet the ground as the QNH decreases - as the pressure descends so do you.

[b_sta]

1 - It was my understanding that the ALT was calibrated with respect to ISA pressure lapse rate. When density or pressure are higher the ALT will over read.

That's incorrect. Remember, the pressure you set in the Kollsman window is the datum above which the altimeter reads pressure and thus altitude. So if you wind it down below actual pressure, the alt 'thinks' the datum is at a higher altitude, and that the plane is closer to it, so the alt shows a lesser than actual height - or underreads.

[Mark1234]

I suspect you're over thinking the question - it's not an altimeter error as such. It could be rephrased more simply:

You have 1027.5 set on the alt, the correct value is 998. How wrong will your alt be?

[Gundog01]

The old saying "wind on hPa, wind on height".

So you've effectively 'wound up' the hPa up by 29.5 and hence the altimeter is indicting higher.