Opinion on Sample Question on Altimetry

[Obs cop]

The easy route here is to know what happens when you change the pressure setting datum on the instrument.

If you "wind the pressure setting up the indicated altitude goes up".

Let's assume you are stationary at an airfield 500' above mean sea level. The airfield's QNH is 1000mb. With 1000mb set on the altimeter your indicated altitude will be 500'.

If you wind up the QNH setting to 1010, then your indicated altitude will also increase. The amount is generally considered to be 30' change per 1mb difference in pressure setting for altitudes below 5000'. Hence winding up our QNH by 10mb in this example will wind up our indicated altitude by 300' (10 x 30) giving an indication of 800' now, even though we havn't actually moved.

Returning to your example and how I would work it out.

Sat at airfield B, with the correct QNH of 999mb set will give the correct altitude of that airfield of 500'. Now we actually have 1020 set, so if we imagine winding up our altimeter from 999 to 1020, we increase our QNH by 21mb and following the winding up theory, we wind up our indicated altitude by 630' (21mb x 30' per mb). Adding this increase in altitude to our airfield B elevation we get an indicated altitude of 1130'.

The elevation of airfield A is irrelevant in the question.

Hope this helps,
Obs