Hey everyone,

Am having a bit of trouble understanding the diagrams Fig. 57 and Fig. 58 on page 3.12. Fig. 56 shows the subscale set to 1020, producing a reading of 2000 feet. If pressure drops by roughly 1hPa for every 30 feet of height gained, then how is setting the subscale to 1010 producing a lower altitude of 1700 feet? The subscale is set 10 hPa lower compared to Fig. 56 so wouldnt that mean you are at a greater height considering for each 30 feet you lose 1hpa? Same for Fig. 58, since the subscale has now been set to 1030 which is 10hPa higher compared to Fig. 56, doesn't that mean the altimeter should produce a lower reading?

Sorry if I sound confusing just struggling to get my head around it.

Any help would be appreciated.

Hi Cessna,

An altimeter has no concept of altitude, all it can measure is the differences in pressure between whatever is around the instrument at the time and the reference pressure you have set in the subscale window. If you set 1020 on the subscale, you are essentially asking the instrument "Show me the difference in pressure between what is around us at the moment and a pressure of 1020 hPa". The hands on the instrument move to indicate 2000ft (in this example).

Change the subscale reading and you are essentially re-calibrating the instrument to measure the pressure difference against a new datum, in this case the 1010 hPa isobar. The 1010 hPa isobar is higher up in the atmosphere than the 1020 hPa isobar and at a rate of 1 hPa / 30ft, it must actually be 300ft higher up.

Now, with 1010 hPa set, you are asking for your pressure difference between outside pressure and a new datum which is now 300 ft "closer" to you in the atmosphere. That is why your altitude indication decreases. You are still 2000ft above 1020 hPa but only 1700 ft above 1010 hPa.

It helps if you draw it out on a piece of paper..

Cheers,

Rich