ATPL Met question

[SkillsToBurn]

Hi everyone,

Im studying my ATPL's at the moment however iv come across a question that has me banging my head of the wall. The question states

The altimiter of an Aircraft at FL40 is corrected for pressure error giving an indicated altitude of 3892ft. if the temp is ISA -12, what is the true altitude?

The answer they have come up with is 3705ft.

However I have NO IDEA how they came up with it.
If anyone could shed some more light on how they got the answer it would be very appreciated.

[keith williams]

For non ISA temperatures use a correction of

4 feet per degree ISA deviation per 1000 feet

Inserting the figures in youir question gives a correction of

4 x (-12) x 3.892 = -186.816

Adding this correction to your indicated altitude gives

True alt = 3892 - 186.816 = 3705.184 feet

[SkillsToBurn]

Hey keith

Tahnks alot for the reply!

Can I ask for when u wrote 4 x (-12) x 3.892

Why was 3892ft converted to a decimal, is this bacuase we are using 4 Degrees C/per 1000ft?

[keith williams]

Yes, we have a correction factor of 4 feet per 1000 feet per degrees ISA deviation, so if our indicated alt is in feet we need to convert it into thousands of feet (by dividing by 1000).

3892 / 1000 = 3.892