altimeter tech question

[meslag]

some of you may recog. this question and where it comes from, but i cannot satisfy myself of the answer. Any offers of explanation please!!

given an indicated altitude of 10,000ft and an actual OAT of -20c, you set your altimeter to a local station setting of 29.62' (about 1003mb i think). If the station elevation is 2500ft, what is your actual altitude?


answer is 9550ft

err. how?

[oxford blue]

There are 2 considerations in converting Indicated Altitude to True Altitude. These are Barometric Error and Temperature Error. The question tells you that there isn't any Barometric Error. You have actually set to the QNH at the station. (The numerical value of the QNH is a red herring in this question).

However, QNH uses the ISA atmosphere for conversion to sea level (because that's what your altimeter uses, and you want to see the correct altitude of 2500 when landing, not the temperature-corrected altitude). So you are only measuring the temperature difference in the column of air between the airfield and 10,000 feet, ie a column of air of 7500 feet.

The temperature correction formula is:

True Alt = Ind Alt + (ISA dev x 4/1000 x Ind Alt)

In this case, substitude 7500 for 10000 feet in the second "Ind Alt" in the above.

So True Alt = 10000 + (-15 x 4/1000 x 7500) = 9550

[Fresca]

Silly me! I was brought up to belive that 1013.25 hPa was 29.92 not 29.95 :D

[oxford blue]

Quite right! Thanks for pointing it out. I'm afraid we all have aberrations from time to time, especially as we get older!

I'll go back and edit the post to correct it.