Altimeter/temperature correction

[tornadoflyer]

Hi,

I recently read a question about altimer error in regard to temperature deviation from ISA. The question is: "What is the error of an altimeter at 10000' PH at ISA -34 degrees? Answer over reads by 14%.

As far as I remember, the rule of thumb is 4' increase/decrease for each degree deviation from ISA per 1000'. Since ISA temperature at 10000' should be -5 degrees there is a difference of 29 degrees. As a result, 29 x 4 x 10= Altimeter overreads by 1,160 '. That is not even 12% certainly not 14%. I do not want to be nitpicking, but is there another formular which is more suitable?

Thank you for any help in advance.

[FLEXJET]

You have to take -34 iso -29, you'll see it works!

The difference with ISA is given in your question, no need to find it!

[tornadoflyer]

Thanks FLEXJET,

I should have more carefully read the question.

Regards.

Tornadoflyer