Wow 455tt, I don't know what to say about your math ...

15 - 13 = 2, not -2 and (-40) - (-2) = -38, not -42.

But (-40) - (2) = (-42) so I'm at a loss.

Right you are! Thanks for posting the correction.

So

ahramin wrote: ↑Sat Feb 17, 2018 9:35 am Wow 455tt, I don't know what to say about your math ...

15 - 13 = 2, not -2 and (-40) - (-2) = -38, not -42.

But (-40) - (2) = (-42) so I'm at a loss.

Here is the math:

The formula requires that you must input the DIFFERENCE between the actual and the ICAO temperature. The ICAO temperature as you state quite correctly is +2 C at 6,500 feet. The actual temperature is -40 degrees C. Thus the difference between these is 42 and this is what you input into the formula. So, 42 X 3.5 X 4 = 588 feet which you deduct from your indicated altitude of 6,500 feet giving you a true altitude of 5,912 and a clearance about the ridge at 5,850 of 62 feet.

Are you OK with this - the true altitude formula can be found in the Air Command weather manual.

I see this example is taken from MC's book where the full explanation and math has been provided.

If ICAO is -2 and ambient is -40 then the differentialis -38.....

ICAO temp is +2.

That is to say, 15 at sea level, minus 13 (6.5 X 2) = +2. And if the actual is minus 40, then the differential is 42. From -40 to 0 is 40, plus another 2 to get to +2 = 42.

Now you need to plug in to the true altitude formula, which says you take the altitude of the air column in thousands of feet, you multiply this by the difference between the ICAO and the actual temperature, and then you multiply by 4 feet, and this is the error you will deduct from the indicated altitude.

Let me try this to explain: the altimeter is calibrated to accurately measure true altitude only in standard conditions. Once the temperature gets really cold, it over-indicates the altitude, and your true altitude will be lower. So, you need to factor in how large this difference between the actual temperature and the ICAO temperature, since the greater the differential, the greater the amount of error and the lower will be the true altitude.

455tt, we understand the formula. It's just that your first post completely fucked up the math, which could be confusing to some, especially since you arrived at the correct answer. Glad we got it sorted out .

That's good to hear ahramin that it all makes sense to you now but what I'd love to know is whether the person that originally asked is all clear now on this subject - so sagabwoy please let us know if we have helped or whether any of this is still confusing.