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Amateur Turbines
21st Jan 2002, 09:49
I've been looking for a more accurate way of calculating altitude corrections due to cold temps. I have found a number of formula on the subject but most are not very user freindly. I am looking for a formula that I will be able to calculate quickly during flight. So please no crazy formulas involving gravity and what not. Thanks in advance!

static
21st Jan 2002, 14:13
4 ft per degree centigrade per 1000 ft altiude.

fokker
21st Jan 2002, 14:56
Density Altitude=Pressure Alt +/- 120T, where 'T' is the ISA deviation.

i.e. : if PA is 13000' and SAT is -31 C, ISA deviation is, therefore, -20. DA is 13000-(120 x 20) =11600'. Easy, innit?

Okay, spot the deliberate mistake (please see below) <img src="rolleyes.gif" border="0">

[ 21 January 2002: Message edited by: fokker ]</p>

Loc-out
21st Jan 2002, 16:50
fokker

Using your Formular and numbers, I make it 10,600' da :) :) :)

fokker
21st Jan 2002, 19:09
******, really? Hold on......

At 13000', ISA temp is -11 (15-26 at 2 degrees per 1000'). If the SAT is -31, it is 20 deg C colder than it should be, so 'T' is -20. 120 x -20 is -2400, so the answer should be DA of 10600'. Looks like I had the wrong abacus out earlier! Thanks loc-out. Anyway, I promise the method's right, even if I'm a cabbage! <img src="wink.gif" border="0">

[ 21 January 2002: Message edited by: fokker ]</p>

Loc-out
21st Jan 2002, 19:52
Take comfort in the thought, that even I make mistakes.

bookworm
22nd Jan 2002, 21:33
I don't think Amateur Turbines was asking about density altitude, but rather the correction required for true altitude.

The quick and dirty formula for that is that your true altitude (strictly, height above datum) is 0.35% lower/higher than your indicated altitude per degree C that the atmospheric temperature is lower/higher than ISA.

e.g. for ISA -10 degC, an indicated altitude of 10350 ft is actually 10000 ft (if the altimeter setting is correct at sea level).

Another way of putting that is 3.5 ft/degC/1000ft. If the difference between that an static's 4 ft/degC/1000ft makes a difference to your health, you're not doing a very good job of height keeping. <img src="smile.gif" border="0">

Checkboard
23rd Jan 2002, 08:37
If you are interested, this was discussed pretty extensively in the <a href="http://www.pprune.org/cgibin/ultimatebb.cgi?ubb=get_topic&f=71&t=000003&p=" target="_blank">Altimeter Correction Cold WX OPS</a> thread, located in the <a href="http://www.pprune.org/cgibin/ultimatebb.cgi?ubb=forum&f=71&SUBMIT=Go" target="_blank">Tech Log Archive</a>