FAF ALT COMPENSATION DUE TO TEMP.

[basics52]

Say your app. plate calls for an outer marker alt. of 1500´
Temp. deviation is ISA + 20

Whats the formula to obtain the corrected OM on GS Alt ?

[Hew Jampton]

This should be in 'Questions'

Look in the Aerad book or your company Ops Manual, although the correction is for very low temps, not high.

[This message has been edited by Hew Jampton (edited 29 October 2000).]

[This message has been edited by Hew Jampton (edited 29 October 2000).]

[basics52]

Well , the question I made is for high temps.

You are correct , very low temps could become critical and for example baro vnav app plates will have a min temp shown in future charts .

So, anyone has a formula ?

Maybe the cold chart if used with opposite compensation will do it...

Could someone post a chart ?

That may be easier than finding the one I had .

[Prof2MDA]

Have an E6-B? Use it! True altitude is on all of them!

[quid]

basics52,

Just out of curiousity sake, what would you use it for?

[reynoldsno1]

The approximate correction is 4ft per 100ft per degree difference from ISA (using the surface temp at the airfield you are approaching).

So, with a surface temp of -10C, at 2000ft the approximate error in your altimeter will be 4 x 2 x 25 = 200ft, i.e. you should be at an indicated 2200ft.

[reynoldsno1]

oops, that should read ... 4ft per 1000ft...

[basics52]

REINOLDSNO :

Hey , you really understood my Q. THANKS !!

A formula like this will help me illustrate how for example in your case at 2000´
ISA- 20 C represents 200 feet error and at 200 ´ = 20´ etc...

Good rule of thumb , TKS

[Checkboard]

From the Tech Log Archive (see the bottom of the forums page, or the bottom of the "go" window below):

Altimeter Correction Cold WX OPS (http://www.pprune.org/ubb/NonCGI/Archives/Archive-000002/HTML/20000604-3-000466.html)

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Tech Log forum moderator

[airbuswhiz]

I co-wrote an article in the American/Canadian ALPA magazine about this very topic. I was also on the team that wrote the FAA directive for implementation of cold temp techniques.

The formula that was used was the old true altitude formula, and in most cases that will still be correct, but there has been a new formula developed. The new formula appears in ICAO PANS-OPS VI, I worked closely with the Canadian officials here and ICAO in Montreal about it's implementation. So I guess one could say that this subject is close to me. I also developed a training course that I used for some in the U.S. Also, I wrote a program that does corrections automatically. Hope this info helps, any more questions, just ask...

Cheers..airbuswhiz