Comments on: Cold Weather Altimetry – Cold Weather Flying

By: Robin Maiden

Robin Maiden — Mon, 21 Nov 2011 15:13:25 +0000

The air is just denser/thicker when it is cold. The atmosphere is literally less tall when it is cold. So, when the you stand on Mt. Everest, on a very cold (ISA minus 30) day, you are closer to the pressure of space than on a hot day (ISA plus 15). An altimeter on the mountain would indicate very high altitude on the cold day and very low altitude on the hot day even though it hasn’t changed its position.

Altimeters described as “temperature compensated” have been designed to maintain the same “ratio” between pressure and indicated altitude across a specified temperature range. This is an engineering feat to make sure the mechanical parts don’t induce errors in the ratio when they expand or contract because of temperature. A “temperature compensated” altimeter does not adjust for different densities of the atmosphere. How could it know? It only “knows” pressure.

In the age of GPS, have you noticed the discrepancy between GPS altitude and indicated altitude? Even on a 29.92″ standard day? This is exactly what I’m talking about with cold weather altimetry.

By: Anson Stelmak

Anson Stelmak — Mon, 21 Nov 2011 07:22:11 +0000

I understand everything that I have read so far, but what I can’t find (or verify) is what is the physics (or chemistry) of the problem? Is it a molecular driven issue due to the cold air on the aircraft metallic instruemets, or is it a density driven issue due to the extreme cold/thick?

By: admin

admin — Wed, 19 Jan 2011 17:32:47 +0000

Thanks for asking. This is pressure altitude.

By: Ron

Ron — Tue, 28 Dec 2010 07:46:17 +0000

Question, so a newly corrected DH for cold temp, is this now a pressure alt reading or a density alt?

By: Mark Imhof

Mark Imhof — Wed, 16 Dec 2009 03:05:36 +0000

I am a CFII and would like to use your video during a presentation. If I cite your work, would you be willing to email your video to imhofm86@gmail.com

Thank you,

Mark Imhof

By: Mark Imhof

Mark Imhof — Wed, 16 Dec 2009 03:03:46 +0000

I am a CFII and would like to use your video during a presentation. If I cite your work, would you be willing to email me your video? Thank you. Mark Imhof

By: Ian Edwards

Ian Edwards — Wed, 22 Apr 2009 06:02:51 +0000

I am flying Glf 550 aircraft in Either Russia or Dubai, so see the two problems, often on the same flight.
The Russians in most airports do not compensate for the low temperature on the below TL numbers, also in the very cold partsd of Russia, there are often severe temperature inversions at 3000 ft, so the temp can change from -40 to only -20 on the approach, in a matter of 100 feet or so. The auto throttles dont like it at all.
Your remarks about alt busts due to over warm days is also valid, and I will ask friends in DXB ATC how this is catered for here.

My original posts on PP were related to the hot corrections, and you clarified that for me.

Thanks.

Ian

By: Bill

Bill — Thu, 11 Dec 2008 18:09:00 +0000

Robin,

I am a military flight school instructor and would like to use your video. If I can get your permission to use it, I want to embed it in a powerpoint slide for a presentation I will us at a safety stand down to enhance flight safety for our future military aviators. Will you email me the flash video so I can put it into my presentation? Full credit will be given to you and I would use a direct link if I could guarantee that I would have internet access every time I give this presentation.

Thank you in advance, I know how busy you must be.

Bill

By: Web Resources for Cold Weather Altimetry - Cold Weather Flying | Robin's Roost

Fri, 29 Aug 2008 13:28:31 +0000

[...] forget to watch my video on the subject here – Cold Weather Altimetry addthis_url = ‘http%3A%2F%2Fwww.robinmaiden.com%2Fcold-weather-altimetry-resources%2F’; [...]

By: Mike Maher

Mike Maher — Wed, 26 Mar 2008 06:30:38 +0000

Hey Robin,
Nice explaination. The real test comes when you are not flying a precision approach with a glideslope (although your barometric altitude will be lower for cold weather, but not by much at 200 ft HAT).

When you’re flying a cold weather non-precision approach with no glideslope (GPS synthetic or otherwise), your real threat is at the intermediate fix altitude, vs the IAF altitude (1000′ ft terrain/obstacle clnc) or the MDA (less deviation closer to the altimeter source, and 250-300 ft obstacle clnc).

At the intermediate fix (500′ terrain/obstacle clnc) you have the most altimeter deviation with the least TERPS clnc criteria, and therefore, you have the worst of all possible scenarios.

I’ll e-mail you some of the cold wx research I did at the USAF Advanced Inst school.
Mike

By: Kent Wien

Kent Wien — Wed, 05 Mar 2008 03:40:37 +0000

Great way to describe it, Robin. Ironically, we covered this today in recurrent. I think I prefer your method though.

Great site Robin!

Kent