Wednesday, December 30, 2009

Cold Temperatures and Low Altitudes

[Note: I wrote the following for my EAA Chapter newsletter. Cold temperature altimeter errors scare me, since most of my flying is in a mountainous northern-tier state. And even if I have written about them before, everyone needs a reminder. So, here it is for everyone to read about.]

We've all noticed that it's Winter, both by the calendar and by the thermometer. Or maybe, like me, you've had your annual furnace failure.

Pilots love winter flying. The cold, dense air makes both your engine and your wings stronger, and you can see forever because the air is so dry.

But the days are shorter, and that means more night flying. The night air is still cold and clear, and the lights shimmer brightly. You need to watch out for fog forming in the valleys, but that usually happens sometime after sunrise unless there's an inversion.

Now imagine yourself flying from Denver to southeast Idaho on a moonless night. Before you left you looked at a chart, whether online or paper, and noticed that the route takes you near Wyoming Peak, elevation 11,378' MSL. So you put on the oxygen and climbed to 12,500. Smart, right?

Well, let's see. Keep in mind that extreme temperatures distort your altimeter readings. Hot temperatures make the altimeter read low, while cold temperatures make the altimeter read high. "High to low, look out below."

People often forget that the altimeter error from cold temperatures increases with altitude. There's a table in the Aeronautical Information Manual (AIM), paragraph 7-2, that illustrates the problem. By the table, when the surface temperature is -20C (which is not all that cold for our winters), the altimeter error is 140 feet when you're 1000 feet above the surface, 280 feet when you're 2000 feet above, and 710 feet when you're 5000 feet above. It's worse when the surface temperature is colder.

Now let's get back to you cruising along at 12,500. You're comfortable in the dark, because your terrain display isn't showing any red. But does it know the surface temperature? What altitude does it use, anyway? Since you don't know, you get the Afton weather, either by tuning the ASOS or with the nifty XM receiver. "Afton Municipal Airport," it says. "0303 Zulu weather. Wind calm. Visibility 10. Sky clear below 12 thousand. Temperature -40 Celsius, dewpoint -42 Celsius, altimeter 3001..." You dutifully set the altimeter, thinking "It sure sounds cold down there!"

That high up, with the Afton temperature -40C, your altimeter error is about 1,000 feet! In other words, while your altimeter reads 12,500, your true altitude is closer to 11,500, putting you within 200 feet of the mountain. Which you can't see. Letting your altitude slip a little could ruin your night.

Pilots are generally carefully about hot conditions. We know to use the usual speeds for takeoff and landing, and to ignore that feeling that we're going too fast, because we're not. We know to allow more runway, too, and more room to climb. Perfect technique in hot conditions keeps you safe.

But perfect technique in cold conditions can kill you. You can pick a good altitude and hold it within an inch and still hit a mountain!

In Canada, Air Traffic Controllers automatically apply the correction to radar vectored altitudes, but in the US we ignore the problem. So, think about this when flying in or near the mountains on a cold night, or while IFR. Choose a higher altitude than you think you need, because in fact you need it. And this isn't just a Rockies thing: look at Mt. Marcy in the Adirondacks (5,344' MSL, nearly 4,000' above the nearest altimeter-setting source), or Mt. Washington in New Hampshire (6,288' MSL, more than 5,000' above the nearest source), or even North Carolina's Grandfather Mountain (5,960' MSL, about 3,000' above Boone, the nearest altimeter source). If there are mountains and winter (yes, even North Carolina has a winter; the ASOS at Boone is showing 01C with visibility 1/2 in snow) then there are cold temperature errors.

(Does anyone know what the situation is in Europe? Do controllers adjust altitudes for cold-temperatures, or do pilots have this information?)

See the AIM for more.

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At December 31, 2009 at 8:38 AM , Anonymous Eric said...

Great article! This is something often forgotten by pilots in the Northwest, and when I was flying in North Dakota it was so flat that it wasn't ever really a factor.

I did want to provide you with a little more information. Terrain displays, at least in Garmin systems, use GPS altitude, which is more or less absolute altitude ±100 feet (depending on the receiver's current accuracy). Here's the line right out of the GNS430W manual: “TERRAIN uses information provided from the GPS receiver to provide a horizontal position and altitude.”

I haven't used any handhelds, but I'd assume it's the same.

At January 11, 2010 at 1:19 PM , Blogger Dr.ATP said...


GPS altitude is almost true altitude but not quite. It's altitude above the geoid, not MSL altitude. There are places where the geoid differs from MSL altitude.


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