altimeter errors from temp

[newmei]

I was reading the post about the GPWS, I realized that errors could be caused from temps. My question is why don't more a/c crash in colder areas (like greenland), for instance on a VOR approach. OR do most flight crews calculate this before they even commence the approach?

Probably a dumb question but most of my flying is is FL and Central America.


Thanks

 

[mar]

I'll take a stab at it.

Hey, that's a fair question. I hope my answer is somewhere in the ballpark. If not, I'm standing by for any corrections or elaborations...

When it's really stinkin' cold in the arctic, like Greenland, it's also (usually) clear and cold with really high pressure.

So (here's my stab in the dark), given the clear conditions there's less opportunity for a CFIT accident.

Here's the problem: In the winter there is more darkness than light. So even if the conditions are clear and cold you may find yourself in IMC (no usable horizon) with an erroneous altimeter.

My conclusion is: In the arctic there are more "unimproved" (gravel, unlighted, VFR) strips than otherwise, so at night the flying drops off and most of it is done during the day when you can see the obstacles.

How'd I do?

 

[DitchDriver]

The altimeter setting typically comes from the field you are shooting the approach to. Colder / warmer than ISA altimeter error is a percentage of height above the altimeter setting station. On the field, it is 0 - set the altimeter to the broadcasted value and you should read the field elevation. On an approach the error is not all that much: let's say, 5% of less than 1000 feet. However, for clearing some hills 10000 feet above the airport while relying on altimeter / GPS-GPWS, 5% will eat into the 1000 ft safety buffer really quick.

 

[typhoonpilot]

Just in case you want the formula for calculating this, it is:

Altitude Diff = Altitude above airport X 4% X ( +/- ISA Deviation/10 )

So for a typical ILS approach with a DA of 200 feet. The difference at ISA -30 would be:

200 X 4% X 30/10 or -24 feet. So without a correction you would bust minimums by 24 feet. That is not really enough to cause an accident.

As DitchDriver pointed out it can make a lot more difference at 10,000 feet, in this case a difference of -1200 feet. So it would be a serious concern in an emergency descent with high terrain around.

Typhoonpilot

 

[Singlecoil]

I don't know about Greenland, but in Canada its SOP to apply a correction to published altitudes. It doesn't matter a whole lot on an ILS as shown above, the biggest danger is doing an arc or procedure turn in mountainous terrain. Look in the AIM. It talks about this, and says crews should apply a correction. It might even publish the chart. I think Jep has a chart somewhere in the introduction section.

If it is 20 below zero, you can have errors of 700-800 feet at an elevation of 3000 agl. A surprising number of people are under the impression that onboard air-data computers correct for this, but they do not.

 

[MooseTrax]

Altimeter Correction

TEMPERATURE ERRORS

a. Pressure altimeteres are calibrated to indicate true altitudes under International Standard Atmospheric (ISA) conditions. Any deviation from these standard conditions will result in an erroneous reading on the altimeter. This error becomes important when considering obstacle clearances in temperatures lower than standard since the aircraft’s altitude is below the figure indicated by the altimeter.

b. The error is proportional to the difference between actual and ISA temperature and the height of the aircraft above the altimeter setting source. Height above altimeter source is
considered to be published HAT or HAA for the approach. The amount of error is approximately 4 feet per thousand feet for each degree Celsius of difference.

c. Corrections will only be made for Decision Heights (DHs), Minimum Descent Altitudes (MDAs), and other altitudes inside, but not including, the Final Approach Fix (FAF). The same correction made to DHs and MDAs can be applied to the other altitudes inside the FAF.

Regards,

 

[metrodriver]

from what I understand is that many european carriers require a cold temp correction