7
$\begingroup$

This pdf from Airbus has a chart from ICAO showing the amount of correction to be applied to altimeter readings for temperatures below 0°C.

  • Do aircraft with flight data computers automatically correct for this?
  • In the US, does ATC account for this correction when assigning vectors?
  • How does a pilot know if they are or not?
|improve this question
$\endgroup$
  • 2
    $\begingroup$ @Jan Thanks for the link. I think I get it: the QNH will be correct at the field, but cold air increases the pressure gradient so error will accumulate at altitudes above the field. Is that correct? $\endgroup$ – TomMcW Oct 28 '15 at 22:17
  • 1
    $\begingroup$ Related. When flying across a pressure gradient - "high to low, look out below" (the altimeter will progressively read higher than the real altitude), "low to high, look up to the sky" (the alimeter will read progressively lower). $\endgroup$ – Simon Oct 28 '15 at 23:27
  • $\begingroup$ The previous comment was removed, so: the phenomenon itself is well explained here. Yes, it means altimeter will read correctly at the field, but error will increase with altitude. $\endgroup$ – Jan Hudec Oct 29 '15 at 19:03
  • $\begingroup$ Related: Do instrument approach procedures sometimes have a minimum temperature?. (Yes, they do). $\endgroup$ – Jan Hudec Oct 29 '15 at 19:04
  • $\begingroup$ Thy PDF link breaketh. $\endgroup$ – Sean May 5 at 3:03
5
$\begingroup$

We all know the saying "From High to Low, Look out below" Some were taught that this only applied to pressure differences and thus were instructed to get a new altimeter setting every 100NM or so. This is a good recommendation.

Temperature differences from standard also causes errors with the altimeter. The same saying above holds true with temperature variations.

An altimeter setting is defined as: (ref. Pilot/Controller Glossary)

The barometric pressure reading used to adjust a pressure altimeter for variations in existing atmospheric pressure or to the standard altimeter setting (29.92).

We also know there is a direct correlation between pressure and temperature and thus an altimeter setting is corrected for both pressure and temperature. This means one thing, the altimeter error is zero at the field elevation. As the aircraft gets climbs higher above the field, the altimeter error will increase.

There is a table in the Aeronautical Information Manual (7-2-8) that lists altimeter error due to temperature as a function of altitude (AGL) and temperature.

ICAO Cold Temperature Error Table

Here is the formula that will give you this table. The formula can be found in Canadian Aeronautical Information Manual (RAC section).

Canadian Cold Weather Formula

The United States Air Force has a PowerPoint that discusses this topic.

The FAA has recently come out with Info 15002 which requires pilots (after 9/17/2015) who fly into "Cold Temperature Restricted Airports" to correct their altitudes for colder than standard temperatures. To determine if your airport is part of this program you must go to the Aeronav FAA website and search for Cold Temperature Restricted Airports. There should be a link at the bottom of the page.

Here is a graphic showing all the airports in the United States that were part of the program in February 2015.

Cold Temperature Restricted Airport - CONUS

Because Lnafziger wanted Alaska.

Cold Temperature Restricted Airport - Alaska

Not only is the pilot required to correct for the segment but they are also required to inform ATC of the new altitudes they will fly. This is to ensure traffic separation with other airplanes who are not temperature correcting their altimeters.

Pictures taken with permission from All About Airplanes blog. All rights reserved.

|improve this answer
$\endgroup$
  • $\begingroup$ Also, some aircraft / FMS's do offer automatic cold temperature compensation, and ATC does not compensate (as you said, the pilot is required to), to finish answering his questions. $\endgroup$ – Lnafziger Oct 30 '15 at 2:45
  • $\begingroup$ Last question, where did you get the graphics? (You should link to the original for proper attribution.) $\endgroup$ – Lnafziger Nov 4 '15 at 17:18
1
$\begingroup$

From ICAO (sorry can't remember word-to-word, but close): Temperature correction must be made to all minimum altitudes at and below MSA when station temperature is at or below 0 degrees celsius.
1. Yes, FMS can make the correction, but at least in some versions you have to enable it for each flight. Also a specific approval is needed to use the FMS correction.
2. Followong applies to EU, not sure about US: Under radar control ATC should make the correction. Without radar they don't.
3. If the controller makes the temperature correction, he/she should notify you by adding "temperature correction made by atc" or "temperature corrected" to the clearance. Ie "descend to 3000ft, temperature corrected"

|improve this answer
$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.