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This question already has an answer here:

I work as ATC in Korea and recently, I have some question about winter altimeter setting.

In my facility, we use winter altimeter adjustment because the aircraft's pressure altitude is lower than true altitude in winter. I know this is due to air density difference in winter and summer.

My question is, can the altimeter setting (inch or QNH) include this adjustment? I know that the altimeter setting which ATC gives to the pilot changes every time according to the ground QNH condition. If the altimeter setting can include the adjustments, it would be convenient to ATC and pilot.

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marked as duplicate by fooot, digitgopher, mins, RockPaperLizard, SMS von der Tann Jan 19 '16 at 1:43

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

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    $\begingroup$ I thought transponders always send the pressure altitude (i.e. altimeter set to 29.92)? $\endgroup$ – kevin Jan 17 '16 at 13:24
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    $\begingroup$ I didn't understand your question completely, can you give some specific examples of the QNH or other altimeter settings? I think that would help to make it clearer $\endgroup$ – Pondlife Jan 17 '16 at 17:29
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    $\begingroup$ This is a great question. I think the answer has to do with the fact that the QNH is meaningful for more than just altimeter settings, so is not adjusted to reflect cold weather conditions. This cold-weather effect also impacts instrument approach minimums. $\endgroup$ – J Walters Jan 17 '16 at 20:18
  • $\begingroup$ Related $\endgroup$ – TomMcW Jan 17 '16 at 21:35
  • $\begingroup$ Related $\endgroup$ – TomMcW Jan 17 '16 at 21:37
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You can't just adjust the QNH setting to compensate. The temperature adjustment is not linear. The error increases with altitude, so when the altimeter is set for QNH the error at field level is zero. But it increases the higher you go. It's not just everything being X number of feet higher or lower.

In your question you say "the aircraft's pressure altitude is lower than true altitude in winter." That's backwards. The actual altitude is actually lower than what is indicated by the pressure alt. This can run the risk of colliding with terrain if the pilot doesn't adjust. In fact, exceptionally hot weather causes an error also, but in the opposite direction. The reason this doesn't need to be compensated is because the actual altitude will be higher than indicated so it doesn't create a danger of hitting terrain.

From answers to another question an aircraft with an FMS can be set to automatically adjust.

The problem seems to be in who's responsible for making the adjustment (ATC or pilot) and being clear on whether the altitude announced is adjusted or not. According to the answer from wbeard52, in the US the FAA has established a list of airports with cold weather procedures that require the pilot to do the compensation and announce the corrected alt to the controller. According to Sami's answer, in the EU the controller makes the corrections under radar coverage, and the pilot makes the correction without. It would appear from your question that in Korea they have decided to have you make the corrections as controller.

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    $\begingroup$ @mins Ah. Was kinda wondering how the heck that would work $\endgroup$ – TomMcW Jan 18 '16 at 16:25

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