Quote:
Originally Posted by lr172
Thanks for the detailed explanation. This is consistent with my initial thinking on how it worked. Given that the local altimeter setting is calculated to give a correct reading in colder temps, why all the warnings about cold weather showing lower than actual altitude readings. It would seem that if the local station is 50% colder than standard and the temp at 4000' AGL is also 50% colder than standard, I would get an altimeter reading in the vicinity of true altitude. I understand that if the temp is 75% colder than standard at 4000' AGL, I will read low and maybe that's where the warning is coming from. Or maybe there is some algebra in there whereby 50% colder than standard at 4000' AGL will still read lower than true altitude eventhough the local station was 50% colder.
Larry
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The issue is that the altimeter has, built into it, a standard model of the atmosphere. By adjusting the Kollsman window you can force the altimeter to read correctly at one altitude in non-standard conditions, but only one. The internal lapse rate model is unchanged. So if going into ktvl (6000') on a warm day, the altimeter will read correctly as you approach 6000'. And because the warm air has expanded up, you will clear the 8000' hill by, say, 1000', even though on a standard day the clearance is just 500'. The problem comes when it is colder than standard. Your altimeter will read 6000' as you approach the airport, if you have the TVL altimeter setting. But as you approach the 8000' hill on the approach, your altimeter will read, say, 9000' (high). If you correct to the published 8500, you'll actually be at 8000 and can hit the hill. So the issue with cold weather approaches is when there are obstacles substantially higher than the airport - your altimeter won't guarantee clearance.
Linear Mode
