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MidlifeFlyer · January 25th, 2004, 12:19

[ QUOTE ]
If you set the correct altimeter setting, your altimeter will read the correct altitude when on the airport surface. This is not the problem. The problem is that if non-standard temperature exist, at any altitude above the surface your altimeter will be in error.

We have all heard that atmospheric pressure decreases by about 1" Hg per 1,000' increase in altitude.

[/ QUOTE ]Great answer. That's the whole point. Let's separate temperature from pressure for a moment.

An altimeter is set to correspond with an altitude on the ground. In the air, it =assumes= that the standard pressure lapse rate is correct. The needles move at the rate of 1000' for each 1" of static pressure change. The problem with the assumption is that the lapse rate that day might =not= be 1"/1000'.

The altimeter that reads 3000' =isn't= saying "you're 3000' above sea level. " It's saying, "Based on the sea level pressure setting you put into the Kollsman window, the static pressure here is 3" lower."

As you pointed out, the problem with the scenario is that the pressure lapse rate may not be standard. If the lapse rate is smaller, the pressure changes coming closer together, that 3" change that makes the altimeter say "3000" will happen at an lower true altitude.

January 25th, 2004, 12:19	#19
MidlifeFlyer Old Skool Join Date: May 2003 Location: Denver Colorado Posts: 3,093	Re: True v Indicated Altitude [ QUOTE ] If you set the correct altimeter setting, your altimeter will read the correct altitude when on the airport surface. This is not the problem. The problem is that if non-standard temperature exist, at any altitude above the surface your altimeter will be in error. We have all heard that atmospheric pressure decreases by about 1" Hg per 1,000' increase in altitude. [/ QUOTE ]Great answer. That's the whole point. Let's separate temperature from pressure for a moment. An altimeter is set to correspond with an altitude on the ground. In the air, it =assumes= that the standard pressure lapse rate is correct. The needles move at the rate of 1000' for each 1" of static pressure change. The problem with the assumption is that the lapse rate that day might =not= be 1"/1000'. The altimeter that reads 3000' =isn't= saying "you're 3000' above sea level. " It's saying, "Based on the sea level pressure setting you put into the Kollsman window, the static pressure here is 3" lower." As you pointed out, the problem with the scenario is that the pressure lapse rate may not be standard. If the lapse rate is smaller, the pressure changes coming closer together, that 3" change that makes the altimeter say "3000" will happen at an lower true altitude.