Student pilot here. I thought I was smarter than this, I’m struggling with this concept. I got the ASA test prep book and there’s a question asking what happens to the indicated altitude when the altimeter setting is changed from 29.15 to 29.85. Answer is indicated altitude increases by 700 feet. Ok, makes sense.

But then I’m reading the aircraft performance chapter in the Aeronautical Knowledge book. It has a chart about finding pressure altitude, and when the altimeter setting decreases on this chart the altitude increases. So, the converse of the concept in the question. I can make either make sense on its own but they seem to contradict each other. What am I missing?

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    $\begingroup$ I think your second paragraph is not as clear as it could be. Which altitude increases? Pressure altitude? What is the reference on the chart? Is it indicated altitude or actual altitude? Do you have a reference? $\endgroup$ – BowlOfRed Oct 30 at 16:09
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    $\begingroup$ Agree, I think you need to edit to " and when the altimeter setting decreases on this chart the PRESSURE altitude increases." Right? $\endgroup$ – quiet flyer Oct 30 at 16:11
  • $\begingroup$ There’s a chart where you find the altimeter setting and it gives you the corresponding altitude correction. Trying to post the pic but not sure how. It has you add altitude to field elevation, as the altimeter setting decreases, to “get pressure altitude.” $\endgroup$ – rbsc Oct 30 at 17:49

Here's the difference in a nutshell:

When you change the number in Kollsman window to bigger number and the atmosphere stays the same, the reading on your altimeter increases.

When you leave the number in the Kollsman window the same (let's say you have it set to 29.92 to see the pressure altitude) and the atmospheric conditions change so that the correct altimeter setting increases--i.e. the air pressure has increased-- then the reading on your altimeter decreases.

You increasing the setting in the Kollsman window is not the same thing as an increase in the prevailing correct altimeter setting. They have the opposite effect on what you see on your altimeter dial. The first thing makes the reading get higher, and the second thing makes the reading get lower. When they both happen at the same time, the reading stays the same.

Clear as mud now?

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    $\begingroup$ Yes. My brain was trying to tell me something like this but I couldn’t quite get there. This absolutely clears it up, thanks so much. $\endgroup$ – rbsc Oct 30 at 17:50

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