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6 Answers

Pressure Altitude Confusion

Asked by: 5309 views ,
Aerodynamics, Weather

Hope you guys can help me out with this one,

 

Here's the scenario, im crusing along at 8500ft with an altimeter setting of 30.50. This altimeter setting is above the standard 29.92  When you do the math of 29.92 - 30.50= -0.58. Multiply that by 1000 and the difference to pressure altitude is -580. According to this it is clear that pressure altitude is below 8500 by -580 which is 7920ft, and to further verify this i recycle my altimeter setting from 30.50 to 29.92 and it reads somewhere close to 7920ft!!!

Now here is my confusion. To my logical thinking if it is 30.50 at 8500ft shouldn't lesser pressure be above me ? Because when you climb pressure decreases and as you descend it increases correct? With this logic at 8500ft with an altimeter of 30.50 a lesser pressure meaning 30.49, 30.48 until eventually reaching 29.92 would be above my current altitude correct ? So i would add the initial 580ft to 8500ft and would place my new pressure altitude at 9080 feet ? Even though this logic feels right and i am doubting myself because why when i reset the kollsman window to 29.92 it read 7920ft placing pressure altitude below rather than above me at 9080ft. 

If any one can explain my crazy thinking i would appreciate it a detailed answer!

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6 Answers



  1. Wes Beard on Feb 09, 2015

    What you need to realize is the frame of reference to which you are asking the question.

    Are you basing the question on the altitude being constant or the altimeter setting being constant? If you answer that, it should clear up an confusion.

    For example, if the altitude is constant (as in your first example) resetting the altimeter back to 29.92 will cause the altimeter to show a decrease.

    If the altimeter setting was constant, as you climb the altimeter would climb indicating lower pressure. As you correctly pointed out, the altimeter climbs with a decrease in pressure at around 1″ per 1000′.

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  2. chucho on Feb 09, 2015

    Well I was at 8500 thought the entire flight and the altimeter remained in the 30.45 to 30.50 range

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  3. Dan S. on Feb 09, 2015

    Try to think of it this way…the higher the atmospheric pressure, the more air molecules acting to produce lift on the wings, and more thrust from the props. This results in better performance. So if the altimeter setting is higher than 29.92, the aircraft will perform as if it were flying at a lower altitude (similar to colder temps giving lower density altitude, and better performance). So when you dial down the setting to 29.92, the pressure altitude will indicate a lower altitude when the altimeter setting is higher than 29.92. Don’t confuse the altimeter setting with the changes in pressure in different levels of the atmosphere. All the altimeter does is measure the difference between current pressure at sea level (your altimeter setting) and the pressure at the altitude you’re currently at.

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  4. Cal on Feb 15, 2015

    I like to think of the setting as the “base level” or “starting level”. When the altimeter is set to the correct value, the base level is sea level. If the correct setting is 30.50 and you set it to a lower value then you’ve set the base level to a higher altitude (lower pressure). Now it’s easy to see that the altitude difference between the new, higher base level and your actual altitude will be less than it was with the coreected setting.

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  5. chucho on Feb 16, 2015

    Thank you Dan S. I had a hard time understanding this concept!

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  6. Alex Weeks on Feb 19, 2015

    It may help to think of the weight of a column of air directly above your altimeter. The more that column of air weighs, the harder it presses on your altimeter and the lower it reads. For example, when you descend 1,000 feet, there is more air above you, that air weighs more and presses harder on your altimeter. As a result, your altimeter reads 1,000 feet lower. Similarly, high pressure presses harder on your altimeter causing it to read lower. To compensate for that pressure, we need to adjust the altimeter up to make up so we see our actual altitude on the altimeter.

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