Wednesday, 24 August 2016

Pressure Altitude and Density Altitude

When asked what pressure altitude is or what density altitude is most pilots will inadvertently dodge the question and instead tell you how to calculate pressure altitude or how to calculate density altitude. This leaves unanswered the questions: What is density altitude? What is pressure altitude?

Here are the answers to those questions:
Dude, Pressure altitude is the altitude in the standard atmosphere at which you would find the pressure in question (usually the observed pressure).
We calculate pressure altitude for an airport by subtracting the current altimeter setting from 29.92 and multiplying the result by 1000. We then add this result to published field elevation
The altimeter setting in Prescott is 30.16 what is the pressure altitude?
Answer: (29.92 - 30.16) * 1000 + 5045 = 4805' MSL

This means that the pressure you are observing in Prescott will be found at an altitude of 4,805 feet in the standard atmosphere.

Now on to Density Altitude
I'm just gonna say it.
Density Altitude is the altitude in the standard atmosphere at which you would find the density in question (usually the observed density).
We calculate density altitude by correcting pressure altitude for non standard temperature using an E6-b (or something fancier).

To better understand Density Altitude, consider the following scenario, which actually happened once:

1. You are kidnapped by a tribe of five-legged aliens.

2. Using magical technology they suspend you at some point in the atmosphere and give you a magical density measuring device.
3. They ask you what your altitude is and place bets on how close your guess will be to reality.
4. They inform you that if you're more than ten percent off that they'll execute you by feeding you to the ravenous Bugblatter Beast of Traal.

...You read the output on the density measuring device and find ambient density to be .9 kg per meter cubed. You bust out your smartphone (which still has service) and follow the above links to a table of the standard atmosphere. You find that in the standard atmosphere a density of .9 kg per meter cubed occurs at about 3km altitude which is 3,000 meters high. You tell the aliens that you are 3,000 meters high. They are Imperial Aliens and demand the answer be given in feet. Being a very crafty pilot you know to multiply 3,000 by 3.28 to get feet. You bust out your smartphone again and multiply 3,000 by 3.28 to get 9,840 ft, you tell the aliens that you are at 10,000 feet MSL.
The aliens acknowledge that you are correct but decide to feed you to the Bugblatter Beast of Traal anyway. You die with a complete understanding of what density altitude is and how to calculate it.

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