Can someone explain Pressure Alt?

[uwochris]

Hey guys,

I seem to get confused when reading about PA and its significance.

From what I understand, PA is simply indicated altitude corrected for non-standard pressure. This all goes back to the fact that the altimeter is calibrated to a standard atmosphere, so when the atmosphere isn't standard, there will be some error. So, by calculating the PA, you are in fact determining the pressure that the a/c thinks it's flying at (or the altitude it thinks its at), assuming standard temperatures. And since all charts are based on PA, it is very important to determine it.

Is my understanding correct? The thing that gets me confused is that we should be more interested in Density Altitude, as it correctes for non-standard temps. I mean, how often can the atmosphere be standard, especially now? Why not worry about DA only, and discard PA? It seems DA is far more relevant, because it gives the altitude the a/c thinks its at, corrected for non-standard temps.

Also, just one more question. I assume that a lower PA is better; is this correct? For instance, if I calculate the PA to be 600' today, but 3000' tomorrow (I know, an xtreme example), it seems that the 600' would give better performance: lower altitudes= more dense air= better performance. Is this correct?

Thanks a lot for the help!

 

[jaybird]

Density altitude is the one you need to be concerned with, I think. From what I understand the only reason you figure PA is to get DA. DA is the altitude the aircraft thinks it's at. PA is simply 29.92. Whenever you put your Kollesman (sp) window on 29.92 you get your PA. If you get the temperature then you can get your performance. Hence PA corrected corrected for non-standard temp is DA. You can be at sea level on a hot day and be at 2000 feet DA. Hope I'm right. Stand by for avbug.

 

[A Squared]

>>PA is simply indicated altitude corrected for non-standard pressure

Actually, no, that's kind of backwards. In a sense, indicated altitude is pressure altitude corrected for non standard pressure. In, other words, indicated altitude (if the altimeter setting is correct) is usually closer to true altitude that pressure altitude.

Pressure altitude is the altitude in the "standard atmosphere" where you would find the same air pressure as where you are. for example; if you are at a sea level airport and the altimeter setting is 30.92, that means the pressure that day is higher than normal. In a standard atmosphere you would find a barometric pressure of 30.92"hg at 1000 ft below sea level....so your pressure altitude, at a true altitude of 0, is -1000'

On the other hand, if the atmospheric pressure is lower than standard, you might have an altimeter setting of 28.92" hg. this would mean that the air pressure at sea level is that same as the pressure that would be found at 1000 ft above sea level in a "standard atmosphere". On that day, your pressure altitude at sea level is 1000 feet.

You are correct that density altitude is more relevant to aircraft performance than pressure altitude. The performance of an aircraft is related to the density of the air it's flying thru. Howver, we can't just skip pressure altitude. determining pressure altitude is an important step in calculating the end result, density altitude. Here's why: we want to know the density of the air for performance calculations. Density is affected by both pressure and temp. higher pressure results in higher air density (remember air is compressible)
higher temperature results in lower density.

First we determine pressure altitude...this accounts for difference in density (from standard) due to pressure, then we calculate density altitude...this accounts for the density difference due to temperature. It's a 2 step process and it wouldn't be complete without both steps.


regards

 

[ILLINI]

You were correct when you said that you need to be more interested in density altitude when looking at aircraft performance. You can't find density altitude without first knowing pressure altitude. Pressure altitude is rather simple to determine as long as you know the current altimeter setting and field elevation or intended cruise altitude. Finding density is a little more complicated, which is why most of us use the E6B or a published conversion chart. When you are using your performance charts to figure out TAS, fuel burn, rate of climb, etc. you are using density altitude; just indirectly. Remember that when you use these performance charts you are asked for the temperature at the surface or at altitude, depending on which performance chart you are using. The chart indirectly uses density altitude to give you the performance numbers you are looking for. You provide the easy to determine pressure altitude, and the chart factors in temperature for density altitude. It just makes it easier to use the chart.

Make sense?

 

[xrjpilot]

If you just use PA to find your DA, you will miss a very imporant part of flying. Terrain Clearance. If you only fly from your airport to the practice area and back only on days when High pressure dominates, You'll be fine. When you start doing long X-c flights, you may cross a front or fly into a different pressure area. If you don't reset your Alt. all your pattern alt will be off. Best case; You'll be high on final. Worst case; you'll be low and slow...Not where you want to be.

You can also use Pressure charts to predict wind and Hazardous WX.

Clear as Mud