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Pressure Alt and Density Alt

hmmm

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Ok embry grads

Old kodger needs some thoughts

There is the Standard Press for a Elev

Say 1000 elev above MSL would be 28.92

And then calculate Actual Press Alt
at 1000 airport elev with a reported alt setting at the field of 30.92 which would calculate out to be negative 1000 below msl for the Actual Press Alt
(-1000)

Am I correct so far?

Now calculating Density Alt

Most online calculators ask for the elev, temp, dew point and reported alt setting.

Formula
DA = PA + (120 x difference between OAT and ISA)

Question
If figuring it out with your trusty dusty calculator....

in the formula, PA is the actual calculated PA, not the uncalculated field elevation. Correct?

in the formula ISA is what the ISA should be at the actual calculated PA, not the ISA for the airport elevation. Correct?

Why I ask.... In the calculators online, for example http://www.pilotoutlook.com/calculators/density-altitude-calculator

it asks for Elevation, Air Temp, Alt Setting and Dew point.

I think I should be putting in the Corrected Press Alt not the elevation, the actual ATIS reported Temp, Alt Setting and Dew Point

But I imagine in their online formula it figures out the Corrected Press Alt then The Density Alt and then only outputs the Density Alt

Forgive the old Cave Man... in the real world for example in the old DC-8-54 as a FE you had about 5 or 6 charts to go through step by step connecting the dots so you didn't really have to think about it and then the other guy checked your figures and then with the ACARS unit = Brain Dead.

Now I gotta re-learn all this stuff to make sure when flying the little ones again

And I may have this all wrong.
 
Last edited:

avbug

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Am I correct so far?

No.

Make it easy on yourself. Pressure altitude is what you read on the altimeter with it's set to 29.92" Hg.

And then calculate Actual Press Alt
at 1000 airport elev with a reported alt setting at the field of 30.92 which would calculate out to be negative 1000 below msl for the Actual Press Alt

Altimeter setting is actual field pressure corrected for the change in elevation. When you get an airport altimeter setting, you've already got an adjusted setting...you don't need to correct it for altitude.

In the example you've cited above, The actual pressure altitude is sea level. Not -1000'.

it asks for Elevation, Air Temp, Alt Setting and Dew point.

I think I should be putting in the Corrected Press Alt not the elevation, the actual ATIS reported Temp, Alt Setting and Dew Point

When whatever system you're using asks for elevation and altimeter setting, it's figuring out pressure altitude. You enter the airport elevation and the altimeter setting, and the calculator is then going to figure pressure altitude. From that, it's going to apply temperature, to come up with density altitude.

If you attempt to come up with your own pressure altitude and hand that to the calculator as elevation, then the computer is going to try to correct that value to pressure altitude, using the altimeter setting you've provided. Your information will therefore come out wrong.

If the computer asks for elevation and altimeter setting, then it wants the elevation indicated on the altimeter when set to the altimeter setting that you provide.

If the calculator wants only elevation and doesn't ask for the altimeter setting, then it's asking for pressure altitude, because the only possible altimeter setting when deriving pressure altitude is 29.92. Pressure altitude is what's read on the altimeter when it's set to 29.92. In the international aviation language, this is simply referred to as "QFE." QFE, then, is pressure altitude.

What you're needing to input into the calculator is called "QNH," or the altitude read on the altimeter with the local altimeter setting put in the kollsman window. If you're on the ground, it's simply field elevation.
 

hmmm

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Thank you

Well... looking at it as if either flying towards destination or on the way to the airport with a weather packet in hand or calling the ASOS on the cell phone. Not sitting on the ground in the airplane.

And tell me again about if the ASOS is reporting 30.92 and the elevation is a 1000 how press alt is "derived" (rule of thumb in your head.)

I was under the impression that every 1000 feet above sea level we lose an inch of pressure so standard pressure at sea level is 29.92 and standard pressure at 1000 above msl would be 28.92

The higher we go the less pressure.

Of course we set our alt to the pressure reported by stations enroute below 180. I say this because invaribly someone will take this at face value.

I like to understand things completely and how they are derived and not take it at face value.

Ok the standard method is for example if den is 5431 and alt is reported at 30.92 - 29.92 is a diff of 1000 actually its -910 (http://www.csgnetwork.com/pressurealtcalc.html)and we just subtract 910 from 5431 = PA of 4521

But what happened to the loss of one inch per 1000 feet?

In the airplanes we use graphs, charts acars and wizz wheels. It would be nice to be able to have an idea how to do it free hand and it doesn't make sense that press alt doesn't take into account the loss of a aprox and inch of ambient press per 1000 ft as for why we have to use turbos in recips to get up high

Cave Man
 
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VNugget

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Thank you

Well... looking at it as if either flying towards destination or on the way to the airport with a weather packet in hand or calling the ASOS on the cell phone. Not sitting on the ground in the airplane.

And tell me again about if the ASOS is reporting 30.92 and the elevation is a 1000 how press alt is "derived" (rule of thumb in your head.)

You take the difference between the reported setting (30.92) and 29.92, and multiply it by a thousand. The answer (1000 ft. in your example) is the difference between elevation and pressure altitude, starting at any given elevation.

Now apply that difference to the elevation we're starting from. In your case, that's a thousand feet minus a thousand feet, giving you a pressure altitude of zero.

I was under the impression that every 1000 feet above sea level we lose an inch of pressure so standard pressure at sea level is 29.92 and standard pressure at 1000 above msl would be 28.92

The higher we go the less pressure.
But, the altimeter setting from an ASOS is already adjusted for field elevation. Meaning, imagine they dug a hole at that airport, down to sea level, dropped a barometer all the way down, and took a reading. THAT is what altimeter setting means.

Let me use a different example from yours, becuase you used the same 1000 feet for two different things, that cancel each other out and make the explanations unclear.

Let's say we're dealing with an airport at 6000' elevation, which would make the pressure there 23.92 on a standard day. But if you set your altimeter to 23.92, it would show AGL. You'd have to manually add the field elevation to get a MSL reading. Pain in the ass, huh? Fortunately, they already DO this for you and make your life easier. On a standard day that airport (and ANY airport regardless of elevation) will report 29.92. So don't try to work elevation changes into your pressure calculation.

Now it's the next day, and let's say a high pressure system moves in. Now that 6000 airport (and any airport close to it) will report 30.12. If we look at the difference between that and the standard altimter setting, it shows an increase by .2, equivalent of 200 feet. So, if you forget to set your altimeter, it will indicate 200 feet lower than yesterday. In other words, the pressure altitude has gone down 200 feet.

To know what the PA is at the airport, you just add or subtract that change from the field elevation, giving you 5800 feet.

So, in summary:

1. Figure the difference between reported altimeter setting and 29.92.

2. Multiply that by a thousand, giving you the P.A. change.

3. Apply that change to the field elevation. (Remember, if pressure increases, pressure altitude decreases; and vice-versa).

----

In most real life situations, the pressure altitude does not change that much (think about the highest and lowest altimeter settings you regularly encounter.) Temperature has a much greater effect on density altitude, and can raise it by several thousand feet. That's what'll kill you in the mountains if you don't figure for it.
 

hmmm

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Edited passed 60 minutes
:
Thank you

Well... looking at it as if either flying towards destination or on the way to the airport with a weather packet in hand or calling the ASOS on the cell phone. Not sitting on the ground in the airplane.

And tell me again about if the ASOS is reporting 30.92 and the elevation is a 1000 how press alt is "derived" (rule of thumb in your head.)

I was under the impression that every 1000 feet above sea level we lose an inch of pressure so standard pressure at sea level is 29.92 and standard pressure at 1000 above msl would be 28.92

The higher we go the less pressure.

Of course we set our alt to the pressure reported by stations enroute below 180. I say this because invaribly someone will take this at face value.

I like to understand things completely and how they are derived and not take it at face value because that the way it is.

Ok the standard method to caclulate PA is for example if den is 5431 and alt is reported at 30.92, - 29.92 is a diff of 1000 actually it's -910 as seen by clicking here(http://www.csgnetwork.com/pressurealtcalc.html)and we just subtract 910 from 5431 = a PA of 4521

But what happened to the loss of one inch per 1000 feet?

In the airplanes we use graphs, charts, acars and wizz wheels. It would be nice to be able to have an idea how to do it free hand and why. Not just face value. It doesn't make sense that press alt doesn't take into account the loss of a aprox an inch of ambient press per 1000 ft. I mean that's why we have to use turbos in recips to get up high.

Cave Man
 

hmmm

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But, the altimeter setting from an ASOS is already adjusted for field elevation. Meaning, imagine they dug a hole at that airport, down to sea level, dropped a barometer all the way down, and took a reading. THAT is what altimeter setting means.

Let me use a different example from yours, becuase you used the same 1000 feet for two different things, that cancel each other out and make the explanations unclear.

Let's say we're dealing with an airport at 6000' elevation, which would make the pressure there 23.92 on a standard day. But if you set your altimeter to 23.92, it would show AGL (you mean MSL?). You'd have to manually add the field elevation to get a MSL(proper?) reading. Pain in the ass, huh? Fortunately, they already DO this for you and make your life easier. On a standard day that airport (and ANY airport regardless of elevation) will report 29.92. So don't try to work elevation changes into your pressure calculation.

.

Now VNugget.... that was a good explanation.

Ok so Den is 5431... its reporting 30.17 so press alt is aprox 5181

Density alt would be then PA 5181 + (120 x diff between OAT 2C and ISA 4C) = DA 5421

But is ISA in that formula for DA based on what ever ISA would be for the actual PA which we just figured out which could affect a big change in the summer? Not a big change tonight of course.

That question is assuming we are not using a computerized calculator online that takes PA into account and we are doing it by hand with the formula above.
 
Last edited:

hmmm

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Den elev 5431 reporting 28.97 33C/24C

PA 6381 (29.92-28.97=.95x1000=950+5431=6381)
DA 10101 (ISA15C - 6.381x2=12.76-15=ISAat6381of2-oat33=31x120=3720+6381=10101)

?



.
 

VNugget

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Correct.
 

hmmm

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Thanks for the help.... Really appreciate it. You have the right attitude VNugget
 
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