Pressure Altitude and Density Altitude

[chr15ru55e11]

This is more of a conformation of formulas but they may be wrong.
The simple way to calculate pressure altitude and density altitude is -

PA=(1013-pressure)x27

This will give a possitive or negative answer which is then added or subtracted from the height of the airfield?

DA=PA +or- (Actual temp -ISA)x120

Anyone any ideas? I know there is a more complex density altitude which does into dew points and what not but just looking for a simple formula?

[mathy]

Before you try to invent another simple formula try this. It is simple but that does not mean easy to do longhand because the subject is quite involved.

It takes the bold step that if we go flying we quickly get into the realms of dry air very quickly and can neglect dew point etc

google richard shelquist + density altitude + calculator

I sometimes find a range of altitudes that work well with richard's approximation then use something like the "polyfit.exe" program for 10 points of interest and the quadratic or cubic function. Google polyfit.exe. On a calculator use the standard deviation test and chose which gives the lower value. Its usually but always the quadratic version. If your home brew formula works it will show up bonzer. If your formula is cr*p stdev will tell you. Bin it!!

My experience of home grown rules is woeful. I make incorrect assumptions! My "rules" fall flat and it is not easy for someone else to spot where I go wrong.

No, let Shelquist do it for you. Then download any interpolation formula, excel will do; test how good the formula is using standard deviation test as I have said; choose the formula that gives the lowest value of stdev. Program your calculator and save.

Look for Richard's section that starts as follows:

Simpler Methods of Calculation...
If you really want to know the actual density altitude, it will need to be calculated in the general manner that has been described above. However, there are simple approximations which have been developed over the years. For example, a particularly convenient form of density altitude approximation is obtained by simply ignoring the actual moisture content in the air. Here is such an equation which has been used by the National Weather Service (see ref 13 (http://wahiduddin.net/calc/density_altitude.htm#b13)) to calculate the approximate density altitude without any need to know the humidity, dew point pressure:

[chr15ru55e11]

Thanks for the reply,

Only reason I was asking for a simple formula was I am attending an interview relatively soon and Ive noticed from other people commenting that one of the things they like to ask you is to calculate the density altitude. Obviously I cant whap out an app or anything to calculate it for me so was just checking if the way I wrote, is acceptable enough to get a round about answer?

[StudentInDebt]

Does this help?
Density Altitude = Airfield Altitude + 1,000ft per 8 degrees above ISA

[chr15ru55e11]

I think as a rough formula that works I think

Say airfield 500 feet, temp 10, dew point 8 at 1012

On a calculator over the internet it says its 91 feet

Through the PA=(1013-1012)x27 =27+500=527
DA you are saying is 1000 for every 8. So as its 5 below that is around 600 off to give -100. Kinda works but dont know if there is a more accurate way (without going into mental formulas)

[mathy]

You are permitter to use a CX-2 or whirly wheel you know!

But try Shelquist anyway. Any time you are asked for Dew Point stick in -40!! That'll be dry enought for you. If any brief formula gives good results it will show up on Shelquist's simple test.

Thanks student in debt.

[A Squared]

Pressure is nonlinear function of altitude, even of pressure altitude. So any formula which is linear (and all posted so far are linear) will be an approximation at best.

[airman13]

the easiest way to find out pressure altitude is to set in your altimeter 1013....

[StudentInDebt]

I'm not being funny but, if I were applying to a company where they expected me, in an interview, to answer a question regarding a Density Altitude question to an accuracy of more than 500ft using mental arithmetic, I'd be grateful to be rejected!