Pressure Altitude

[C.R.O.]

ok, back to our original topic....PA.

 

 

I am having issues with the concept of "true altitude" and "calibrated" altitude, and how to calculate them.

 

Here's an actual performance problem my CFI had me work thru. I needed alot of "hand holding" thru true/calibrated altitude, but otherwise did fine with it. I am only including the data for PA, DA, and True/Cal altitude and leaving out everything else.

 

 

Departure site:

Elevation 2000 ft

ATIS: 285@20, Temp 12C, BAR 27.84"

 

 

Destination - To overfly a site (photo shoot) by 1,000'

Site elevation 7000'

Temp at site: M23C

No wx report, only temp.

 

 

Calculate (at altitude we are overflying site):

PA

DA

True Altitude

Calibrated Altitude

 

 

How would you work thru this problem?

 

 

Sandy,

 

If you want some help, I will stop by the pilot lounge after I am done flying @ 1:30pm tomorrow, while you are waiting for your flight, and we can go over all the stuff.

 

Conor

[tattooed]

No one wants to take a stab at this and help me work thru it? (other than Conor....thanks, dude!...catch yah soon....)

 

 

I guess my specific issue now is PA, True Alt., and Cal Alt. on the E6B.

 

 

So, I come up with departure PA of 2,080' above MSL

DA (after dialing 2,080 and 12 degrees into E6B) = 2,500' above MSL

 

 

 

 

 

To estimate PA of "the spot"..or more accurately of my 1,000' overfly (making it 8,000')

1) I took the departure station BAR of 27.84 and figured we loose 1" per 1,000' of altitude, so I subtraced 6" = 21.84

 

2) I subtracted that from standard datum 29.92 - 21.84 = 8.08 x 1,000 = 8,080 above MSL for overfly PA.

 

3) (and here is where I get stuck) I dial 8,080 and M25 into the LEFT PA window on my E6B. Now what?

Do I find my true altitude of 8,000' (overfly) and look for Cal Alt in middle circle? I then get 8,800 calibrated.

Is that correct?

[tattooed]

Question -

 

If I have my PA, then I can determine OGE and IGE hover. Now I know my performance.

 

I have TRUE altitude...in this example 7,000 surface and 1,000' overfly. Keeps me from bumping into things.

 

Why do I need CALIBRATED altitude?

[justfly]

editing

[Kelly N.]

Was reading this thread again and decided to post what I came up with for my final ground exam. It's really just several equations boiled down to three that are pretty simple to remember (they're kinda symmetric to me).

 

Equation #1: PA = FE + 1000(ISAPress - CurrentPress)

Equation #2: TV = |(FE*.002) - 15| Note: you just want the absolute value of TV so if it's negative, ignore the sign.

Equation #3: DA = PA + 120(OAT - TV)

 

PA = Press Alt (duh!)

FE = Field Elevation

ISAPress = 29.92

CurrentPress = Field Pressure

OAT = OAT

TV = Temp Variation from std based on field elevation

DA = DA

 

You do them in order like so:

 

Given:

FE = 369

CurrPress = 30.01

OAT = 26

 

PA = 369 + 1000(29.92 - 30.01) = -90 + 369 = 269'

TV =|(369*.002) - 15|= |-14.262|

DA = 269 + 120(26 - 14.262) = 1678' (rounded)

 

 

The formulas are symmetric if you think that the parens alternate from being at the end (Eq1) to the beginning (Eq2) to the end (Eq3) and the first operation in each equation is +, *, +. The last operation is always subtraction. After that, it's just a matter of remembering the parens * 1000 for Eq1 and * 120 for the last (Eq3).

 

It may sound like a lot, but if you just remember

 

PA

TV

DA

 

You can then fill in the parens for each, the constants and the major operators:

 

PA = + 1000( 29.92 - )

TV = ( *.002 ) - 15

DA = + 120( - )

 

And then the variables:

 

PA = FE + 1000(29.92 - CurrPress)

TV = (FE*.002) - 15

DA = PA + 120(OAT - TV)

 

Maybe this helps someone else. Definitely helped me and another student on our ground school exam.

 

Now, if you're still awake, you're an avgeek.

 

Kelly

 

P.S. I didn't check my notes, but I'm pretty confident I remembered the formulas correctly. Definitely double check using other methods and let me know if you get a different result. We checked using an electronic flight computer and got very close (it used a slightly different value for the temp variation calculation).

[tattooed]

Hmmmmmmmmmmmmmmmm.....

 

 

Me likey.

 

 

 

Must mean I'm an "av-geek".....but I think I already knew that.

 

 

Thanks for posting that....definately helpful. That's goin' in my notes.