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Methods to calculate DA


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I'm a lower time CFI, and I'm currently going thru my notes and lesson plans. This weekend I am going thru DA and PA notes.

 

I just wanted to be able to document the various way one can calculate DA...other than the "whiz wheel" and digital e6b (no cheating with your iPhone!).

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See, a method! LOL!

 

Great, now I am hearing that dude's voice in my head. Thanks. Like a bad song.

 

 

 

1. e6b/Flight Computer

2. POH DA chart

3. Call ASOS

Edited by tattooed
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It can be done mathematically too, as you probably know.

 

For example, using the formula, we find the following:

PA= HA + PAV

PA = 1,500 + 1,000(29.92 - 29.41)

PA = 1,500 + 510

PA = 2,010 feet

 

DA = density altitude,

PA = pressure altitude at the level you desire density altitude,

120 = a temperature constant (120 feet per 1°C),

andVt = actual temperature minus standard temperature at the level of the pressure altitude.

 

For example, let's say the surface temperature is 30°C and your pressure altitude is 2,010 feet. find the standard temperature corresponding to 2,000 feet. You should find 11°C. Plug these values into the formula to find the following:

DA = PA + (120 Vt)

DA = 2,010 feet + [120(30°C - 11°C)]

DA = 2,010 + 120(19)

DA = 2,010 + 2,280

DA = 4,290 feet

 

I prefer an iphone or some sort of computer! ;)

Edited by Trans Lift
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Straight from my lesson plans: This is set up for the airport I trained at so just change your field elevation where needed.

 

Computing Density altitude via equation –

 

1. Pressure Altitude- your standard pressure is 29.92”hg – subtract the current pressure from standard and then multiply by 1000, add positive numbers and subtract negative numbers from your field elevation.

 

So if you had a pressure reading of 30.06 you would subtract 29.92 – 30.06 =-0.14 multiply this by 1000 = -140 so subtract 140 from 1382 = 1242’ PA

 

The 1000 represents the lose of horse power, for every thousand feet of altitude. We loose 7 HP of engine performance.

 

2. Density Altitude – Pressure altitude corrected for non standard temperature.

 

Standard temperature is 15C,

First step will be to find our standard temperature for our elevation, 1382 @ IWA

Multiply 1382 by. 002 = 2.764 now subtract this from the standard temperature of 15C

12.236C is what our standard temperature is. Now you will need to figure out how much warmer it is than standard, so subtract our standard temp from current, lets say 35C = 22.764C above the ISA, This will effect the performance substantially, multiply your answer 22.764 by 120 = 2731.68 the 120 represents 120’ loose of helicopter performance for every 1C over the ISA.

 

Now we take our pressure altitude and add to our temperature equation 1242 + 2731.68 = a Density altitude of 3973.68 feet.

 

 

Easy right??

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... or, in simpler terms:

 

1) calculate your pressure altitude

 

2) find the ISA temperature for that pressure altitude (15ºC at sea level, ~2ºC less per 1,000ft AMSL)

 

3) Look at the ambient temperature.

For every ºC warmer than the standard temperature, add 120ft to the PA to get the DA.

If it is colder than standard, subtract 120ft per ºC.

Edited by lelebebbel
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oh you people! I am trying to teach STUDENTS, no support AT&T. LOL.

iPhone NOT allowed when you are my student.

 

 

 

(guess what I use to calc DA....iPhone)

 

 

At ChprPlt, Trans Lift, and lelebebbel (Hi D!!)

THANK YOU!! That is exactly what I need.

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  • 2 weeks later...

I just tune up the ASOS/ATIS and listen to the last element of the weather. If I bother at all. I don't really care about the DA numbers, just whether not not the ship will fly, and if it will hover, it will fly.

 

True, but.....

 

It flew when you took off, but here in Colorado it is not unusual to have the temp increase by 3-10 degrees C during the course of a flight, especially if there is a brief shower with our high evaporative cooling.

 

Another scenario: You took off out of KBJC at 10:00 AM and it was 30 deg. C., but you're landing in the mountains at 9200' msl and the helo's OAT says its 22 degrees C. What's your DA?

 

Remember, your Pax will freak out if you let go of the controls and start rummaging through the POH....

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