DENSITY ALTITUDE CALCULATIONS FOR FAA TEST

[k9flyer]

I am studying for FAA written, My question is regarding density altitude calculations long-hand. I know I can use e6b BUT my learning style is to understand process longhand first.

Given numbers to determine densisty altitude (DA)
1. altimer setting = 30.35
2. runway temp in faherenheit = +25
3. Elevation = 3894 msl

Formulas used:
a) Fahrenheit to celsius conversion = F temp substract 32 multiplied by .5556
b) Pressure Altitude calcs (PA) / PA = (std pressure 29.92 minus altimeter) multiplied by 1000 plus field elevation
c) Density Altitude formula (DA) / Pressure Altitude plus (+) [120 (OAT outside air temp minus std temp 15degrees celcius)]

Found PA to be = 3464
PA = (29.92-30.35) x 1000 + 3894

My calcs for DA:
DA = (PA) 3464 + [120 (-3.89 c - +15 c)]
DA = (PA) 3464 + [120 x 11.11]
DA = (PA) 3464 + 1333.2
DA = 4797.2

ANSWER IN BOOK IS 2000'

WHERE DID I GO WRONG IN CALCS? ALL HELP APPRECIATED!

 

[asicer]

-3.89 - +15 is not 11.11

Even so, std temp is not +15c at 3894MSL

 

[AggieMike88]

I know I can use e6b BUT my learning style is to understand process longhand first

I can appreciate wanting to know the scientific mathamatology behind this....

but you're making it waaaay to hard and are not focusing on the learning of how to get the right answer so you can then do something with the answer.

Just learn to use the flipping E6B and you'll have the answer in less than 15 seconds.

The way you are doing it here creates to many chances for errors and wasting time while taking the exam. Plus, are you really going to do this calculation with pencil/paper in real world flying?

 

[alfadog]

Temperature. You are saying it is warmer than standard, book says it is colder than standard. Which is it?

 

[Alex Hardt]

1) 30.35 in Hg is larger than the standard 29.92 and results in a pressure altitude lower than the runway elevation:

(29.92 - 30.35) * 1000 = -430 ft

PA = 3894 - 430 ft = 3464 ft


2) 25°F is below freezing and is a negative number in degrees Celsius:

(25 - 32) * (5/9) °C = -3.89 °C


3) Runway elevation is 3894 ft. The standard temperature of 15°C has to be adjusted to account for the 2°C/1000 ft altitude lapse rate:

3.894 * 2 °C = 7.79 °C correction for lapse rate

The resulting standard temperature to use for the density altitude calculation is :

STD at 3894 ft = (15 - 7.79)°C = 7.21 °C

The density altitude formula is then:

DA = PA + 120*(OAT - 7.21)

DA = 3464 + 120*(-3.89 - 7.21)


DA = 3464 - 1332 = 2132 ft

 

[dmspilot]

You need to reference the standard temperature for the pressure altitude in question, neither 15°C nor standard temperature for the actual elevation would be correct.

Additionally, the FAA expects you to use Figure 8 if they so refer you to it, the resulting answer may be more or less accurate than other methods but will get you the correct answer on the test.

 

[AggieMike88]

Using the online version of the ASA CX-3 flight computer.

 

[Clip4]

You do know there is a density altitude graph in the private test supplement?

 

[midlifeflyer]

I am studying for FAA written, My question is regarding density altitude calculations long-hand. I know I can use e6b BUT my learning style is to understand process longhand first.

BUT you are not. For example, that 120 you are using is a rule of thumb approximation, not the real D-Alt formula.

Go find Ed Williams Aviation Formulary if you want to play with the real calcs.

 

[dmspilot]

BUT you are not. For example, that 120 you are using is a rule of thumb approximation, not the real D-Alt formula.

Go find Ed Williams Aviation Formulary if you want to play with the real calcs.

So is the pressure altitude rule of thumb that he's using.

 

[midlifeflyer]

So is the pressure altitude rule of thumb that he's using.

True. In either case, the RoT calculation does get you pretty close and I use it in the real world. Applying it to @k9flyer's question, and rounding the temperatures to whole numbers, 2,144. Assuming the test question doesn't have answers closely surrounding 2,000, it would work if applied correctly.