density altitude: dewpoint?

[ScottC]

Hi,
I'm a student with about 6.5 hours and studying for the written. Here's my question:

If I use the app I have on my iPhone, "mycopilot," to find the density altitude, I have to enter the dewpoint. The test questions don't give a dewpoint, yet the answer will differ substantially on the app with different dewpoint values. So my question whether there is a standard dewpoint that should be entered to get the "correct" answer. If the answer can vary widely, why wouldn't dewpoint be part of the question?


thanks all

 

[Cpt_Kirk]

Find a METAR. Enter the temp, dewpoint, and pressure. I'm not sure why dewpoint is a part of the calculation, though.

METAR KATL 182152Z 19003KT 10SM FEW065TCU SCT090 BKN200 BKN250 32/17 A3010 RMK AO2 SLP180 TCU OHD-N T03170167

 

[bobmrg]

The answer is to use another method of determining density altitude. The DA solution has two components: pressure altitude and temperature; dew point does not enter into the calculation. Sad to say, not everything in this world works on an iPhone.

Bob Gardner

 

[ssonixx]

Use the density altitude charts in your book. Convert atmospheric pressure to pressure altitude and correct for temperature. Humidity affects atmospheric pressure so it is already factored in.

Use an E6B.

I would start with FAA tools and materials and then move from there. You are doing this kind of backwards if you start with your own tools.

 

[flyingron]

As Bob aludes, while humidity does fit in to DA calculations, it's much less of a contributor than temperature. Most of the test questions assume dry air. One common approximation is DA = pressure_atltitude + 120 x (TEMP - ISA_standard_temperature). A rule of thumb on the humidity calc is add 100 foot for each 10% of relative humidity.

 

[AggieMike88]

iThingie can't be used during the written exam. But this chart is in the figures book the test center supplies. Start learning how to apply it now and you'll be just fine.