Skyview Density Alt. Calculation

I have noticed over the pased 100 hrs flying behind the Skyview that it shows a higher DA calculation than almost all other E6B computers. Not sure about BA or AFS.

I narrowed the error down to only one possibility that I could think of and that is what are they using for humidity being most EFIS units or calculators for that matter do not allow for a Relative Humidity input.

I recieved a responce from Dynon on the matter here.




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Sorry we never got back to you on this. The answer is that we calculate density altitude using 100% humidity (zero dew point spread).

This is the worst case (highest density altitude) but clearly isn't going to match any other calculation that uses a different baseline or uses the real humidity.


So it appears that the only time the DA calculation from the Skyview is accurate is when I am flying IMC (100% RH). Any other time the actual DA is something less than the Skyview number.

This also effects the TAS calculation by the Skyview.

If you use the E6B that is included in the utilitys section of your Garmin GPS you will notice much lower DA calcuation and a lower TAS calculation.

Interesting after doing some digging I found this.

http://www.faa.gov/library/manuals/aviation/pilot_handbook/media/PHAK%20-%20Chapter%2010.pdf

If you read pages 10-4 to 10-5 you notice the first thing it says under the "Effects of Humidity" is "The preceeding paragraphs are based on the presumtion of perfectly dry air."

I am assuming my garmin uses 0% RH so am I correct that the actual DA and TAS will usually be somewhere between the one calcuated by "Most" E6B calculator and Dynon's calculation?

When I recieve conflicting data from seperate sources it makes me want to know why. Not that one or the other is wrong they may both be right but I want to know what I am looking at.

Maybe I should install a seperate temp probe with a wick and a bottle of water to keep it moist and then wire that into the Skyview for a more accurate calculation.

Interesting.

In my many years of flying I've never run across a chart used to determine DA that even takes humidity into account. The site dsays that it does make a slight difference but gives no info as to how much.

Density Altitude changes with relative humidity

Reference the Density Altitude Calculator - using selectable units and relative humidity from http://wahiduddin.net/calc/calc_da_rh.htm.

A change in relative humidity from 100% to 0% changes the DA. The change amount depends upon tempratures and altitude. A 100% change in RH at SL, 0°C changes the DA by 79', 15°C changes the DA by 219' and 30°C changes the DA by 538'. At 6000' the changes are: 0°C - 94', 15°C - 259' and 30°C - 639'.

A correction here: we mis-spoke. SkyView uses a standard "dry air" model. We flipped a couple of numbers in our response, and that changes the entire meaning. So the answer really is:

The answer is that we calculate density altitude using a dry air calculation that is the general "standard" equation for density altitude calculation when you don't know the humidity.

This doesn't get you the highest possible density altitude (which is at 100% humidity) but clearly isn't going to match any other calculation that uses a different baseline or uses the real humidity.