density altitude

[milehigh]

I knowthis should be in technical talk but I was having a hard time trying to start a new thread there.

question: let say current alt is 30.08 and airport elevation is 1000 so my pressure altitude would be 840ft. If im flying in the area at 4000 does that mean i subtract the same amount from my indicated alt that i did from my airport elevation to get my density alt at 4000?????

[SteveC]

<relocated>

[JDMcFly]

Originally Posted by milehigh

I knowthis should be in technical talk but I was having a hard time trying to start a new thread there.

question: let say current alt is 30.08 and airport elevation is 1000 so my pressure altitude would be 840ft. If im flying in the area at 4000 does that mean i subtract the same amount from my indicated alt that i did from my airport elevation to get my density alt at 4000?????

I'm not sure I understand the question.

Density alt = Pressure altitude corrected for non-standard temperature.

[av8or91]

Originally Posted by milehigh

I knowthis should be in technical talk but I was having a hard time trying to start a new thread there.

question: let say current alt is 30.08 and airport elevation is 1000 so my pressure altitude would be 840ft. If im flying in the area at 4000 does that mean i subtract the same amount from my indicated alt that i did from my airport elevation to get my density alt at 4000?????

Get a temperature and use your E6-B

[scottyboy75]

Ok time for a helicopter pilot to step in. DA rules my life these days.
Find your pressure altitutde.
Find temp for the area.
Find the difference from standard.

Easy formula DA= PA+/-(temp*117) All the books on it I have read say 120 but 117 gets you within 50 ft.
I always add my correction to any alt I am at so I know for sure what my hover ceilings are at and my Vne. It would suck having to explain to the boss why I had to do a running takeoff with private student on a cross country.
Hope this helps
Shane

[tgrayson]

Originally Posted by milehigh

If im flying in the area at 4000 does that mean i subtract the same amount from my indicated alt that i did from my airport elevation to get my density alt at 4000?????

Yes, assuming you meant pressure altitude. You would need to correct the result for temperature, just as you would at the airport elevation to get density altitude.

[MidlifeFlyer]

Originally Posted by scottyboy75

Easy formula DA= PA+/-(temp*117) All the books on it I have read say 120 but 117 gets you within 50 ft.

Only at certain altitudes and temperature combinations. The error generated using any rule of thumb changes with both pressure altitude and temperature. So your 117 will result in only a 59' error at 6000 msl and 20°C. Raise the temperature to 25°C and you have a 98' error. Run some calculation comparisons and you'll see.

120 is probably used because (1) it consistently results in a higher number that the real calculation and (2) multiplying by 20 an easier mental calculation than multiplying by 17 (for a lot of folks, anyway).

[scottyboy75]

Makes sense most of the da I have worked with have only been within 500' agl. I don't like to fly any higher than that.
Shane

[ElyJs]

I use temp Fahrenheit above standard / 15 *1000 for a quick estimate in my head. So at my home airport of 5 grand if its 85 out we are about 50 F above standard / 15 equals about 3 thousand+ feet added to my PA so somewhere around 8500 DA

-Jason

[caliginousface]

The 120 or 117 correction factor is still much, much better than using an E6B. I teach both, but the 120 factor better accuracy.