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Density Altitude and Humidity

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Michael Bennett
Young Eagles Pilot or VolunteerHomebuilder or Craftsman
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#1 Posted: 4/29/2011 09:12:03

 I am currently working on my private license.  During my studies, humidity was identified as one of the factors that affect density altitude. All the conversion charts take into account field elevation, barometric pressure and temperature.  However humidity is not.  I guess my question is how much does humidity really affect takeoff and landing distances?  More practically what would be the difference in performance if I operated in Georgia at 90F at 90 percent humidity compared to operating in Arizona at 90F at 30 percent humidity?  Of course both at the same field elevation and barometric pressure.  If it does not have a real effect why is it addressed?

Thank you for your help in advance.

Mike



Bob Meder
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#2 Posted: 4/29/2011 12:34:08

Well, the water in the air displaces the air molecules making the air less dense.  I've never really looked at a rule of thumb, because I've routinely added a generous margin to take off and landing distances and teach my students to do the same; not just for this, but to take into account the fact that the numbers were derived by a test pilot in perfect conditions with a perfect plane, none of which applies in normal operations.

Anyway, I found this link to the Flying magazine site that gives some numbers to use for 100% humidity.  As the author states, go with the maximums (e.g. add 32% to the distance at sea level) if you're concerned about it - which goes back to my point about generous margins....

 

http://www.flyingmag.com/pilot-technique/tip-week/correcting-density-altitude-humidity



Bob Meder "Anxiety is nature's way of telling you that you already goofed up."
Michael Bennett
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#3 Posted: 4/29/2011 12:59:19

Thanks Bob!

I thought it was strange that it was not factored in on the conversion charts.  32% is a very good thing to know on short feild landing!

Thanks again!

Mike



Bill Greenwood
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#4 Posted: 4/29/2011 17:41:35

Mike, good question.

I am not aware of any chart that actually calculates for humidity. There may be some ,perhaps in jet flying where loads vary a lot and the result is vital to know.

I have never heard of 32% as a safety factor to add to your calculation for wet air.

Dry air is denser than wet air, all other factors being the same, But in the real world I guess that humidity is not a big factor on density altitude. normal calculations are for altitude corrected for temperature.. Both make a BIG difference in performance. A 172 will barely climb out of ground effect on a summer day at Leadville, Co. at almost 10,000 feet. 

In the real world, you can go early in the day when temps are cooler or keep the plane as light as possible when it is tight. Don't try to rotate and climb before reaching enough indicated airspeed in any case.



Ron Dillard
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#5 Posted: 4/29/2011 19:02:11

Mike,

 

Here is a calculator that takes into account Dew Point which is a measure of air humidity.

 

http://wahiduddin.net/calc/calc_da.htm

 

I recently took the Helicopter CFI add on written and one of the questions was calculating the DA and the chart used had dew point as one of the variables.

 

Ron

advancedtailwheeltraining.com



Jerry Rosie
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#6 Posted: 4/30/2011 11:27:05

Great link, Ron.  Thanks.



Cheers, Jerry NC22375 out of 07N
Michael Bennett
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#7 Posted: 5/2/2011 15:55:00

Thank you everyone for your help!  I played with the outstanding link Ron provided.  It seems that humidity only affected dencity altitude by houndreds of feet as opposed to temp which changed things by thousands.  This is well within any safty margin used and beyond my accuracy I have in my takeoffs and landings.  goggles 

Thanks again!!!

Mike



Graeme Smith
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#8 Posted: 5/6/2011 06:02:46 Modified: 5/6/2011 06:03:33

Ha!  I asked exactly the same thing in ground school and ended up putting quite a lot of background work into finding formulae that would help me figure this.  I finally got a working formula that I have built into a spreadsheet.  Entering Field elevation, amnd the ATIS information including Temp and Dewpoint does the math.  The sheet also does my performance, weight and balance, crosswind and reject take off speed/distance.  Be happy to share but you need Excel 2007 as a minimum to handle some of the formulae which are not in 2003.

 

Or you could use this from this week's AOPA newsletter on the subject:

 

“Humidity should not be ignored but almost always is because aircraft performance charts do not include it as a factor, but should,” wrote AOPA Pilot columnist Barry Schiff in his July 2007 Proficient Pilot column. “Although even the most humid air is not that much lighter than dry air, it causes reciprocating engines to lose considerable power.”

He offered an alternative way to estimate humidity’s effect: “Because the precise effect of humidity involves complex calculations and is difficult to determine, pilots can compensate by raising the calculated DA by 1,000 feet on hot, humid days and decreasing calculated performance by a conservative fudge factor of 10 percent.”