Density Altitude and Humidity

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

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

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.

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

Great link, Ron.  Thanks.

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.   

Thanks again!!!

Mike

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.”