Density Altitude

[avanti blade]

I was taught that density altitude is the altitude the aircraft thinks it is at and performs in accordance with. My understanding is that density altitude is affected by pressure, temperature and relative humidity.
In my experience, in most cases, aircraft takeoff and landing charts take into account actual pressure and temperature but not relative humidity.
In this day and age that information is readily avaliable and I would think would be easy to input if required, does anyone know why relative humidity is ignored? Is the effect on aircraft performance (both engine and airframe) so small thats its not worth taking into account or is reference to it just missing from the relevant legislation so ignored as matter of convenience?

Thanks in advance
Avanti

[ZeeDoktor]

The effects of humidity on air density are very small, less than 0.2% between -10C and 50C, and not much greater outside that.

The effects of humidity however on friction coefficients however is very large, hence different landing charts for dry plus the various forms of wetness on the ground...

Cheers

Doc

[Old Smokey]

The various legislations from the regulatory states DO consider one standard Relative Humidity when applying the certification rules to aircraft performance.

RH, of course, varies enormously, but be assured that it is considered (albeit a standard one) within the various legislations.

I don't happen to have my various reference materials with me at the coffee shop from which I am replying, I'm sure that by the time I return home someone will have provided the figure.

Regards,

Old Smokey

[Old Smokey]

Here it is from FAR 25.101

25.101 General.

(a) Unless otherwise prescribed, airplanes must meet the applicable performance requirements of this subpart for ambient atmospheric conditions and still air.
(b) The performance, as affected by engine power or thrust, must be based on the following relative humidifies;

(1) For turbine engine powered airplanes, a relative humidity of -

(1) 80 percent, at and below standard temperatures; and

(11) 34 percent, at and above standard temperatures plus 50' F.

Between these two temperatures, the relative humidity must vary linearly.

(2) For reciprocating engine powered airplanes, a relative humidity of 80 percent in a standard atmosphere. Engine power coitections for vapor pressure must be made in accordance with the following table:

Specific humidity w
Altitude H (ft) Vapor pressure e (in (Ib moisture per lb Density ratio p/cy
Hg) dry air) 0.0023769

0 0.403 0.00849 0.99508
1,000 0.354 0.00773 0.96672
2,000 0.311 0.00703 0.93895
3,000 0.272 0.00638 0.91178
4,000 0.238 0.00578 0.88514
5,000 0.207 0.00523 0.85910
6,000 0.1805 0.00472 0.83361
7,000 0.1566 0.00425 0.80870
8,000 0.1356 0.00382 0.78434
9,000 0.1172 0.00343 0.76053
10,000 0.1010 0.00307 0.73722
15,000 0.0463 0.001710 0.62868
20,000 0.01978 0.000896 0.53263
25,000 0.00778 0.000436 0.44806

(c) The performance must correspond to the propulsive thrust available under the particular ambient atmospheric conditions, the particular flight condition, and the relative humidity specified in paragraph (b) of this section. The available propulsive thrust must correspond to engine power or thrust, not exceeding the approved power or thrust less -

It's a text 'cut and paste', not a scan, so my apologies if the formatting looks a bit strange.

Hope that this helps.:ok:

Regards,

Old Smokey

[avanti blade]

Thank-you both for your prompt and informative replies.

Regards,

Avanti Blade

[FlightPathOBN]

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