Bre 901,
I agree that the effect is not enormous, but I don't think its negligeble either. (and neither does the FAA, which dedicated an entire issue of the Aviation Safety magazine a couple of months ago to illustrate the effect of moisture on density altitude).
I did some number crunching and came up with values very similar to your's: about a 1.9 - 2.4% reduction in density.
This might not seem much, but to but it into perspective, on a hot day (35C) it is equivalent to the density reduction created by an additional 8degrees C. In other words, on a very humid day, when the reported temperature is 35C, your aircraft is performing as if the temperature was 43C.
On the aircraft I fly (normally aspirated high performance light twin) this translates into a 13% increase in Takeoff distance, as well as a 25% loss in single engine climb performance (well, not sure we can talk of single engine climb performance on light twins
) or in other words the same performance loss due to a 25% increase in payload.
Now, I do agree that the above example is more of an extreme situation (35C with 85%RH), and that more realistically, we are talking about a "density penalty" equivalent to a 5-6C temperature increase. Furthermore I understand that in many areas of the world this spread is further reduced since the temperatures never get this high. But in tropical areas (where I fly), as well as other climates during the summer months, humidity DOES affect take-off performanve in a non-trivial way.
This can be noticed by seeing an increased deviation between the performance numbers in the POH/AFM and the actual performance you get in the aircraft (as opposed to a lesser difference under cooler and dryer conditions).
Although I've never flown jets, I would imagine that under certain conditions, large amounts of water vapour could affect performance. (I am thinking about some central/south american airports at high elevations, high temperatures, just a couple of hours following a brief-but-intense afternoon shower, high TEMP/RH combination, not so long rwy, don't you think that the "density penalty" due to moisture of about 5C would cause a non-trivial hit on the rwy limited TO weight? I would think so, and probably that's why the airlines don't want to know about it
)
I took a dispatching course but was never taught to incorporate humidity in the performance calculation. Does anyone know if any airlines do consider humidity?
I know that the ops specs list the rwy limited weight in a tabulated format that has temperature in one degree celsius increments. That means that the engineers are concerned with up to one degree accuracy in the field temperature but do not even consider humidity
palgia
PS. Correct me if i'm wrong, but the standard atmosphere engineers use is 21% oxy, 78% nitrogen and a few other gasses. No moisture? Are they calculating nubers for DRY AIR?