Windchill effect on Holdover times?
 

Back in my Air Force days some one once said don't forget to apply the Wind Chill Equivilant Temperature to de-icing holdover tables. This makes total sense but I've searched on the net for a reference but to no avail. Does anyone have a reference or does your company SOP's take wind chill into account?

Rgds WS

I've never heard of wind chill factors being used. Wind chill is important to "living" things, not structures. In any event, holdover times are pretty conservative.

Huh??

I always thought that wind chill factor was another way of describing the effects of convective heat loss from a heated surface as in a living body temperature vs outside air.

For a surface near the outside air temperature, I doubt that film effects make much difference.

A human being is warm. It therefore warms the air immediately around it. If there is a slight breeze or a strong wind, the warming effect on the air around it will be greater, because the warmed air is rapidly removed and replaced with air at the ambient temperature. There will therefore be the sensation of being colder than with no air movement.

A lump of metal, not possessing a Central Nervous System and not relying on oxydisation of internal fuel to maintain its body temperature, will feel no colder with a wind. It therefore does not notice any "wind chill effect".

Furthermore, the rate at which ice either forms or melts or sublimes from a surface is dependent solely upon the temperature of the solid body and the ambient air. After standing overnight, the metal will be at the same temperature as the ambient air, more or less. After a long descent from colder air it may even be significantly colder. Wind over the parked aircraft will not, therefore, assist.

In short, whoever told you to apply a Wind Chill Effect is either taking the mick, or is a total idiot.

My understanding too is that Windchill Equivalent Temperature is merely an index of human comfort. Have a look at this publication.
Windchill
Note especially the reference to inanimate objects in the first paragraph. However if an object is wet the increased evaporation brought about by airflow could cause the temperature do drop below ambient. The same effect causes a wet bulb thermometer to record a temperature below ambient depending on relative humidity. Click here to see what the FAA have to say about wind effect. Scroll down to Appendix 3 paragraph 7(j).
FAA AC20-117
I think there is a misprint here in that ‘practical’ should read ‘impractical’

Regards

Xacto

If you read UK AIC 32/1998 (Pink 169) dated 24 March, you will see than on Pages 3 and 4, after each of Tables 1, 2, and 3, it says "......high wind velocity.........may reduce holdover time below the lowest time stated in the range". Obviously, the CAA thinks that an increase in wind velocity increases evaporation rate. However. they don't actually quantify the effect in numerical or percentage terms.

That is another story all together. The de- and anti-icing fluid giving you a holdover time, are designed to sheed of during takeoff roll (at least that was the intention), and that is why high wind velocity and even jet-blast can reduce your holdover time. Wind-chill has nothing to do with holdover time.