Sublimation

[bluesky74656]

I was having a conversation with my instructor regarding icing, and specifically what our outs would be on a particular IFR lesson. Cloud bases were from 3000-4000, with tops around 8000. Temps on the surface were below freezing.

He said that if we started picking up ice, we would descend below the bases to get out of it, which I agreed with. I then said that although that would stop the accumulation of ice, we had no way of getting rid of it since temperatures were below freezing at the surface. He said that wasn't correct because the ice would sublimate. I thought you needed sunlight for sublimation to happen, but he said there was enough air moving over the wing to provide energy for sublimation. Is that true?

Also, the flight was at night. Would that make a difference with regard to sublimation?

 

[dell30rb]

He's correct, although its not going to come flying off. It will slowly erode away.

At least that is my understanding, have not experienced it for myself.

 

[bkspero]

Sublimation is the direct conversion of solid water (ice) to water vapor without going through the liquid water state. Sublimation will happen whenever ice is in contact with air that is not saturated with water vapor. The rate of sublimation will depend on three things, 1) how much below saturation the air is, 2) how much energy is available to drive off the water vapor from the ice (i.e., how high is the temperature and is there sunlight shining on the ice), and 3) how much air is sweeping by the ice (is the airplane flying or standing still on the ground).

In your case flying at night, getting out of the clouds into unsaturated air either above or below the cloud will begin sublimation. And since it's night time, there is no heating by sunlight (whereas during the day it would somewhat favor flying in the sun above the clouds...although the potential for collecting more ice in the climb counterbalances that factor). So the rate of sublimation either above or below the cloud will depend upon the temperature and dewpoint spreads in the two different locations. The higher the temperature and lower the dewpoint, the faster the loss of ice to sublimation. Of those two factors, my guess is that higher temperature will trump lower dewpoint...as long as there is a reasonable dewpoint temperature spread.

 

[pericynthion]

Sublimation (solid -> gas) behaves somewhat similarly to evaporation (liquid -> gas). I'm sure you can imagine that a damp towel hung out on a windy, but cold and overcast day will eventually dry. Same thing with ice on a wing. I imagine though it would be a very slow process at night in below-freezing temps.