sinkrate3278

I was taught that after you pick up icing on your wing, it can be removed through the process of sublimation by simply flying out of visible moisture.

Does sublimation work at any OAT? Can it ever be too cold out for sublimation to work?

How quickly can ice be removed through sublimation?

Does it work at night?

As always, thank you for your time!!

TheFly

Is the aircraft in question certified to fly in known icing conditions?

Also, I can tell you from experience that yes, ice will remove (sometimes the boots won't get rid of all the ice) when flying in warmer air, even if it's a few degrees below freezing.

rickair7777

The preferred way to eliminate ice is with your de-icing equipment. Not certified for FIKI? Then don't fly in ice. But if you inadvertently pick up ice...

Two things are getting confused here.

Melting: This is when ice is heated above freezing and turns to water (on an airplane in flight it then blows away or evaporates). Melting happens when you fly into warmer air such that the airflow over the ice is above freezing. This will melt the ice, and quickly. Note that on fast airplanes the airflow will be warmer than OAT so this can occur below zero OAT (ie total temp > zero).

Sublimation: This is a natural (but slow) process where individual frozen water molecules are knocked off of the ice by energetic air molecules or sunlight. It never turns to liquid water but goes straight from ice to a free water molecules (water vapor). Before they had clothes dryers, people would hang their clothes out to dry...this worked even in the dead of winter because sublimation works even far below zero.

If you have ice and are in a hurry to get rid of it, you want to get to air which is warm enough to just melt the ice (total temp > zero). You'd probably run out of gas waiting for sublimation to work on a significant ice layer, although I imagine airflow increases the sublimation rate somewhat even well below zero.

UAL T38 Phlyer

Sublimation is a two-way street: it is the process of turning a solid directly to a gas, or a gas directly to a solid.

Rick, mostly true, but the physical/chemical explanation is that all molecules/atoms have a bell-curve distribution of temerature or energy [temperature is the kinetic energy of the vibrating molecule or atom]. That is, when it is 32 F, some of the molecules are "high-energy" 32 F, and some are "low energy" 32F. Or, some of the ice is at 31 F, and some is 33 F--but the average---which we measure---is 32.

For Mike's question (ice directly to a gas), the higher-energy molcules get a few extra calories of heat (from wind-friction, sunlight, or random-chance) and evaporate.

Mike: I flew the A-320 one late afternoon BOS to DIA. BOS had severe ice from 4000-6000 ft. The captain and I delayed departure for over an hour, making sure we were legal to actually takeoff. Turns out, we were.

Airborne, cleaned-up, all anti-ice on. Quick zoom through the ice layer; clear on top...in fact, clear the rest of the way to Denver.

We got some ice on the wings and nacelles, which was gone within about 10 minutes with the heat on.

There is a temperature probe on the 320 that sticks out of the center windshield post. It is about 4 inches long, 3/8 of an inch in diameter.

It had an ice-ball on it about 3 inches in diameter.

It sublimated slightly over the next 3 hours, but not much....maybe down to two and a half inches.

We didn't lose it until we put the gear down (in air well above freezing).

Of the sources of energy that will cause sublimation, temperature is the strongest, sunlight second, and I would put wind-ablation at last (unless you are flying in the transonic region where TAT actually starts to rise from friction!)

JamesNoBrakes

Was always taught that the reverse process was "deposition", a gas directly to a solid.
http://en.wikipedia.org/wiki/Deposition_(physics)
Anyways, I think stagnation points and L vs H pressure has to be discussed. Go look at the pressure distribution on an airfoil in ANA and you'll see why the big horns and shapes form on the leading edge. This is also why ice doesn't form in the middle of the top of the wing, except if it's wet and "runs back", then it's simply a volume issue (too much moisture). I've watched frost on top of the wing "dissapear" on rotation due to sublimation (being exposed to low pressure), and of course I've watched ice stay on the leading edge and other places long after the encounter. Ice in low pressure area=will sublimate eventually if out of conditions and drag and other performance factors are not too affected. Ice in high pressure area=won't readily go away, even out of icing. Low pressure is one of the reasons sublimation occurs, or think of the reverse, the only way to keep a block of dry-ice from sublimating (evaporating basically) is to keep it under extremely high pressure. Expose it to atmospheric and it basically evaporates away.

UAL T38 Phlyer

James:

Good call; I was taught sublimation was either way, but deposition makes perfect sense.

tomgoodman

I remember watching the locals play a form of "Trivial Pursuit" on Ascension Island. Their "Brigadier" drew the question: What, in the chemical sense, is the meaning of the word "sublime"?
He replied: "It is something which...falls short of being lime."

trent890

Your memory is correct UAL Phlyer. The "current" 1975 revision to the DOT/FAA publication "Aviation Weather" does in fact state: "Sublimation is the changing of ice directly to water vapor, or water vapor to ice, bypassing the liquid state in each process."

I first learned about the deposition term for the changing of water vapor to ice in my collegiate aviation weather courses in the mid-1990's. Couldn't tell you when the first use of deposition came to be used as the reverse of sublimation in the aviation weather community.