What is "clear air ice"?

jnmeade

jnmeade

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Jim Meade
Had a Lockheed briefer today give me reports about "clear air ice". It is a term I'm not familiar with. After a short discussion, he assured me he knew the difference between "clear ice" and "clear air ice". I thanked him for his help.
I did a quick google search and didn't get any useful returns to "clear air ice". Can someone point me in the right direction?
I guess another way to put it would be how does one pick up ice in the absence of visible moisture? (Let's assume one is not flying underneath a rain cloud)
 
Doing a quick search, the closest I could find was the following statement,

"There are basically two types of in flight icing. Rime ice is formed when super cooled water freezes on contact with the airplanes wing. Rime ice looks like granular form white type with entrapped air. The other type is clear air ice. Clear air is a glossy form of ice a little harder to notice visually. Its a slow freezing of large super cooled water droplets. It forms a sheet of ice."

I would be interested in a little more detailed description as well.
 
I'm no weather expert here, but could this be icing that could form in the absence of visible moisture, ie. in clear air and not in a cloud...is there such a thing?

I would presume that if the Temp/Dew-point spread is small, a plane coming out of the flight levels could itself be both below the dew-point and freezing and could accumulate ice in the absence of a cloud, couldn't it? In level flight, absent a significant change in temperature, frictional heating of the plane should keep it at or above ambient temp, making that unlikely.
 
I realize that "fog" is somewhat contrary to "clear air" but I think I've experienced "freezing fog" when the visibility was legally VMC. Would that be an example of "clear air ice"?
 
scottd said:
Cold soaking is a known phenomenon, but not one that is serious. I know of no icing accidents or incidents for airborne aircraft caused by cold soaking. Yes, the thermal momentum of a cold aircraft could cause it to accrete ice right at the moment of impact with moisture that is at above freezing temperatures, but not likely different enough in temperature to cause deposition of ice crystals on the airframe in clear air.
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I have situations on the ground where I landed with a cold-soaked wing and cold-soaked fuel in the wing and have had rain turn to a sheet of ice and frozen droplets. Enough to need deicing. I can see how frost might not happen in flight because of the airflow but do you think rain falling on a cold-soaked wing in flight might cause it to glaze over? I guess if there have been no accidents because of this it must not be a problem.
 
Since the formation of structural icing requires visible moisture (even haze is visible moisture of extremely low density), I don't see how one can have structural "clear air icing." I've certainly never heard the term before in my 40 years of flying. I'd like to hear that briefer's explanation of the term and what it means.
 
gismo said:
I realize that "fog" is somewhat contrary to "clear air" but I think I've experienced "freezing fog" when the visibility was legally VMC. Would that be an example of "clear air ice"?
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As have I. I've picked up enough ice in VMC conditions to where it had a noticeable effect. DA-20-C1. It was a night, I wasn't in the clouds, and I had way more then VMC visibility.

I first noticed it when the airspeed indicator's airspeed did not make sense with the RPM indication. I then held a light up to the canopy and saw it iced over. There was also ice on the wings. The airspeed indicator then quit indicating much of anything.

I diverted and landed without the airspeed indicator, mostly by just carrying way more power down final then I'd generally need.

It never dawned on me that it did not have pitot heat, until, I went to turn it on.
 
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scottd said:
What kind of aircraft?
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Lear 35. Just to make it clear, the ice formed on the wing after we landed when the wing was cold soaked and the rain was falling on it. It made droplet-shaped bumps as well as covering the wing which is why I knew it happened on the ground. The air temperature on the ground was above freezing but not by much. I've also seen frost form on the wing on the ground in similar circumstances, actually this is much more common, but can't imagine that would happen in the air.
 
scottd said:
Something else must have been going on. Not likely you developed structural icing in "clear air" - certainly not enough to cause your airspeed indicator to ice up. Most likely was freezing rain or freezing drizzle aloft. And it is possible to have good visibility in these conditions.
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Possible. Whatever it was, I couldn't really see it happening until it happened.
 
Personally, I wouldn't consider "freezing fog" to be "clear air," especially since they don't use the FG term until the vis is below, IIRC, 1 mile; above that, it's "mist" or "haze." And since "clear air ice" isn't in the FAA's lexicon, if this person is an FAA-authorized briefer, s/he should stick to the FAA-approved terms.
 
Everskyward said:
Lear 35. Just to make it clear, the ice formed on the wing after we landed when the wing was cold soaked and the rain was falling on it. It made droplet-shaped bumps as well as covering the wing which is why I knew it happened on the ground. The air temperature on the ground was above freezing but not by much. I've also seen frost form on the wing on the ground in similar circumstances, actually this is much more common, but can't imagine that would happen in the air.
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The issue of cold-soaked fuel chilling the surface of a wing so much that structural ice/frost forms in temperatures well above freezing is a known issue -- one that even led to an accident involving a DC-9. If you're coming out of the flight levels, or have just refueled with fuel from underground tanks, this is a concern when entering clouds or other visible moisture, or even on the ground when the fuel is cooler than the dew point. But I've never heard of this phenomenon being referred to as "clear air icing," since it only happens in flight in visible moisture.
 
Ron Levy said:
The issue of cold-soaked fuel chilling the surface of a wing so much that structural ice/frost forms in temperatures well above freezing is a known issue -- one that even led to an accident involving a DC-9. If you're coming out of the flight levels, or have just refueled with fuel from underground tanks, this is a concern when entering clouds or other visible moisture, or even on the ground when the fuel is cooler than the dew point. But I've never heard of this phenomenon being referred to as "clear air icing," since it only happens in flight in visible moisture.
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I was just giving an example of cold soaking for people who might not have encountered it. I wasn't implying that it was "clear air icing", whatever that is.
 
scottd said:
That's the first I've ever heard of any accident. Do you know the specifics? NTSB record? I've asked three leading icing experts in the country about this (including one who works closely with NASA) and they are unaware of a cold-soaked aircraft causing any significant problem.
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IIRC, it was a freight outfit's DC-9 out of Rochester or Buffalo NY. I'm not finding the specific incident on line, but I have found an AD related to it:
http://www.airweb.faa.gov/Regulator...b85075f8bead86256ced00732339/$FILE/022106.pdf. And in this article, there is reference to an accident at Cleveland.

BTW, here's a briefing I found on the issue.
http://www.smartcockpit.com/data/pdfs/plane/boeing/B737/instructor/B737-Cold_Soak_Fuel_Frost.pdf
 
Ron Levy said:
Personally, I wouldn't consider "freezing fog" to be "clear air," especially since they don't use the FG term until the vis is below, IIRC, 1 mile; above that, it's "mist" or "haze." And since "clear air ice" isn't in the FAA's lexicon, if this person is an FAA-authorized briefer, s/he should stick to the FAA-approved terms.
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I guess I was stretching the FAA definition of "freezing fog" quite a bit but what I meant was that I've seen occasions when it's slightly misty/hazy and the temp was such that some of the water vapor was condensing on cold surfaces as frost.
 
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