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7 Answers

Frost vs. ice

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General Aviation, Student Pilot

What is the technical difference between aircraft icing and frost?

-In order for aircraft icing to occur, there needs to be visible moisture present.  But what about temperatures?  Do temperatures HAVE to be at or below freezing for icing to occur?  What OTHER conditions are necessary for ice to occur? 

-As for frost...The collecting surface must be at or below the dewpoint of the air..and the dewpoint has to be below freezing. 

 

 

 

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7 Answers



  1. Paul Tocknell on Dec 14, 2010

    Frost is a form of ice. They are both considered “frozen contaminants”. We tend to use the term “frost” to apply to static structures on the ground and “ice” to apply to conditions encountered in flight (or actively occurring on the ground such as freezing rain)

    Your right, frost will occur on aircraft surfaces if the aircraft’s surface temperature is below the ambient dew point and the surface temperature is below freezing.  Frost can occur even when the surrounding air temp is above freezing.  For instance, an airplane parked overnight will radiate heat and become cooler than the ambient air.  If it cools to it’s dewpoint (and it’s dewpoint is below freezing) then frost will occur. 

    As far as your other question.  According to AC 91-74 “nearly all icing occurs in supercooled clouds. These are clouds in which liquid droplets are present in temperatures below 0° C.   At temperatures near 0°, the cloud may consist entirely of such droplets with few or no ice particles present.”

    Besides the temperature, the other things that need to occur is the right type and size of water droplet.  Large droplets tend to be more conductive to in flight icing conditions. 

    There is a lot more that goes into it and AC 91-74 is a great resource to read for more information

    All I know is that “Rime Ice, Ain’t Nice”

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  2. Wesley Beard on Dec 14, 2010

    Do temperatures have to be below zero for icing to form?  NO.  The Aeronautical Informatoin Manual states in 7-2-21 that icing is most prevalent in temperature ranges from +2deg C to -10deg C.  In most corporate jets I fly, the manufacturer considers icing conditions exist when inside visible moisture and the temperature is between +10deg C and -40deg C.
     
    The reason for this is temperature drop of the air as it moves over the top of the wing.  Remember Bernoulli pointed out lower pressure and Gay-Lussac’s Law makes pressure and temperature  proportional.  Thus the air on top of the wings is colder than static air.
     
    Other times icing can occur is with a cold soaked wing or more precisely cold soaked fuel inside the wing.  It is possible with high humidity for that moisture to freeze to the underside of the wing when the fuel is below freezing.  This is probably improbable for small airplanes but I thought I would mention it.
     
    You are correct on frost.  I hope this helps.

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  3. Micah on Dec 15, 2010

    I will agree and disagree with Wesley.  Yes, the temperature must be freezing or below for ice to form.  But remember, this is the temperature of the surface on which the ice forms–not the temperature of anything else.  Like Wesley said, you can be in +5 air but be in freezing conditions because your wing surface is cooled to 0 or less.  

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  4. Brian on Dec 15, 2010

    …Qt:Micah…Like Wesley said, you can be in +5 air but be in freezing conditions because your wing surface is cooled to 0 or less….
     
    Not without violating the second law of thermo dynamics you cannot. 🙂 I learned this the hard way when I argued to the contrary a little over a year ago.
     
    Wiki: Second Law of Thermodynamics 
     
    In other words, if you stuck a wing in a wind tunnel with a temperature of +5 degrees and turned the fan on, the airfoil would never fall below +5 degrees. The temperatures would equalize, that is what this law states.
     
    What can happen, however, is super cooled water droplets. These will freeze when impacting a surface, even if that surface is slightly above freezing. There may be other ways to get ice above freezing, but I can’t think of any at this time.
     

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  5. Wesley Beard on Dec 15, 2010

    Quote:  In other words, if you stuck a wing in a wind tunnel with a temperature of +5 degrees and turned the fan on, the airfoil would never fall below +5 degrees. The temperatures would equalize, that is what this law states.
     
    This is pretty much what I am saying but have gone more in depth.  The static air is +5deg C but as that air is accelerated over the top of the wing, the pressure has to drop and at the same time, the temperature of the air also drops a certain amount.  The more camber on the wing, the more the pressure and temperature drops as air is accelerated over the top of the wing.  Thus we have a localized phenomena on the wing.  The top of the wing is cooler than the bottom of the wing (all other variables remain the same) and it is quite possible ice can form on the top of the wings and leading edge with static air temperature higher than freezing.
     
    There are a lot more variables than just visible moisture and temperature in determining whether or not ice will form.  The size of the droplets also matter.  Droplets smaller than 40 mu meters don’t have a chance on sticking they are simply too small.  Conversely, droplets that are too big and are not supercooled probably will not form ice either.

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  6. Brian on Dec 18, 2010

    Wesley,
     
    Thank you for clearing that up for me. I’ve been doing some reading over the past few days to try and understand this better. I think I have it now, seems I just missed pressures effect on molecular movement. Realizing, again, that temperature is just a measure of molecular movement I think is what did it for me.
     
    Thanks again,
     
    ~Brian

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  7. Micah on Mar 23, 2011

    Reading an Air Canada pilot’s blog and he recently posted this. Reminded me of our conversation. Probably a little more complex than most of us will face in bugsmashers, but certainly worth considering.
     

    Icing at plus 16 C?

    This morning saw me back in the saddle after two weeks. It was a “little thing” day. Starting by spilling my treasured coffee on my freshly laundered uniform. Of course there were no napkins in the car. 

    The first two legs were warm up “Rapidairs” to Montreal and back launching a four day mission.

    Nothing out of the usual…until we receive a call from the “back end” stating some pilot near the wing claims there is ice on the wings while passing through 16,000 feet. WTF? The TAT (Total Air Temperature) displayed +16 C and the ambient temperature confirmed  plus 4 C. I crisply retorted, “is this guy an A.C pilot?” Not that it matters who reports the ice, but heck it was Monday morning.

    But it got my meteorological side of the brain grinding its gears. There are about 30 parameters which may cause airframe icing. One of them…skin temperature must be zero Celsius or less. With a TAT (skin temperature) of plus 16 then this wasn’t the case. 

    When I checked the log book (captain’s duty for every aircraft) I assumed it had not flown that day. However, when I checked the brake temperatures they told me someone heated the brakes. Hmmm?

    I rechecked the log book. It indeed flew a “red eye” from Vancouver meaning the fuel sat in temperatures at -57 C for at least four hours. Plus it arrived with lots of fuel so the fueler did not add warm fuel for the short trip to Montreal.

    Do you see where I’m going? Then Captain “D” checks the fuel temperatures. Minus 5 !!!!
    Bingo! With us flying in moist conditions, the cold soaked wing created it’s own ice and the low pressure over the wing (think additional cooling) exemplified things. 

    I radioed to the “de-ice co-ordinator” to give him a heads up. But all the aircraft were checked.

    The pilot from “another airline” met with me after the flight and he sent me some pics later on in the day.

    This ice formed well aft of the leading edge and posed little threat because of the area involved and it’s location. The ice melted nicely in Montreal. 🙂
    (http://www.fromtheflightdeckbook.com/2011/03/back-in-saddle.html)

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