Clarification on Frost

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Stets656

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Ben
Looking for some help to get a more advanced understanding of frost and how it forms on airplanes. Is it possible for an airplane to develop frost in flight? How would that differ from just regular icing?
I always see other pilots rushing to get airborne before frost can accumulate, why would being airborne help to negate any accumulation?
Thanks.
 
Frost is water vapor that sublimates directly from vapor to solid without going through the liquid phase. It results when the surface falls below the air's frost point. That often happens on a clear night (no overcast) as heat radiates off the surface into space and the surface falls below the ambient temperature of the air.

There are people that don't believe the physics here. They don't think that the top of a wing, for instance, can get colder than the air around it. That's kind of strange, considering that the top surface can get a lot hotter than the air when the sun radiates on it, the reverse of outgoing radiant heat off the wing.

Frost won't form if the airplane has a roof over it, or if the sky is overcast, or if it's windy, or if the airplane is in flight. The air moving over the wing keeps the wing at ambient temperature.

An article on it: https://www.linkedin.com/pulse/frost-result-radiational-cooling-arjen-piest
 
Sublimation. The H2O goes directly from solid to gas.
Its important to get rid of the frost on an airplane's flying surfaces. Its like coating the wings in sandpaper. Most important, the FAA says to.
When I flew helicopters that had been tied down out doors, I removed the frost from the windshield and side windows first. Never use hot water for this. Didn't worry about the fuselage because its not a lifting surface. The rotor blades are. Just check for chunks of ice that would cause an unballence and start the engine. Frost is gone in seconds. Frost on the fuselage is gone seconds after take off.
I once saw a ski equipped J3 Cub start up and taxi into position with at least 12 inches of snow piled on the wings. Must have increased take off weight couple of hundred pounds. He just started down the runway and all the snow disappeared ten feet upward in a cloud. I could see that its wings were clean when it lifted off. I do not recommend this.
 
Maxnr said:
I once saw a ski equipped J3 Cub start up and taxi into position with at least 12 inches of snow piled on the wings. Must have increased take off weight couple of hundred pounds. He just started down the runway and all the snow disappeared ten feet upward in a cloud. I could see that its wings were clean when it lifted off. I do not recommend this.
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Won’t always happen that way...all I’m sayin’.
 
Maxnr said:
I once saw a ski equipped J3 Cub start up and taxi into position with at least 12 inches of snow piled on the wings. Must have increased take off weight couple of hundred pounds. He just started down the runway and all the snow disappeared ten feet upward in a cloud. I could see that its wings were clean when it lifted off. I do not recommend this.
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The Russians seem to do this on commercial airliners.. you'll see a little square area that they plowed off, presumable to look for ice, and if non found they just punch it..:

 
It destroys lift. It's that simple. Can it occur in flight, I don't think so. That's icing. Frost on a wing can reduce the airflow across the wing, and thus disturb the lift characteristics s of the wing. Bottom line, remove all frost before takeoff. That includes the bottom of the wing as well. Fuel put into a wing that is cold soaked can induce frost when you least expect it. So look after fueling as well. MD-80's were especially prone to that icing. Caused an SAS crash many years ago.
 
Frost occurs when the collecting surface is below freezing and is below the dew point. Water falls out of the air (below the dew point), and the below freezing collecting surface turns the dew into frost.
 
For deposition of frost (that's the term for the opposite of sublimation) the object surface needs to be colder than the dew point. This can happen if the object is radiating heat energy away, cooling the object below the dew point. Metal objects have low heat capacities, so they cool in temperature quite rapidly with radiative heat loss.

In flight, the aircraft is nearer thermal equilibrium with the surrounding air, so unless the atmosphere is below the dew point (i.e., visible moisture), deposition isn't possible.
 
To the comment frost can't form under a roof? It's common to find frost on tools and toys inside my unheated sheds. It's common to find frost inside fuel tanks. Find a steel door frame on a cold day and you may find frost on the inside. So it goes with airplanes. A colder plane exposed to warmer air will frost up before your eyes.
 
For deposition seems I remember frost point (instead of dew point)?
 
Stewartb said:
To the comment frost can't form under a roof? It's common to find frost on tools and toys inside my unheated sheds. It's common to find frost inside fuel tanks. Find a steel door frame on a cold day and you may find frost on the inside. So it goes with airplanes. A colder plane exposed to warmer air will frost up before your eyes.
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I didn’t post that but can attest to it - I park outside right next to a row of shade hangars. My plane will be covered in frost, the ones parked under the shade hangars will not. It’s very strange, especially considering that the shade hangars are only a roof - no walls, no door, etc.
 
AA5Bman said:
I didn’t post that but can attest to it - I park outside right next to a row of shade hangars. My plane will be covered in frost, the ones parked under the shade hangars will not. It’s very strange, especially considering that the shade hangars are only a roof - no walls, no door, etc.
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Heat rises. A roof keeps the heat from rising.
 
Salty said:
Heat rises. A roof keeps the heat from rising.
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Yeah I get that of course, you just wouldn’t think it would make that much difference, especially with open sides, a tiny touch of breeze or air movement... but it definitely does.
 
AA5Bman said:
Yeah I get that of course, you just wouldn’t think it would make that much difference, especially with open sides, a tiny touch of breeze or air movement... but it definitely does.
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As mentioned earlier in the thread, even a small breeze is usually enough to prevent frost, roof or no roof.
 
The FAA used to say if you can't remove the frost then polish it. Then the FAA says no frost, period. The logic escapes me. If you polish frost to a surface as smooth as the underlying wing surface then why not fly? Perhaps the FAA feels a pilot cannot judge smoothness. Or maybe just bureaucratic CYA.
 
Snowmass said:
The FAA used to say if you can't remove the frost then polish it. Then the FAA says no frost, period. The logic escapes me. If you polish frost to a surface as smooth as the underlying wing surface then why not fly? Perhaps the FAA feels a pilot cannot judge smoothness. Or maybe just bureaucratic CYA.
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Isn't "polished" frost just another term for a thin layer of ice?
 
Snowmass said:
Or maybe it reflects radiated heat back to the wing.
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That's what it does. An overcast reflects radiated heat back to the surface as well. Infrared light.
 
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Stewartb said:
To the comment frost can't form under a roof? It's common to find frost on tools and toys inside my unheated sheds. It's common to find frost inside fuel tanks. Find a steel door frame on a cold day and you may find frost on the inside. So it goes with airplanes. A colder plane exposed to warmer air will frost up before your eyes.
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An uninsulated metal roof can do that.
 
WDD said:
Frost occurs when the collecting surface is below freezing and is below the dew point. Water falls out of the air (below the dew point), and the below freezing collecting surface turns the dew into frost.
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Nope. No liquid phase in frost formation. No dew. Frozen dew is ice, not frost. Different crystalline structure. Snow is another form of vapor-direct-to-solid.

upload_2021-2-11_12-21-37.jpeg

Frost formation is often known as deposition rather than sublimation. Sublimation more correctly refers to the evaporation of frost.
 
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Salty said:
Heat rises. A roof keeps the heat from rising.
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Heat is a form of energy. It is not a physical thing subject to buoyancy. A roof will slow radiative cooling (infrared emission) by reflecting it back.
 
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