Icing Airmets

Mike Rapoport ) wrote:

You can't unless your thermometer is wrong.

Are you replying to Dave's first sentence, which is:

I'd like to know how you get in icing when the temperature is +5.

Or his second:

I have never seen ice until the the gauge reads 0 or below.

Or both?

If you are replying to his second sentence, this is contradictory to my
understanding that the temperature right at the leading edge of an airfoil
could be up to a few degrees colder than the surrounding air, which is why
I learned that airframe icing (excluding freezing rain) can occur in air
temperatures above freezing.

I should point out here that I am simply seeking clarification of your
comment, not challenging it (as I look to you as one of the more
experienced in these groups).

--
Peter












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Both sentences actually. Recent studies of airframe icing have not been
able to produce icing above 0C. (Sorry I don't have a reference). If the
static air temperature is above 0C then the water isn't supercooled, so even
if the airplane is at or slightly below freezing you won't get ice. The
wing's leading edge experiences a temperature increase, not a decrease.

I suppose that it is possible to locate the temp probe in an area with a ram
temperature rise greater than the wing. In that case, it is possible for
the temp guage to read above 0C but still have the static air temperature
and portions of the airplane below 0C. That would lead to icing but it is
an instrument error not icing above 0C.

Mike
MU-2


"Peter R." wrote in message
...
Mike Rapoport ) wrote:

You can't unless your thermometer is wrong.

Are you replying to Dave's first sentence, which is:

I'd like to know how you get in icing when the temperature is +5.

Or his second:

I have never seen ice until the the gauge reads 0 or below.

Or both?

If you are replying to his second sentence, this is contradictory to my
understanding that the temperature right at the leading edge of an airfoil
could be up to a few degrees colder than the surrounding air, which is why
I learned that airframe icing (excluding freezing rain) can occur in air
temperatures above freezing.

I should point out here that I am simply seeking clarification of your
comment, not challenging it (as I look to you as one of the more
experienced in these groups).

--
Peter












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Newsgroups
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The temperature above the wings and below the elevators could be
slightly below ambient due to the lower pressure. I don't have a
number on what the temperature drop is on these surfaces, but
technically it is possible to have icing on the lifting surfaces when
the ambient temperature is above freezing. Sort of like carb icing in
above-freezing temperatures.


"Mike Rapoport" wrote in message hlink.net...
You can't unless your thermometer is wrong.

Mike
MU-2

"Dave" wrote in message
ink.net...
I'd like to know how you get in icing when the temperature is +5. I have
never seen ice until the the gauge reads 0 or below.




"ArtP" wrote in message
...
On 26 Jan 2004 10:08:44 -0800, (Andrew
Sarangan) wrote:

For example, right now the freezing level is at 5000 ft,
MEA is at 2500 ft, yet we have an airmet Z that goes all way from SFC
to 18k.

Since the published icing conditions occur from +5 to -20 C, a
freezing level at 5000 would mean icing potentially below 3000 feet.
The MEA may be 2500 but the normal minimum IFR altitudes would be
either 3000 or 4000 feet depending on direction and that is known
icing conditions. I am sorry this frustrates you but depending on
where you live a big chunk of winter is off limits to small GA
aircraft trying to fly IFR.

In article ,
(Andrew Sarangan) wrote:

The temperature above the wings and below the elevators could be
slightly below ambient due to the lower pressure. I don't have a
number on what the temperature drop is on these surfaces, but
technically it is possible to have icing on the lifting surfaces when
the ambient temperature is above freezing. Sort of like carb icing in
above-freezing temperatures.

You're talking about two very very different things.

When a gas undergoes adiabatic expansion, it gets cooler. There is no
doubt that this happens at the leading edges of airfoils, but at the
pressure drops we're talking about in any kind of airplane I'm likely to
fly is very small. How small is very small? I'm not sure, but I can't
imagine more than a degree or two.

Yes, you in the back? What's that? You think I'm trying to befuddle
the issue with big-sounding words like "adiabatic"? OK, all adiabatic
means is that there's no exchange of heat. We all know that gasses get
hotter when you compress them. You probably learned Boyle's Law and
Charles's Law in high school chemistry, or maybe the Ideal Gas Law.
These are all just different ways of saying that if you've got a certain
amount of gas which contains a certain amount of energy, if you know any
two of pressure, volume, and temperature, you can figure out the third.
As long as you don't add or subtract energy (i.e. heat), you can play
with those three variables to get all sorts of different combinations.

All those "no heat lost or gained" transitions are adiabatic. That's
what happens at the leading edge. The air moves from an area of high
pressure to an area of lower pressure on top of the wing. As it does,
it expands and cools, but the total amount of energy in a given parcel
of air stays the same.

This is not to say that adiabatic cooling can't cause very large
temperature drops. Anybody who has ever fired off a CO2 fire
extinguisher knows that the gas coming out is VERY cold, and that is an
adiabatic process. But it's also undergoing a pressure drop orders of
magnitude bigger than what goes on at the leading edge of a spam can.

Carb icing is a totally different fish. What's going on inside a
carburator is liquid gasoline is evaporating and turning into vapor.
There's a phase change. It takes a huge amount of energy to effect a
phase change. The air that enters the carburator is NOT undergoing an
adiabatic process; it's giving up energy to the gas to make it vaporize.
That's why you get huge temperature drops inside the carb, and can get
carb icing at ambient temperatures way above freezing.

"Andrew Sarangan" wrote in message
om...
The temperature above the wings and below the elevators could be
slightly below ambient due to the lower pressure. I don't have a
number on what the temperature drop is on these surfaces, but
technically it is possible to have icing on the lifting surfaces when
the ambient temperature is above freezing. Sort of like carb icing in
above-freezing temperatures.


There has been a lot of hypothisizing about this in the past but NOBODY has
EVER been able to reproduce it.

You don't get icing just because a portion of the airplane is below 0C. I
doubt if droplets even touch any part of the airfoil where the
temperature/preasure is below ambient without running back. If they run
back, the first encountered the heated portion of the wing (leading edge).
You need supercooled water to get airframe icing.

It is another of aviations myths

Mike
MU-2
..

It is possible to get icing if you were recently in colder air such that the airframe got cold-soaked and has not warmed to above
freezing yet. I've picked up ice in a descent that way before, but it doesn't take long before it goes way.

--
--Ray Andraka, P.E.
President, the Andraka Consulting Group, Inc.
401/884-7930 Fax 401/884-7950
email
http://www.andraka.com

"They that give up essential liberty to obtain a little
temporary safety deserve neither liberty nor safety."
-Benjamin Franklin, 1759

Any time air is accelerated, as it is when passing over a small-radius
surface, its temperature drops...so it is entirely possible to accrete ice
when the temp is above zero. That's why you look at those surfaces
first...OAT guage, struts, lower edge of windscreen where there is a lip
rather than a flush surface, etc. That is also why tail feathers begin to
accrete ice before the wing's leading edge does.

Bob Gardner

"Dave" wrote in message
ink.net...
I'd like to know how you get in icing when the temperature is +5. I have
never seen ice until the the gauge reads 0 or below.




"ArtP" wrote in message
...
On 26 Jan 2004 10:08:44 -0800, (Andrew
Sarangan) wrote:

For example, right now the freezing level is at 5000 ft,
MEA is at 2500 ft, yet we have an airmet Z that goes all way from SFC
to 18k.

Since the published icing conditions occur from +5 to -20 C, a
freezing level at 5000 would mean icing potentially below 3000 feet.
The MEA may be 2500 but the normal minimum IFR altitudes would be
either 3000 or 4000 feet depending on direction and that is known
icing conditions. I am sorry this frustrates you but depending on
where you live a big chunk of winter is off limits to small GA
aircraft trying to fly IFR.

"Bob Gardner" wrote in message
news:XKARb.132385$nt4.579289@attbi_s51...
Any time air is accelerated, as it is when passing over a small-radius
surface, its temperature drops...so it is entirely possible to accrete ice
when the temp is above zero.
first...OAT guage, struts, lower edge of windscreen where there is a lip
rather than a flush surface, etc. That is also why tail feathers begin to
accrete ice before the wing's leading edge does.

Bob Gardner


While small radius objects do collect ice better than larger redius objects,
temperature drop has nothing to do with it. Small radius objects have a
higher "collection efficiency" meaning more of the droplets in their path
will impact the surface. They have a higher collection efficiency because
they don't project a "bow wave" as far in front of them as larger. You NEED
supercooled water for airframe icing.

Mike
MU-2

"Mike Rapoport" wrote in message
ink.net...

"Bob Gardner" wrote in message
news:XKARb.132385$nt4.579289@attbi_s51...
Any time air is accelerated, as it is when passing over a small-radius
surface, its temperature drops...so it is entirely possible to accrete
ice
when the temp is above zero.
first...OAT guage, struts, lower edge of windscreen where there is a lip
rather than a flush surface, etc. That is also why tail feathers begin
to
accrete ice before the wing's leading edge does.

Bob Gardner


While small radius objects do collect ice better than larger redius
objects,
temperature drop has nothing to do with it. Small radius objects have a
higher "collection efficiency" meaning more of the droplets in their path
will impact the surface. They have a higher collection efficiency because
they don't project a "bow wave" as far in front of them as larger. You
NEED
supercooled water for airframe icing.

Not exactly. Small objects and small water lead to the best conditions,
from a statistical standpoint, for gathering ice.

"Dave" wrote in message
ink.net...
| I'd like to know how you get in icing when the temperature is +5. I have
| never seen ice until the the gauge reads 0 or below.

Icing occurs when liquid water freezes on an airplane surface that is below
freezing. The aircraft may have been cooled earlier when it flew through a
layer.

Per your next reply -- if you don't trust Peter R.'s gauge to say it is +3
when he is getting icing, what makes you think you can trust your gauge?