Supercooled Water - More on Icing

This question is a question on physcial phenomena, NOT on regulation (so
I am starting a new thread).

As I understand it, icing happens between +2C and -10C. Assuming this
is correct, I have three questions.

(a) Does that mean one is relatively safe if the surface temperature is
below -10C (and there is no temperature inversion, meaning that the
temperature is known to decrease as one goes up in altitude)?

(b) Is icing from 0C to +2C a possibility only when your aircraft skin
is colder than 0C (probably because you are descending from altitude)?
Or is there some other condition where this is possible?

(c) I have been told that icing is possible from -10C to 0C because
water sometimes get "super-cooled" (which I assume means that water gets
below freezing, but does not form ice for some reason). Is that right?
If so, under what atmospheric conditions does water get super-cooled?

"O. Sami Saydjari" wrote in message
...
This question is a question on physcial phenomena, NOT on regulation (so
I am starting a new thread).

As I understand it, icing happens between +2C and -10C. Assuming this
is correct, I have three questions.


MOST icing occurs between 0C and -10C but icing has been shown to occur
at -40C. Icing in cumulus clouds virtually always extends below -10C.and
often to below -20C


(a) Does that mean one is relatively safe if the surface temperature is
below -10C (and there is no temperature inversion, meaning that the
temperature is known to decrease as one goes up in altitude)?


Ice is less likely to be a problem than if it was 0C at the surface but,
again, icing can occur at temperatures much lower than -10C particularly in
clouds with vertical movement (cumulus).
..
(b) Is icing from 0C to +2C a possibility only when your aircraft skin
is colder than 0C (probably because you are descending from altitude)?
Or is there some other condition where this is possible?

This was the conventional thiinking buy a recent NASA study was unable to
document ANY ice formation above 0C.

(c) I have been told that icing is possible from -10C to 0C because
water sometimes get "super-cooled" (which I assume means that water gets
below freezing, but does not form ice for some reason). Is that right?
If so, under what atmospheric conditions does water get super-cooled?

If you do a search on supercooling you will find a lot of info. Basically
water almost NEVER cools to 0C and then starts crystalizing. It usually
cools to -7C before crystalizing starts. Once the crystalization starts the
temperature increases as the latent heat is released and when the last water
freeezes the temp is at 0C again. You can verify this with a glass of
water.and a thermometer. This assumes that the water is still. It the
water is turbulent then the temperature will go even lower before
crystalization starts.

Mike
MU-2

Mike Rapoport wrote:
"O. Sami Saydjari" wrote in message
...

This question is a question on physcial phenomena, NOT on regulation (so
I am starting a new thread).

As I understand it, icing happens between +2C and -10C. Assuming this
is correct, I have three questions.



MOST icing occurs between 0C and -10C but icing has been shown to occur
at -40C. Icing in cumulus clouds virtually always extends below -10C.and
often to below -20C



(a) Does that mean one is relatively safe if the surface temperature is
below -10C (and there is no temperature inversion, meaning that the
temperature is known to decrease as one goes up in altitude)?



Ice is less likely to be a problem than if it was 0C at the surface but,
again, icing can occur at temperatures much lower than -10C particularly in
clouds with vertical movement (cumulus).
.

(b) Is icing from 0C to +2C a possibility only when your aircraft skin
is colder than 0C (probably because you are descending from altitude)?
Or is there some other condition where this is possible?


This was the conventional thiinking buy a recent NASA study was unable to
document ANY ice formation above 0C.


However, keep in mind that most thermometers have some error in them.
So, even though ice doesn't form above 0C, it may form above 0C as
indicated on your airplane thermometer.


Matt

"Matthew S. Whiting" wrote in message
...
However, keep in mind that most thermometers have some error in them.
So, even though ice doesn't form above 0C, it may form above 0C as
indicated on your airplane thermometer.


Matt

That is true and in addition there is some aero heating of the airplane as
it travels through the air and different parts of the airplane will
experience different amounts of heating. Most turbine airplanes have a
chart that shows OAT guage error at different altitudes and airspeeds.

Mike
MU-2

"Matthew S. Whiting" wrote in message
...

However, keep in mind that most thermometers have some error in them.
So, even though ice doesn't form above 0C, it may form above 0C as
indicated on your airplane thermometer.

Absolutely true, but remember that your thermometer is one of the easiest
instruments in the aircraft to calibrate. The ATIS gives the temperature on
the ground before flight -- it's well worth a check.

Julian Scarfe

It is not just a calibration problem (accuracy of themometer), nor is
it just the problem of getting an accurate static temperature (due to
moving air). There are low pressure areas on the wing surfaces, and
lower pressure causes the air temperature to go down, in these places.
So these are all reasons why we don't use zero degrees (which IS
required to freeze water), but use some temperature a little higher.

I use the airmets as my guide. If there is an airmet for icing, I
don't go. Even then, it is possible to pick up ice. I have also broken
this rule when I talked to an incoming pilot who flew the other way
through the clouds, and he informed me that not only did he not pick
up ice, he didn't think there was any ice in those clouds. It was a
bit risky, but he was right, no ice. It was the tail end of the front
(I have found less ice in the tail end of the fronts than the leading
part), and the icing airmet hadn't gone away, yet, but the clouds were
"dry". None of the PIREPS indicated icing either. It think I was legal
as I had more complete information than the airmet (debatable). I
agree with MU-2 Mike in that we do get ice below -10 degrees Celcius.
It is also possible to get ice when there is no airmet, so always have
an out.

One big reason the airliners can deal with ice is they have the climb
ability to outclimb the ice. A turbine or at least a turbo charged
aircraft (with either oxygen or pressurization), has protection at
least as good as having deice system. Also keep in mind, that if you
go fast enough (300 knots or so), you wont get ice due to friction
warming of the wings (specific info needed on the type from the mfg
for this, no general rule).

One final piece of information. The smaller diameter of the surface
area, the more ice will accumulate. This is why antennaes and struts
get more ice. This is due to the physics of the air in front of the
surface. Large diameter surfaces deflect the air coming toward the
surface more thand smaller diameter surfaces.



"Julian Scarfe" wrote in message ...
"Matthew S. Whiting" wrote in message
...

However, keep in mind that most thermometers have some error in them.
So, even though ice doesn't form above 0C, it may form above 0C as
indicated on your airplane thermometer.

Absolutely true, but remember that your thermometer is one of the easiest
instruments in the aircraft to calibrate. The ATIS gives the temperature on
the ground before flight -- it's well worth a check.

Julian Scarfe

Doug ) wrote:

snip
I
agree with MU-2 Mike in that we do get ice below -10 degrees Celcius.
It is also possible to get ice when there is no airmet, so always have
an out.

During this season (my first full winter winter since receiving my IFR
rating last March), I have picked up ice at +2 C, -12c, and in areas that
were outside of icing Airmets by hundreds of miles.

In my limited experience, one fact seems apparent to me: There are no
reliable rules pertaining to ice except, perhaps, plan for possible icing
from October to March (at least downwind of the US Great Lakes).

--
Peter















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Julian Scarfe wrote:
"Matthew S. Whiting" wrote in message
...


However, keep in mind that most thermometers have some error in them.
So, even though ice doesn't form above 0C, it may form above 0C as
indicated on your airplane thermometer.


Absolutely true, but remember that your thermometer is one of the easiest
instruments in the aircraft to calibrate. The ATIS gives the temperature on
the ground before flight -- it's well worth a check.

Julian Scarfe



Yes, but where is their thermometer? On the ground near the level of
your plane, or on top of the tower? Makes a lot of difference. Also,
the error may be a slope error, not just a offset. It may be off by 2
degrees at 70 degrees, but less than that at 32 (or more, but typically
error gets less at lower temps).

I prefer to watch things very carefully when the temp is between about 0
and 35F. Most ice I've picked up was between 26 and 32 as indicated by
my Skylane thermometer. No idea how accurate it was, but I think it was
within a couple of degrees which is close enough for most purposes. I
make decisions based on what is happening on the wing, not what the
thermometer says, generally anyway.


Matt

"Matthew S. Whiting" wrote in message
...

Yes, but where is their thermometer? On the ground near the level of
your plane, or on top of the tower? Makes a lot of difference.

Fair point. At least in my part of the world, reported temps are supposed
to be 2 metre temps (measured about 6 ft above the ground). But as others
have pointed out there may be differences because of the local measurement
environment.

I still think it's well worth making a comparison between your OAT and the
reported temperature. It's not difficult to spot consistent deviations of a
couple of degrees.

Julian Scarfe

Julian Scarfe

Absolutely true, but remember that your thermometer is one of the easiest
instruments in the aircraft to calibrate. The ATIS gives the temperature
on
the ground before flight -- it's well worth a check.

Although the temperature at the ATIS station and the temperature where you
thermometer is located may quite a few degrees off. Kinda like setting your
Heading Indicator when lined up on the runway.

Hilton