Supercooled Water - More on Icing

Gotta wonder why the AWC icing page in ADDS offers the user to choose
between a graphic display of "all icing" and of "SLD icing" if Supercooled
Liquid Droplets are not a hazard. The Roselawn accident (ATR-72, if I recall
correctly), was attributed, in part, to SLD icing.

Bob Gardner

"Tarver Engineering" wrote in message
...

"Mike Rapoport" wrote in message
nk.net...
You still have it partly backwards on droplet size.

Let me provide you with a more probabilistic view of the data you are
relying on, being "observed". Now in fact FAA funded a study of icing
from
large droplets, as that was the "observed phenomena". Consider an
experiment where 50 pilots encounter large droplet icing and 50 pilots
encounter small droplet icing. At the end of the experiment, the group
with
"observed" large droplet reports 49 incidents, while the small droplet
"observed" group has only 5 advocates.

O. Sami Saydjari wrote:
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)?



1. How the cloud got formed is as important as the current temperature
of the cloud.

As many have pointed out, cumuloform clouds often retain supercooled
water well below -10.

But not only that, the original air that formed the cumuluus cloud may
have come from low, warm, humid levels. If the air in the cloud started
out at, say, 20 degreec C with a 15 C dewpoint, it will have condensed
out about 11 grams of liquid water per cubic meter by the time it cooled
to -10. (This is also why the worst icing in cumuloform clouds is often
near the top.)

If the air started with a dewpoint of -5, it will have condensed about 1
gram per cubic meter... only about a tenth of the above scenario.

If the air in the cloud STARTED with a dewpoint of -10 or less, the
cloud will have condensed less than 2 grams of liquid water per cubic
meter NO MATTER HOW COLD IT GOT.

2. As Roy mentioned, water condenses only onto a nucleus of some type.
In nature these nuclei are MOSTLY the wrong shape for ice crystals to
form, so it is natural for the condensation to stay liquid as long as
possible, and -10 or so appears to be "common". (By the way, Roy, the
latent heat release when it DOES freeze just goes to warm up the air a
tiny bit). Typically, once a few drops DO freeze, or when snowflakes
are fall into the liquid layer from above, then the liquid will migrate
to the ice fairly quickly...

Below -10, it is common that at least SOME ice crystals will form, and
this usually erodes the water content quickly. However, in vertically
developing cloud (Cumulus, etc.) the updrafts may prevent the
introduction of ice crystals for some substantial period of time.

"Mike Rapoport" wrote in message
ink.net...
I have no idea of what you are trying to say.

OK.

Aircraft performance in icing
is measured in both wind tunnels and in flight. It doesn't matter what a
bunch of pilots think,

There you go.

the data is quantified

Yes.

and it shows that the
performance degradation is highest with large droplets which form ice in
ridges aft of the leading edges.

But icing events that lead to catastrophic failure are more likely to occur
in small droplets forming rapid acretion on the tail surface. (rudder
reversal)

"Bob Gardner" wrote in message
news:xkRAb.329519$9E1.1623342@attbi_s52...
Gotta wonder why the AWC icing page in ADDS offers the user to choose
between a graphic display of "all icing" and of "SLD icing" if Supercooled
Liquid Droplets are not a hazard. The Roselawn accident (ATR-72, if I
recall
correctly), was attributed, in part, to SLD icing.

Management of acretion may have been a factor in the ATR-72 AP problems, but
we know now that f the operator disables the autopilot in such conditions
that the airplane remains in controlled flight. There is a second issue
with certain German Litton gyros and their switcher power supplies passing
through 0 C, but that is something clearly referenced on the equipment's
boiler plate.

"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

"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.

(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)?

Sami, I believe you are in Wisconsin - in the Midwest this is
generally the case, but as you know, with icing there are no
guarantees.

-Nathan

O. Sami Saydjari wrote:
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)?

If there is any chance you will pick up ice at some altitude, you'll want to be
sure there is some altitude you can get to where you can shed the ice before
landing, unless you can land with an iced-up windscreen.

For me, that means somewhere within range where the temperatures are above
freezing at the surface.

This is probably not possible in Wisconsin for a big part of the year.

I've never had the experience of accumulating ice and then not being able to
descend to above-freezing temperatures, but I'd think it would be a pretty
serious situation without a heated windscreen. When I've had an iced-over
windscreen, the defroster wouldn't touch it. It was only descent into warmer air
that allowed me to see again.

Maybe someone who has landed with ice when the surface temps are below freezing
can commment on the windscreen issue.

Dave
Remove SHIRT to reply directly.

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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Mike Rapoport wrote:

[...]
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).

I've been wondering why the vertical movement makes a difference.

This assumes that the water is still. It the
water is turbulent then the temperature will go even lower before
crystalization starts.

Is that the answer? The vertical movement counts as "turbulence" in this
context?

Why does the "activity" of the water alter the temperature at which freezing
starts? The kinetic energy of such movement? The friction which results
from such movement? But wouldn't the friction merely raise (or slow the
decrease) of the temperature?

- Andrew