ICING IN ABOVE FREEZING TEMPS?
Craig Beaty
I'm trying to learn more about icing and when it can happen in flight.
I've read that airframe structural icing can occur anytime you have
visible moisture and the temp of the collecting surface (the wing, etc.) is
at or below freezing. Visible moisture would be cloud droplets, drizzle,
or rain. Scary moisture would be freezing drizzle, freezing rain, and
'supercooled large water droplets in clouds. I'm curious about icing
forming in normal stratus type clouds where the temp is above freezing, say
+01 C to +05 C or more. My question is how do you know when the wing is at
or below freezing, because the wing or tail surfaces can cool to freezing
or below due to aerodynamic cooling, even if the temp is above freezing.
So, does anyone have experience or info on what temp range above freezing
under which you can expect the wing and tail surfaces to cool to freezing
or below? Please, no responses concerning freezing drizzle or freezing
rain, we're in the clouds. What about any personal experience, wisdom or
guidelines anyone has learned? It's prime icing season so if you can help
please respond and teach us all a thing or two. Thanks.
--
Craig Beaty
Aero-Mechanical Engineer
Career Pilot Hopeful
My latest homemade phrase:
"If you want to be a pilot, you have to act like a pilot;
if you want to act like a pilot, you have to think like a pilot."
Paul Lawrence Hamilton
In my experience, the ice forms when the OAT is at or below 0 C and
melts quickly if the OAT is anything greater than 0 C. In twelve
northeastern winters, I have *never* seen ice above 0 C.
Wisdom -- don't have any of that! A few tips:
know where the freezing level is
know where the tops are
over mountains, assume all clouds contain ice
an O2 tank is sometimes a "go/no-go" item
always have an out:
VFR beneath the clouds
above the clouds
below the freezing level
up where the temp is -20 or below (except over mountains)
In general, avoid subzero clouds unless you know they are ice -free.
Spend as little time in them as you can. The hours pass slowly when
you are shining a flashlight on the strut every few minutes.
I find that I cancel or delay only a few trips a year, all due to ice.
Take care,
Paul
"Craig Beaty" <CBSG...@msn.com> wrote:
Paul Lawrence Hamilton, WWW.METROFLIGHT.W1.COM
Samis & Hamilton
Airport and Aviation Consultants
(301) 299-3573
Bob Gardner
I don't think that it is possible to generalize about this subject
because aerodynamic cooling is going to vary with airspeed, location on
the aircraft, and structures that affect airflow. At the icing
conference in 1996 there was quite a bit of sentiment in favor of ice
detection on surfaces not subject to impact icing, like the aft surface
of the wing or horizontal stab where ice can accrete due to cooling but
where ice detectors are not normally located. In my opinion, some of the
most promising efforts were aimed at measuring flow separation rather
than ice accumulation---that would cover bugs, dents, and bird doo-doo,
as well as ice--although some of the devices that use capacitance or
conductance as detection media are also promising efforts.
The biggest problem, again in my opinion, is the size of the potential
market versus the cost of certifying such devices.
Bob Gardner
THE COMPLETE ADVANCED PILOT
Charron
> I don't think that it is possible to generalize about this subject
> because aerodynamic cooling is going to vary with airspeed, location on
> the aircraft, and structures that affect airflow.
Practically, (and theoretically) there is no ice in above-freezing
temperatures that you have to worry about. Airspeed will raise the
skin/surface temperature, not lower it.
--
Guido Lepore
Remove the NOSPAM in header to reply
Cgarr
Thats funny?? my wings get hotter the faster I go! not cooler The only
thing that would make it freeze at a higher temp would be the lower air
pressure. Have you ever forgot to take a beer out of the freezer then when
you open it, it freezes very quick, well think of just the reverse in a
low pressure area. You ever boil water in the mountains?? Big difference
in boiling point, aint there.
Charron <gle...@NOSPAMdirect.ca> wrote in article
<glepore-1101...@van-52-0918.direct.ca>...
Rod MacLean
I don't want to touch the 'aerodynamic cooling'' part of your question
because that gets into the intractable problem of "heat of evaporation".
But it does bring up something to watch out for on the descent of a
"chilled" aircraft into above freezing moisture. If you've been flying at
25C, then descend into a cloud which is ,say, +1C, you're a candidate for
"intantstalloverfrost", which has brought more than 1 aircraft to grief.
aardwolf wrote in message
<34b98d1b...@news.calgary.telusplanet.net>...
>On Fri, 09 Jan 1998 18:33:10 -0800, "Craig Beaty" <CBSG...@msn.com>
>wrote:
>
aardwolf
>....I'm curious about icing
>forming in normal stratus type clouds where the temp is above freezing, say
>+01 C to +05 C or more. My question is how do you know when the wing is at
>or below freezing, because the wing or tail surfaces can cool to freezing
>or below due to aerodynamic cooling, even if the temp is above freezing.
I don't believe this is correct. If, by "aerodynamic cooling", you're
referring to the "wind chill" factor, it's my understanding that the
wind chill does not reduce the actual temperature, it merely
accelerates the rate at which heat is lost. A drop of water at 1 deg C
will not freeze, even if the wind chill factor is -15, because the
wind cannot reduce the temperature below the ambient air temperature.
Disclaimer: I'm not a physicist or a meterologist, so this is just my
understanding of it.
Will Lorimer
Bob Webster
You're wrong. I don't know why it happens, but I have collected ice at 1 to 3
degrees C. I have 2 temparature guages, one varied from 1-2 and one from 2-3.
And I didn't like it one bit, because couldn't descend out of it at the time.
Maybe evaporative cooling?
xxxalan....@aquila.com.xxx
In <34b98d1b...@news.calgary.telusplanet.net>, on 01/12/98
>Will Lorimer
As a meteorologist and ATP I can offer the following ...
WIND CHILL is not relevant ... see bottom of this text!
However...
If the AMBIENT air is at 0C then the airframe will be slightly
warmer... due to compressional heating! Note that I have highlighted
AMBIENT in the above line!
Some typical values of airspeed/temp & temp rise are
IAS/FL/Temp TEMP RISE Indicated
(KNOTS/100FT/C (Deg C) Temp (C)
100/FL100/-5 2 -3
200/FL200/-25 10 -15
300/FL200/-25 20 -5
400/FL300/-45 50 (Mach 1.02!) +5
M1.5/FL350/-55 110 +55
NOTE ... your outside air temperature gauge WILL read high!!! The
above values are just typical... depends on current atmosphere.
Icing detection is tricky! The rate at which objects in a stream of
air collect moisture (read ICE here!) depends greatly on the shape of
the object! Objects like the typical OAT gauge on light aircraft have
a very high collection rate and hence are one of first objects to
start collecting/showing ICE. Hence ... watch the tip of your OAT
gauge and at the first sign of ICE take action! PITOT heat and
anti-ice should be applied well in advance of any likely hood of
icing!
As a note to the above ... I have often observed light aircraft that
have been cold soaked (operated at high altitudes well below 0C) to
accumulate ice in CLEAR air at temperatures slightly ABOVE 0C when the
dewpoint depression is low ... (read near 0). This is the result of
the airframe being colder than the dewpoint ... and the moisture
sublimates out onto the airframe as FROST... Assuming you have warmer
air below you (and altitude between you and the ground) this is not a
significant threat as eventually the airframe will warm up (due to
convective heat transport from the air to airframe) and the frost will
sublimate back off the the airframe!
WIND CHILL is the result of the "convective" transport of heat from an
object due to a fluid (the air) moving by the object. If the object
is at the same temperature as the fluid then there will be no heat
transfer... and hence no "Chill" effect.
A person on the other hand will try to maintain his/her body
temperature (via his/her metabolism) and hence there will be a heat
loss (wind chill) due to the fluid (air) moving by. The wind chill
temperature is the temperature at which a person in calm air will lose
heat at a rate equal to the rate of loss in the current conditions!
In other words if the wind chill temperatures are equal then the rate
of heat loss is equal.
-----------------------
From Alan Robertson
Fax @ 630-443-9968
-----------------------
brewster
I would have to agree with the Mr. McLean's statement about descent through
clouds. I've seen it many times. I've landed in desert's with frost on the
wings.
Concerning the change in skin temp due to a/s, there is a heat rise
correction worked into CR-2 and CR-3 whiz wheels for the apparent rise in
temp due to skin friction. You have to subtract the HRC from apparent temp
to calculate actual ambient temp.
As a side note, if you are flying above clouds and can see your shadow below
you, look for a rainbow around the silhouette. Rainbow = liquid moisture =
possible icing.
No rainbow = ice = no accumulation. Picked that little tidbit up a while
back. Seems to have worked for me.
Good luck!!
T. Weddle
Festlover
Craig,
I've had two experiences with icing in (what I thought were) above freezing
conditions. They both involved super-cooled drizzle drop moisture where ice
was neither forecast nor reported. The air temp was about 35'F. In both
situations I was flying in IMC in a very "heavy", wet cloud. All at once (and
I mean like "now!") the moisture on the airframe froze and began accreting
incredibly fast. It seemed as though there was a "cold spot" in the cloud
where what once was water turned immediately to thick, clear ice. One such
occasion happened on the glideslope and resulted in a stall in IMC above a 250'
ceiling. Needless to say, a situation that I now take great steps to avoid.
These days I don't fly in IMC in the winter unless the air temp is below 5'F or
above 40'F and the ceilings are plenty high. Any ice produced at the colder
temps seems to be of the rime type and accumulates much slower and at a more
predictable rate. With super-cooled drizzle drops one runs out of options
unbelievably quick.
rich
Matthew S. Whiting
I believe that "supercooled" refers to water that is below the freezing
temperature but which has not yet frozen. Typically, any disturbance
will cause virtually instantaneous freezing. I don't think it is
possible for water to freeze at temperatures above freezing and
certainly that condition would not be described as supercooled.
Matt
Bob Gardner
Good reply. I have asked Dr. Marcia Politovich, the "Ice Queen" at the
National Center for Atmospheric Research for her comments on this
subject and will inform the group when I get her reply.
Bob Gardner
Dave
wind chill factor refers to how cold it feels on your skin. it
has not effect on when water freezes.
Andrew Boyd
alan.ro...@aquila.com.xxx wrote:
> If the AMBIENT air is at 0C then the airframe will be slightly
> warmer... due to compressional heating!
My father, in the RCAF, used to fly a non-deiced T-33 in all
sorts of icing wx - he used to do a weekly cross-continent
commute.
The way he dealt with icing was just to keep the speed up
over 300K. No icing due to compressional heating.
--
ab...@igs.net ATP
Festlover
Matt wrote:
>I believe that "supercooled" refers to water that is below the freezing
>temperature but which has not yet frozen. Typically, any disturbance
>will cause virtually instantaneous freezing. I don't think it is
>possible for water to freeze at temperatures above freezing and
>certainly that condition would not be described as supercooled.
To which I reply:
Matt,
You're right. The term 'supercooled' was hastily chosen on my part. The
situation I wanted to discribe was that of flying in above freezing conditions
and, quite suddenly entering an isolated area of significantly (3 to 5'f) lower
temperatures. This has happened to me twice and I don't believe that I've seen
anything about 'cold patches' (my terminology) in the popular weather
literature. The icing resulting from such an encounter is dramatic and I would
be interested in knowing if anyone else out there has had a similar experience.
Rich