It's that time of year again. Time to shift your operational concerns from thunderstorms to icing. But where does airframe icing come from? How does it form? Where? How can I recognize the danger of icing during my flight planning and what do I do about it? In this first part of a series, AVweb contributor R. Scott Puddy tackles these questions and more. Consider it a primer for what's to come.
July 10, 2002
The FAA's Airport and Aircraft Safety Research and
Development Division, Aircraft Safety Research and Development Branch, issued
a fact sheet on September 6, 2000, which begins:
"Aircraft icing continues to be one of the major safety threats to
aircraft operations during hazardous weather conditions and can result in
catastrophic accidents unless adequate precautions are
I'm sure that news is as shocking to you as it was to me. I guess we'd
better prepare to take "adequate precautions" for icing during the upcoming
winter flying months.
Icing occurs when liquid moisture comes into contact with an object that is
at a below-freezing temperature. The temperature of the liquid moisture
doesn't matter. Step outside on any bitter cold winter morning. Using your
moist, above-freezing tongue, lick the below-freezing, metal street post.
That's icing. (Don't try this alone or you could be "stuck" outside for
You can't make it through an article about icing conditions without reading
about "supercooled" water droplets and how they are THE cause of aircraft
icing. Although they are not THE cause, supercooled water droplets do play a
major role. As discussed below, if not for supercooled water droplets you
usually would not encounter liquid moisture at altitudes where the outer skin
of your airplane is at a below-freezing temperature. Liquid moisture is one of
the prerequisites for icing. However, icing occurs because of the temperature
of the airframe. If the temperature of the airframe is above freezing (such as
on the surface of a windshield anti-icing panel) ice will not form.
You will pick up ice anywhere liquid moisture impacts any
part of the airframe that is below freezing. The temperature of the airframe
is a function of the ambient air temperature and the warming that results from
the friction of the air passing over the surface. The techies would point out
that the air molecules immediately adjacent to the airframe are not moving at
all and that the friction actually occurs within the boundary air layer — but
that is a distinction without a difference. The friction within the boundary
air layer heats the air that heats the airplane skin and elevates the surface
temperature to some extent. Just how much the temperature is elevated depends
on the speed of the airframe in general as well as the speed of airflow across
the skin at specific locations (and, one for the techies, the thickness of the
boundary air layer).
The faster the airplane, the higher the skin temperature. The Concorde
boasts a skin temperature of between 91-127 degrees Centigrade (196-261
degrees Fahrenheit) in a Mach 2 cruise at altitudes where the ambient air
temperature is well below freezing. No need for anti-ice on the Concorde, at
least in cruise. Similarly, transport-category aircraft climbing out at over
200 knots could encounter no ice in conditions that would load up a GA
aircraft climbing out at under 100 knots.
At subfreezing temperatures water vapor sublimates to ice crystals, so
where is all this liquid moisture coming from? If you have watched videos of
airplanes being certified for flight in known icing conditions, you know that
one possible source is a large tanker aircraft flying directly in front of you
trailing a water nozzle. If you ever find yourself in that position, radio the
pilot ahead and tell him you will catch up on your child support as soon as
you get paid for completing this delivery of checks for the Federal
In all other cases, the two possible sources of liquid
moisture are the above-freezing air below you and the above-freezing air above
you. Most discussions of aircraft icing classify ice as either "clear,"
"rime," or "mixed." If the source of liquid moisture is the air below, rime
ice will form. If it is liquid moisture falling as rain from above, clear ice
will form. If it is attacking you from both direction (such as in a cumulus
cloud) the icing will be mixed. "Rime" is shorthand for "bad"; "clear" is
shorthand for "really bad."
In order to be able to predict how "bad" the icing you may encounter might
be, you need have a detailed understanding of the phenomena that cause liquid
moisture to be present at temperatures below freezing in order to answer the
real issues, which are: "How much moisture is there?" and "How much of it is
likely still to be in liquid form?" In order to be able to predict the answers
to those questions, you need to understand the basic steps of the icing
All moisture in the atmosphere comes from evaporation, the process through
which water changes from a liquid to a vapor (a gas). Oceans (e.g., the
Pacific Ocean and the Atlantic Ocean) and other large bodies of water (e.g.,
the Gulf of Mexico and the Great Lakes) are the most prominent sources of the
water vapor that starts the icing process.
Air has a limited capacity to hold water vapor and that capacity diminishes
as the temperature drops. The most usual cause of condensation is a lifting of
the airmass. As the ambient pressure decreases at higher altitudes, the
airmass expands and cools. When the air cools, its capacity to hold water
vapor decreases, the airmass becomes supersaturated, and the vapor condenses
on condensation nuclei (or existing cloud droplets) and forms cloud droplets.
Not to be confused with rain drops, "cloud droplets" are tiny little suckers
that range in size from a few micrometers to a few tens of micrometers. A
single raindrop (one millimeter in radius) would consist of approximately 1
million cloud droplets with a 10-micrometer radius. A typical cloud contains
100 to 1,000 cloud droplets per cubic centimeter.
In warm (liquid) clouds, drops of water form from water droplets through
the process of coalescence. Water droplets of different sizes travel at
different speeds and hence tend to run into one another. If the collision is
strong enough to overcome the surface tension of the droplets, the two
droplets combine (coalesce) as one. After as many as a million such unions,
the drop of water eventually becomes too heavy for the cloud to support and it
falls from the cloud as rain or drizzle. The size of the drop depends on the
type and strength of the cloud. There is not a lot of circulation in a stratus
cloud and the coalescence process proceeds slowly. Stratus clouds also lack
the vertical circulation necessary to support larger drops of water and tend
to produce drizzle (drops 0.50 millimeters in diameter or smaller). In
convective clouds, in contrast, the complex circulation patterns afford ample
opportunity for collisions and coalescence and the vertical circulation
supports much larger drops (up to more than 6 millimeters in diameter).
(Option 1: Supercooled Water Droplets)
The wing of a NASA
Twin Otter after landing. This looks to be clear icing or perhaps mixed.
Notice the runback well past the leading edge and on the underside of
Click image for larger
version. Photo copyright NASA-Lewis.
Minute supercooled water droplets could exist at below-melting temperatures
if a stratus cloud is lifted to cooler, higher altitudes. We all know from
common experience that smaller bodies of water freeze more quickly. A large
lake may remain unfrozen throughout the coldest of winters. A smaller lake may
freeze over after a few weeks of subfreezing weather. The mud puddle next to
your driveway will freeze in a couple hours. Absent antifreeze, the spray from
your automobile windshield washer will freeze in the time it takes the wiper
blades to complete a cycle. Anything as small as 1/1,000,000th the size of a
rain drop must therefore freeze instantly if the temperature dips to 31
Wrong. That's why scientists have to go to graduate school. The temperature
we commonly refer to as "the freezing point" should actually be called "the
melting point." Frozen water always melts at temperatures above 32 degrees,
but the freezing process is much more complex. Freezing requires the formation
of ice crystals which in turn requires the presence of "freezing nuclei" or
"ice nuclei." In contrast to the mud puddle in your front yard, the immaculate
little water droplets were formed through sort of a scientific Virgin-birth
process and consist of water of a purity that the Culligan Man could only pray
for. No freezing nuclei here. Hence, no crystallization and no immediate
freezing when the temperature drops below "melting." For the pure water that
exists in the atmosphere, there is no set "freezing point."
Sponsored by those who would like to harvest rice in the desert, Ph.D.
types have performed countless scientific studies relating to the spontaneous
freezing point of water droplets of various sizes and the most efficient means
of causing those water droplets to coalesce or crystallize and fall to Mother
Earth as measurable rainfall. The "spontaneous" freezing temperature for
undisturbed water droplets has been observed to be as low as -40 degrees
Fahrenheit. Supercooled water droplets are liquid but unstable. If they
encounter "freezing nuclei," "ice nuclei," or "your airplane," they will
Considering that the average cloud has up to 1,000 unstable droplets in
each of its cubic centimeters, you might imagine that there is some variation
between them. Although they all come into life in a virginal state, some
droplets bite of the forbidden fruit and freeze. Once some of the droplets
have frozen, they constitute the nuclei that will cause other droplets to
freeze. Given enough time and enough cold, the supercooled water droplets will
depart their unstable liquid state and reach their stable frozen state.
(Option 2: Supercooled Drops of Water)
Close-up of a test
wing section. A 16-inch portion of a wing being tested is lifted through
a port in the Twin Otter's fuselage and exposed to supercooled liquid
water then pulled back inside and photographed.
Click image for larger
version. Photo copyright NASA-Lewis.
The studies suggest that the much larger drops of water are not as
resistant to freezing as the minute water droplets. Drops of water
nevertheless can exist in liquid form at below melting temperatures, usually
in convective clouds. The strong updrafts in convective clouds lift liquid
drops of water from the warmer temperatures at lower altitudes to the
below-melting temperatures above. Until the drops of water have an opportunity
to freeze, they remain in liquid form. Strong thunderstorms can produce liquid
moisture well into the flight levels in this manner.
(Option 3: Supercooled Raindrops)
Liquid raindrops, supercooled or not, can exist where rain falls from
above-melting air into cooler below-melting air. If you've ever run out of ice
at a party and refilled the ice cube trays in a futile effort to replenish
supplies before your guests request a second round, you know that there is a
time element to the freezing process. Where there is rainfall aloft and ice
pellets below, there will be a layer of supercooled water (freezing rain) in
Hence there are at least three ways that liquid water could find its way to
your below-melting flight altitude. The next question is, "Do you care how it
got there?" Absolutely, positively, yes, you do.
Icing from above (freezing rain) is a component of the natural selection
process. Out west, those conditions occur perhaps a couple times a year. The
pilots who fly in those conditions are eliminated from the aviator gene pool
and you no longer have to avoid them while they are taking off from taxiways
at uncontrolled airports.
Freezing rain requires a strong temperature inversion. It is usually
associated with an advancing warm front but can also occur in conjunction with
a cold front or an occluded front. In an advancing warm front, the warm air is
elevated as it advances over the underlying cooler air and it expands and
cools. Water vapor condenses into rain in the higher above-melting air, falls
into the lower, below-melting air, locates your airplane and turns it into a
flying popsicle. Supercooled raindrops are huge in comparison to the
supercooled water droplets that you might encounter flying through a cloud and
they generate a correspondingly greater volume of ice when your plane runs
into them. Furthermore, the larger drops of liquid readily spread before they
freeze (that is why the ice is "clear.") That can create problems even for
airplanes with deicing equipment if the clear ice spreads back on the airfoil
to beyond the deice-protected regions. For most GA pilots, the appropriate
precaution for freezing rain is to avoid it at all costs. On the other hand,
if you and four of your drinking buddies just left the Raiders Monday night
football game and departed from an unlighted taxiway in a C-172, go for
of the test wing section.
Click image for larger
version. Photo copyright NASA-Lewis.
During the wet months, the Pacific Coast is a veritable ice-making machine
and the conditions will require you to draw upon your knowledge of icing and
to exercise your judgment as PIC (assuming you do not intend to mothball your
plane until spring training). In the case of icing from above, the "Go/No-Go"
decision could be made using a breathalyzer. In contrast, when conditions
would support icing from below there usually is no forecast, pilot report,
regulation, or other tool that will make the "Go/No-Go" decision for you. It
all comes down to your exercise of judgment based on your knowledge of the
prevailing conditions, your understanding of the causes of icing, your
personal level of risk aversion, and your willingness and capability to tackle
a tough flight on any given day.
The Pacific Ocean supplies ample water vapor to the air masses passing over
it. When an airmass hits the Pacific Coast, the coastal mountain range
provides a topographical mechanism that lifts the airmass to cooler altitudes
aloft and promotes condensation and water droplet formation. If the airmass is
unstable, the initial topographical lifting will trigger convective activity
which will provide yet another lifting mechanism. A little more than a hundred
miles inland, the airmass will encounter the inland mountain range which
provides still another lifting mechanism. If the airmass is a front, the front
itself will provide a lifting mechanism.
The first step to tackling a flight in winter conditions anywhere that
icing is commonly a problem is to check to see what the forecasters think
might be in store. The forecasts and AIRMETs for icing conditions may or may
not be any more accurate than they used to be, but they are much easier to
interpret now that the NOAA has made graphs available. Check out the following
sites for graphic representation of the areas covered by all AIRMETs and
Don't worry if you lose the URLs. They are available in AVweb's weather section.
As long as you're online, check out the rest of the weather as well at :
Out west there usually is a forecast or an AIRMET for occasional light to
moderate rime and mixed icing in clouds and precipitation and it usually
covers an area of thousands of square miles. Whether used in a forecast or an
AIRMET, "occasional" is defined to mean a MORE than 50/50 chance of occurrence
during LESS than half of the forecast period. A forecast of "occasional" icing
is therefore a prediction both that icing will probably exist and that icing
will probably not exist at any given time and place in the coverage area. You
are expected to understand that icing is frequently a localized and transitory
phenomenon. A forecast of icing in an area consisting of thousands of square
miles is not intended to suggest that icing is likely to exist throughout that
area at any given time. Rather, it is a "heads up" that you could encounter
icing. A PIREP is likewise just another clue. It tells you where icing was
encountered a little while ago.
Armed with the knowledge that there is a risk of icing, your next step is
to determine the probable location and severity of the condition along your
route of flight. The essential element to icing is that your airframe be at a
below freezing temperature. The winds aloft forecast will give you the
freezing levels which you can compare to the MEAs along your route.
You will want to determine the active lifting
mechanism(s) because the more lifting, the bigger the drops, the more severe
the icing. As discussed above, lifting results from terrain, frontal activity
and/or convective activity. The lifting effects are additive so the biggest
supercooled water drops will occur at the highest altitudes inland from where
the airmass is unstable and a front is crossing the coastal mountains. The
surface analysis and prog charts will show you the current and forecast
frontal locations. The synopsis in the Area Forecast will alert you to
convective activity. Laying your VFR sectional chart alongside your IFR
low-altitude chart will assist you in identifying areas where your planned
route crosses areas where rising terrain will provide topographical lifting.
If the synopsis would support developing of icing on shore (over the coastal
mountains) and your route of flight is offshore, you might determine that
icing is not probable along your route.
The final necessary component for icing is visible moisture. Check out the
satellite photos on line. At times, although icing is forecast over a broad
area, only a small portion of the area is actually covered by clouds. On a
recent flight up to Seattle, icing was in the forecast because of predicted
cumulous development. Sure enough, upon my arrival there were numerous
towering cumulus clouds upwards of 10,000 feet tall. There were PIREPs of
icing from several inbound pilots. At the same time, the basic conditions were
VFR. As the controller told me, "Why don't they just fly around those
Once you have determined the specific areas where you might likely
encounter icing conditions, you need to assess your available escape routes
from those areas. If there is an easy escape, you can be more aggressive on
your go/no-go decision making. If there is no easy escape ... well ...
surviving nine flights out of ten just doesn't cut it.
Once you're en route, keep an eye out for the first indication of ice.
Movement of air across the skin surface heats the skin and tends to sweep the
liquid moisture away before it contacts the aircraft skin. The first
indications of icing will therefore occur in areas of dead air, facing the
airplane's flight path, such as at the base of the glare screen, the area
around the OAT probe and the leading edge of the wing. In those locations, the
airflow is not across the skin surface because of obstructions to the airflow
(such as around the OAT probe), because the air cannot flow smoothly along the
surface (such as at the intersection of the cowling and the glare screen), or
because the surface is perpendicular to the airflow (such as at the stagnation
point along the wing's leading edge). That keeps the skin surface relatively
cool at those locations and prevents the airflow from sweeping the liquid
moisture out of harm's way. If you see it coming, get out of Dodge.
Based on all of the above, despite forecasts for icing, AIRMETs for icing,
and PIREPs of icing, you might determine that icing presents a manageable risk
for your flight and that it is appropriate to proceed. You have taken and can
take "adequate precautions" to complete the flight safely, but is it legal?
Keep a watch at AVweb — we'll discuss that issue in a couple