ICING
OVERVIEW
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ICING TYPES (3)
ICING SEVERITY (4)
ICING FACTORS
SUPERCOOLED LARGE DROPLETS
FAVORED AREAS
LOCAL AIRCRAFT RESTRICTIONS
ICING TYPES
RIME ICING
Rime ice grows when droplets
rapidly freeze upon striking an
aircraft. The rapid freezing traps air
and forms a brittle, opaque, and
milky-colored ice. Rime ice grows
into the air stream from the forward
edges of wings and other exposed
parts of the airframe.
ICING TYPES
CLEAR ICING
In clear ice formation, only a small portion
of the drop freezes immediately while the
remaining unfrozen portion flows or
smears over the aircraft surface gradually
freezing. Since few air bubbles are
trapped during this gradual process, the
end result is ice less opaque and denser
than rime ice that can appear either as a
thin smooth surface or as rivulets,
streaks, or bumps of clear ice.
ICING TYPES
MIXED ICING
Due to small-scale (tens of kilometers or
less) variations in LWC, temperature, and
droplet sizes, an airplane can encounter
both rime and clear icing along its flight path.
Known as mixed icing, this can appear as
layers of relatively clear and opaque ice
when examined from the side. Mixed ice is
similar to clear ice in that it can spread over
more of the airframe's surface and is more
difficult to remove than rime ice.
ICING SEVERITY
TRACE
The trace category is used when the rate of ice accumulation is just
slightly greater than the rate of loss due to sublimation. This category of
icing is not hazardous. De-icing, anti-icing equipment, or an altitude
change are not necessary unless this category is encountered for one
hour or more.
ICING SEVERITY
LIGHT
The light icing category means that the rate of ice accumulation may create a problem if
the aircraft remains in this environment for one hour or more. Occasional use of de-icing
or anti-icing equipment is necessary to remove or prevent accumulation. When
prolonged flight in this environment is likely, a heading or altitude change becomes
necessary.
ICING SEVERITY
MODERATE
The icing category is classified as moderate when the rate of ice accumulation is so
great that even a short encounter can become hazardous. The use of de-icing or antiicing equipment is necessary. Often a heading or altitude change is also required
especially if the aircraft remains in the moderate icing environment for more than a very
short period.
ICING SEVERITY
SEVERE
Icing is severe whenever the rate of ice accumulation is such that deicing or anti-icing equipment cannot control or reduce the hazard.
Typically an immediate heading and/or altitude change is necessary.
ICING FACTORS
AIRFRAME
Icing severity and type depends on the properties of the aircraft as well as the
atmospheric conditions. Although forecasters cannot be expected to know and
understand all the aircraft-specific icing influences, they should understand
some basics;
Shape and Size: Ice accumulates faster on large, non-streamline airframes
with rough surfaces than on thin, smooth, highly-streamlined airframes.
Cargo –VS- Fighter
Airspeed: Rate of ice formation increases with airspeed, however at higher
(supersonic) airspeeds friction creates heat that melts structural icing. Typically,
icing is negligible at speeds above 575 kts.
Cargo –VS- Fighter
ICING FACTORS
ENVIRONMENTAL
After considering the supported airframe, forecasters need to focus on
diagnosing the icing environment. The meteorological quantities most
closely related to icing severity and type are, in order of importance:
1. Liquid water content (LWC)
2. Temperature (Altitude)
3. Droplet size
ICING FACTORS
LIQUID WATER CONTENT
LWC is the most difficult factor to accurately forecast in the icing formula. LWC
can vary significantly in the vertical and horizontal planes of a seemingly uniform
cloud deck. The LWC levels weigh more heavily on icing severity than on icing
type. Forecasting Rule-of-Thumb: Increases in LWC levels are indicative of
increased accumulation. Although, some research aircraft data contradicts the
previous ROT.
--LWC is higher in cumuliform
clouds than stratiform clouds
--Higher levels in orographic
and frontal lifting situations
--Clouds with warm bases
ICING FACTORS
TEMPERATURE
Generally, temperature can be a good indicator for diagnosing the type of icing
expected. However, there are often instances where the temperature-icing type
relationship is not well defined. The relationship between temperature and icing
type that is typically used is outlined
in the following table:
TYPE
Clear
TEMPS
0° C to -10° C
Mixed
-10° C to -15° C
Rime
-15° C to - 40° C
These relationships are meant to be used as general
guidance and should not be used explicitly without
taking into consideration the effects of LWC and
droplet size, and other aircraft-dependent factors.
Rime icing is the most frequently reported icing type.
ICING FACTORS
DROPLET SIZE
Although droplet size variation can have an influence on icing, it has not been
found to be as important as LWC and temperature variations unless the droplet
size is larger than those classified as cloud droplets. Cloud droplets are
generally considered to be those with diameters smaller than 40 microns.
Droplet size influences the collection efficiency of drops on the airframe. Small
droplets have little mass and momentum. As the airplane flies through the air,
these tiny drops tend to be swept around the airframe, following the airflow
streamlines. If drops are to impact at all, they will likely do so near the leading
edges where the air diverges to go around the airfoil.
As drop size increases, so does the tendency for drops to cross the
streamlines and collide with the airframe where they can accrete. Larger
drops that cross the streamlines can impact farther aft along the airfoil. Deicing and anti-icing devices are typically located near the forward edges of
airfoils, so ice forming aft is difficult to remove in flight unless melting or
sublimation are taking place.
SUPERCOOLED LARGE
RDROPLETS (SLD)
Icing caused by supercooled large droplets (> 40 microns in diameter) is
extremely hazardous to aviation. Large droplets form a very lumpy textured ice
(similar to glass in a bathroom window) . This lumpy texture significantly disrupts
airflow and the aerodynamics of the aircraft. Further, these drops can flow along
the airfoil for some distance prior to freezing.
The net result is ice accreted on surfaces
beyond the reach of de-icing equipment.
SLD are either formed through melting of
ice and subsequent supercooling of the
drops (warm layer process), or through
droplet growth processes within a
supercooled environment (collisioncoalescence). The presence of freezing
precipitation at the surface is a good initial
indicator of SLD aloft.
SUPERCOOLED LARGE
R DROPLETS (SLD)
Even a small amount of ice on the lower and upper surfaces of the airfoil can
seriously disrupt its aerodynamic properties.
The bottom panel illustrates one of the
unique properties of this ice type to form
aft, beyond the reach of de-icing
equipment's effectiveness. The result is
ice remaining on the airfoil continuing to
disrupt the airflow and reduce the aircraft's
aerodynamic integrity.
In extreme cases, turbulence and flow
separation bubbles can travel along the
airfoil and inadvertently activate the ailerons
(movable flaps on the wings used to control
rolling and banking movements), creating
dangerously unstable flying conditions.
FAVORED AREAs
SLD
SLD, especially freezing drizzle, tend to occur in
areas where gradual lifting, shallow, saturated
conditions and relatively warm cloud tops (-12° C)
coincide.
Preferred regions are:
•Ahead of warm fronts
•Behind low pressure centers
•Post-frontal upslope
These regions appear to share the common
element of a change in horizontal wind speed and/or
direction with height at some level within the cloud.
This is most common ahead of the warm frontal
zone within a synoptic-scale cyclone. However, post
frontal upslope conditions are often associated with
a narrower shear zone near cloud top that can also
enhance SLD growth.
Turbulent motions and mixing result in larger drops
that grow either by enhanced condensation or collision
processes.
LOCAL AIRCRAFT RESTRICTIONS
A-10 Aircraft cannot operate in forecasted Moderate or Severe Icing.
F15 Aircraft may penetrate areas of Icing. However, Loitering in these areas is
Prohibited.
F-16 Flight in Areas of Icing should be Avoided whenever possible. Aircraft
should Minimize Duration of Time in Icing Conditions.
F-15/16 Aircraft Performing at Supersonic and Transsonic Speeds Have A
Built-In Ice Prevention System for Wing and Empannage Surfaces Due to
Aerodynamic Heating.
MC-130 Icing >= Moderate Intensity Prevents Flight Operations and can Cause
Damage to the Aircraft
Rotary Aircraft should Avoid all Areas of Any Icing Conditions.
General Flight Rules: Pilots will not Takeoff with Ice on the Aircraft. Deicing is
required before Takeoff.
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