ICINGICING

OVERVIEWOVERVIEW

• ICING TYPES (3)ICING TYPES (3)

• ICING SEVERITY (4)ICING SEVERITY (4)

• ICING FACTORSICING FACTORS

• SUPERCOOLED LARGE DROPLETSSUPERCOOLED LARGE DROPLETS

• FAVORED AREASFAVORED AREAS

• LOCAL AIRCRAFT RESTRICTIONSLOCAL AIRCRAFT RESTRICTIONS

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 TYPESICING TYPES

RIME ICINGRIME 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 TYPESICING TYPES CLEAR ICINGCLEAR 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 TYPESICING TYPES MIXED ICINGMIXED ICING

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 SEVERITYICING SEVERITY TRACETRACE

LIGHTLIGHT

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 SEVERITYICING SEVERITY

MODERATEMODERATE  

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 anti-icing 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 SEVERITYICING SEVERITY

SEVERESEVERE

Icing is severe whenever the rate of ice accumulation is such that de-icing or anti-icing equipment cannot control or reduce the hazard. Typically an immediate heading and/or altitude change is necessary.

ICING SEVERITYICING SEVERITY

 

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 FACTORSICING FACTORSAIRFRAMEAIRFRAME

 

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 FACTORSICING FACTORSENVIRONMENTALENVIRONMENTAL

--LWC is higher in cumuliform clouds than stratiform clouds

--Higher levels in orographic and frontal lifting situations

--Clouds with warm bases

ICING FACTORSICING FACTORSLIQUID WATER CONTENTLIQUID 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.

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:  

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.

TYPE TEMPS

Clear 0° C to -10° C

Mixed -10° C to -15° C

Rime -15° C to - 40° C

Rime icing is the most frequently reported icing type.

ICING FACTORSICING FACTORS TEMPERATURETEMPERATURE

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. De-icing 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.

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.

ICING FACTORSICING FACTORSDROPLET SIZEDROPLET SIZE

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.

SUPERCOOLED LARGE SUPERCOOLED LARGE RDROPLETS (SLD)DROPLETS (SLD)

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 (collision-coalescence). The presence of freezing precipitation at the surface is a good initial indicator of SLD aloft.

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.

SUPERCOOLED LARGE SUPERCOOLED LARGE R DROPLETS (SLD)DROPLETS (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.

FAVORED AREAsFAVORED AREAs SLDSLD

Turbulent motions and mixing result in larger drops that grow either by enhanced condensation or collision processes.

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.

LOCAL AIRCRAFT RESTRICTIONSLOCAL AIRCRAFT RESTRICTIONS

Questions?Questions?