SofemaOnline (SOL) www.sofemaonline.com Considers Aspects of Aircraft Ground Icing & the potential for Performance Degradation.

Introduction - There is no such thing as an insignificant amount of ice.

As we understand any roughness on an aerodynamic surface (caused by ice, snow, or frost) is likely to disrupt the airflow over the lift and control surfaces of an aircraft.

Frost, ice, or snow formations on an aircraft may decrease the lift and alter the stall and handling characteristics. Aircraft may become airborne in ground effect but be unable to climb.

• Loss of lift could be severe with increased drag and impaired maneuverability and is particularly evident during the take-off and initial climb phase.
• Ice can also

»  Affect the movement of control surfaces

»  Add to the weight of the aircraft

»  Has the potential to Block Pitot Static Ports or interfere with Stall Vanes

Cold Soaking - Can cause frost to form on the upper and lower wing under conditions of high relative humidity.

After a flight, the temperature of an aircraft and the fuel carried in the wing tanks may be considerably colder than the ambient temperature.

• Cold Soaking may occur in above freezing weather at airports where there is normally no need for de-icing equipment.
• Typically occurs where the fuel in the wing tanks becomes cold-soaked to below freezing temperatures because of low-temperature fuel uplifted during the previous stop or cruise at altitude where low temperatures are encountered.

»  Frost can form on the under and upper sides of the fuel tank region during the ground turn-around time and tends to reform quickly even when removed.

»  Post Flight the temperature of an aircraft and the fuel carried in the wing tanks may be considerably colder than the ambient temperature.

Clear Ice

Cold Soaking can cause clear ice to form on wing areas above the fuel tanks.

• Such ice is difficult to see and, in many instances, cannot be detected other than by touch with the bare hand or by means of a special purpose ice detector such as a Ground Ice Detection System (GIDS).

Note 1 - A layer of slush on the wing cannot be assumed to flow off the wing on take-off and must be removed. ( A layer of slush can also hide a dangerous sheet of ice beneath)

Note 2 - Sheets of clear ice can dislodge from the wing or fuselage during take-off and impact control surfaces or be ingested by rear-mounted engines

Ice Accretion

The amount of ice forming will depend on

• Type, depth, and liquid content of precipitation,
• Ambient air temperature and
• Wing surface temperature.
• Contributing factors such as

»  Low temperature of the fuel uplifted by the aircraft during a ground stop

»  Duration of Previous Sector resulting in a situation where the remaining fuel in the wing tanks is sub-zero.

»  Fuel temperature drops of up to 18C have been recorded after a flight of two hours;

Note 1 - Heavy freezing has been reported during drizzle or rain even in a temperature range between +8C to +14C.

Note 2 - Ice or frost formations on upper or lower wing surfaces must be removed prior to take-off. (lower wing frost may be allowed in accordance with TCH Instructions.)

Freezing Rain Caution

Hold Over Times for aircraft anti-icing fluids have not been evaluated under moderate and heavy freezing rain conditions and aircraft have not been certified to fly in freezing rain conditions.

The operation of an aircraft during freezing rain conditions should be avoided whenever possible.

Freezing Drizzle

Aircraft anti-icing fluids provide greater protection for freezing drizzle than for freezing rain. Avoid if possible

Exercise Caution as high winds or high taxi speeds can increase the effective precipitation rate for freezing drizzle. (Freezing drizzle can also be so light that it is almost unnoticeable)

Ice Pellets - May be spherical, irregular, or conical in shape. Ice pellets usually bounce and make a sound upon impact.

Internationally recognized & known as

• Sleet - Generally transparent, globular, solid grains of ice that have formed from the freezing of raindrops or the refreezing of largely melted snowflakes when falling through a below-freezing layer of air near the earth’s surface.

• Small Hail - Generally translucent particles, consisting of snow pellets encased in a thin layer of ice. The ice layer may form either by the accretion of droplets upon the snow pellet or by the melting and refreezing of the surface of the snow pellet. It is believed that the ice pellets are capable of penetrating the fluid and have enough momentum to contact the aircraft's surface beneath the fluid.

Snow Pellets

• A kind of precipitation, which consists of white and opaque grains of ice.

»  These grains are spherical or sometimes conical; their diameter is about 2-5 mm. Grains are brittle, easily crushed. They do bounce and break on hard ground.

Hail

Precipitation of small balls or pieces of ice with a diameter ranging from 5 mm to greater than 50 mm falling either separately or agglomerated.

Caution:

• The capability of anti-icing fluid to tolerate a heavy snowfall rate has not been evaluated;
• Holdover times for heavy snow conditions have not been generated.
• The holdover time performance of an anti-icing fluid in the presence of ice pellets has not been evaluated, (however to note that it is expected to be extremely short).

Frost on the underside of the Wing

• May conduct a take-off in an aircraft that has frost adhering to the underside of its wings that is caused by cold-soaked fuel, if the take-off is conducted in accordance with the aircraft manufacturer's (TCH) instructions for take-off under those conditions.

Hoarfrost

• A uniform thin white deposit of fine crystalline texture, which forms on exposed surfaces during below-freezing, calm, cloudless nights with the air at the surface close to saturation but with no precipitation.
• The deposit is thin enough for surface features underneath, such as paint lines, markings, and lettering, to be distinguished.

Frost on the Fuselage

it may be acceptable for aircraft to take-off when hoarfrost is adhering to the upper surface of the fuselage (If it is the only remaining contaminant) - Again I.A.W. TCH Instructions

• Provided all vents and ports are clear. Also, aircraft with wing-mounted engines can take off in such conditions however, for both types this can only be conducted in accordance with the aircraft manufacturer’s instructions.

Snow

The highest snowfall rates occur near 0 C. The Visibility in Snow versus Snowfall Intensity

• The snowfall rate will be used to determine which HOT Guideline value will be appropriate for the fluid in use.

Next Steps

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