15 Icing

Question 1

Super cooled water droplet sizes

Answer 1

• 0 degree C to -15 degrees C: Larger SCWD

- -15 degrees C to -40 degrees C: Smaller SCWD

Question 2

Types of clouds and their dangers of Icing:

Answer 2

• High level cloud: Nil
• medium level cloud: Light to moderate
• Stratus: Occasional light to moderate
• Stratocumulus: Light to moderate
• Cumulonimbus, Nimbostratus and Cumulus: Moderate to severe

Question 3

Types of Icing

Answer 3

• Hoar Frost
• Rime Ice
• Clear / Glazing Ice
• Mixed Ice
• Freezing Rain
• Cloudy Ice / Packed Snow

Question 4

Hoar Frost

Answer 4

• Light and brittle
• Caused by sublimation (deposition) when below 0 degrees C
• Normally occurs on the ground or when flying from an area of cold air to an area of warm humid air, where moisture freezes on contact
• Disappears when airframe warms

Question 5

Rime Ice

Answer 5

• Formed by small SCWD freezing on contact
• Builds on the leading edge of the wing
• Opaque (air bubbles) and brittle
• De-icing boots remove this
• Occurs on an airframe below 0 degrees C, in high altitude clouds and in stratus. On the ground it forms in freezing fog

Question 6

Clear / Glazing Ice

Answer 6

• Formed by large SCWD freezing on contact with an airframe below 0 degrees C.
• The initial freezing releases latent heat which allows the remainder of the droplet to flow back across the wing freezing as it goes.
• Clear / transparent so hard to see
• Forms quickly
• Occurs in low to medium convective (cumuloform) clouds or nimbostratus with orographic intensification.

Question 7

Mixed Ice

Answer 7

• Combination of Rime and Clear Ice
• Creates a ‘V’ shape on the leading edge
• Cloudy/milky colour

Question 8

Cloudy Ice / Packed Snow

Answer 8

• Ice crystals stick to wet airframe
• Ice freezes the water forming a rough cloudy ice
• If snow is present, same process occurs producing deposit that looks like tightly packed snow

Question 9

Freezing rain / drizzle

Answer 9

Requires a unique set of meteorological events to occur:

• Clouds producing precipitation are in a warm air mass (>0)
• The rain falls down into an area of cold air (<0)
• If this rain hits an airframe below 0 this will cause icing

Question 9

Orographic Intensification of Icing

Answer 9

Orographic uplift can greatly increase the severity of ice accumulation because,

• Droplet size and quantity increase due to the increase in vertical support of the air
• 0 degrees C isotherm lowers, which increases the depth of the potential freezing zone

Question 10

Carburettor Icing

Answer 10

• Build up of ice in the venturi which restricts air flow.
• Caused by the vaporising of fuel and the drop in pressure at the venturi.
• Most acute in warm, moist conditions with the throttle at or close to idle

Question 11

Intake Ice

Answer 11

• Also known as Impact Ice
• Ice builds up around the intakes which reduces air flow and restricts engine power
• Ice can break off and be ingested by the engine causing damage to the turbines

Question 12

Fuel Freezing

Answer 12

• Water suspended in the fuel freezes

- Can block fuel pipes