icing

Question 1

What conditions allow for supercooled water droplets to form?

Answer 1

• Temp below 0º.

- RH reaches 100%

Question 2

What happens to supercooled water droplets below -40ºC?

Answer 2

They freeze immediately.

Question 3

What happens to supercooled water droplets above -40ºC?

Answer 3

Droplets may remain liquid for longer depending on their size and purity.

Question 4

Do larger, supercooled water droplets exist longer than smaller ones at lower temperatures?

Answer 4

No.

Question 5

What are supercooled water droplets?

Answer 5

Supercooled water droplets are those that remain liquid below 0 degrees.

Question 6

What is one important characteristic of super-cooled water?

Answer 6

Its instability.

Question 7

What happens when supercooled water and prior and during encountering a solid surface?

Answer 7

• Prior, it remains in a liquid state.

- When it encounters a solid surface, it automatically freezes.

Question 8

What are the several factors affecting the way ice forms?

Answer 8

1. ) Latent heat.
2. ) Droplet size.
3. ) Kinetic heating.

Question 9

What energy is released when water freezes?

Answer 9

Heat energy known as latent heat is released as water freezes.

Question 10

How does ice form with regard to latent heat?

Answer 10

• As each water droplet meets an aerofoil, it starts to freeze.
• A network of ice needs form surrounded by water.
• Latent heat produced from the freezing process warms the remaining part of the droplet, which then flows over the aerofoil.
• This liquid portion then freezes by conduction.

Question 11

What determines at which ice freezes due to latent heat?

Answer 11

The rate depends on the initial temperature of the water droplets.

Question 12

How many super-cooled water droplets form when they freeze at 0ºC?

Answer 12

Only a small portion of the supercooled water droplet freezes on impact with the aircraft.

Question 13

What determines the severity of icing?

What happens when severity is increased?

Answer 13

• Severity is increased if the droplets are large.

- Inertia will be greater and less likely to follow the airstream.

Question 14

Which aerofoil accumulates more ice, a thin or thick aerofoil?

Answer 14

Thin aerofoil.

Question 15

What are the conditions necessary for airframe icing? (3)

Answer 15

• Air temp below 0ºC.
• Aircraft surface temperature below 0ºC.
• Presence of water droplets at temps below 0ºC.

Question 16

How does kinetic heating affect ice formation?

Answer 16

• As air passes an aerofoil, kinetic heating raises the temperature near the leading edge.
• This results in the temperature varying across the length of the aerofoil.

Question 17

What is the most hazardous type of ice formation? Why?

Answer 17

• Clear ice.
• Because it can form a solid layer over the airframe and it can seriously affect the aerodynamic properties of the wing.

Question 18

What are the conditions required for clear ice to form?

Answer 18

• Large water droplets.

- Cloud with a high liquid water content.

Question 19

What determines the severity of clear ice formation?

Answer 19

When temps are close to zero, clear icing is more severe.

Question 20

How fast is the freezing of super-cooled water droplets?

Answer 20

• Relatively slow.

- Some droplets combine to form a compact sheet of ice.

Question 21

What happens if clear ice mixes with solid precipitation?

Answer 21

The deposit can become rough or uneven and disturb the laminar flow.

Question 22

What is rime ice? Where does it occur?

Answer 22

• Rough opaque deposit.

- Accumulates slowly in exposed areas - mainly the leading edge of aerofoils.

Question 23

What conditions are required for rime ice to form?

What does this result in?

Answer 23

• Low temps.
• Very small water droplets.
• The number of large, supercooled water droplets are small.
• These water droplets freeze almost instantaneously into small ice particles with air trapped between them.

Question 24

When is latent heat mainly released in terms of rime ice?

Answer 24

During the freezing process.

Question 25

Why does rime ice not spread beyond the leading edge?

Answer 25

Because it freezes quickly.

Question 26

Where does rime ice build-up instead? Why is this bad?

Answer 26

- Leading edge into a nose shape. | - If not removed, it can seriously affect the aerodynamic properties of the wing.

Question 27

What is hoar frost?

Answer 27

- A light crystalline deposit often found glistening on rooves and pavements after a clear winter's night. - Winter equivalent of dew. - Only type of airfame icing that occurs in clear air.

Question 28

What is hoar frost the product of?

Answer 28

De-sublimation (deposition).

Question 29

How does hoar frost occur? (4)

Answer 29

- Occurs when descending rapidly from a cold layer, into a warm and moist cloud free layer. - Other way it occurs is when an aircraft takeoffs from a marked surface inversion from a cold airport. - The unsaturated air in contact with the aircraft is cooled to below zero, and below its frost point. - Desublimation occurs and the ice crystals form immediately onto the cold airframe.

Question 30

How does hoar frost persist?

Answer 30

- Remains as long as the temperature of the surface remains below zero.

Question 31

How thick is hoar frost?

Answer 31

- Very thin layer, still affects flight surfaces.

Question 32

What causes carburettor icing to form?

Answer 32

- High humidity due to mist, fog, clouds, or precipitation, may cause ice to accumulate in the engine air intake and carburettor.

Question 33

How does carburettor ice form?

Answer 33

- As air from the intake passes through the carburettor, the Venturi produces pressure and temperature decrease. Further cooling is generated by the injection of fuel into the air stream. Combined cooling effect of the venturi and fuel evaporation will convert the humidity present into ice, accumulating on exposed surfaces of the air intake.

Question 34

Why is carburettor icing bad for aircraft?

Answer 34

- Disturbs or reduces the supply of air to the engine. | - Can result in loss of power or even engine failure.

Question 35

When is carb icing most frequent and less frequent?

Answer 35

- Most frequent in air temps of 10 to 15ºC combined with any form of precipitation. - Less frequent in low freezing temperatures.

Question 36

What are conditions required for serious carburettor icing?

Answer 36

- In the temp range of 0ºC to 15ºC, with dew point depressions of less than 9ºC

Question 37

What are the other conditions which allow for carburettor icing?

Answer 37

A reduced throttle setting in the cruise and on descent, increases the temperature range for carb icing.

Question 38

What is the temperature range for carb icing?

Answer 38

Between 38ºC and -15ºC.

Question 39

What are the symptoms of carb icing and how can you get rid of it?

Answer 39

- Rough engine running. - Loss of manifold pressure (or fluctuating RPM). - Engine vibration. - Turn carb heat on to get rid of carb icing.

Question 40

What is impact icing and how does it occur?

Answer 40

Ice blocking the engine air take | -Due to the engine air intake being blocked up with ice.

Question 41

How can you get rid of impact icing?

Answer 41

Clear the ice by turning the alternate air on and consider descending the aircraft below the freezing level.

Question 42

What type of icing is prevalent at temps below -25ºC?

Answer 42

Ice crystals.

Question 43

What type of liquid/icing is prevalent at temps between -25ºC and -12ºC.

Answer 43

Mixture of water droplets and ice crystals.

Question 44

What type of liquid/icing is prevalent at temps between -12ºC and 0ºC.

Answer 44

Typically pure water, with a small number of ice crystals.

Question 45

What type of ice is associated with stratus clouds? | What is the risk of severe icing with stratus clouds? Why?

Answer 45

- Light rime ice. | - Low. Because of its thin layer.

Question 46

What type of ice is associated with cumulus clouds? What is the risk of severe icing with cumulus clouds? Why? Where can the most serious icing conditions be found in cumulus clouds?

Answer 46

- Clear ice. - Significant because cumulus clouds have large water droplets and a high water content. - Updraughts.

Question 47

Where is clear icing most commonly found when flying through cumulus clouds?

Answer 47

On leading edges.

Question 48

What happens when the temperature is higher at the cloud base level?

Answer 48

- Results in higher content of supercooled water droplets in the freezing part of the cumulus or cumulonimbus cloud.

Question 49

What clouds are associated with warm fronts? | When is icing most likely to occur during warm fronts?

Answer 49

- Stratus. - Precipitation from warm front keeps water content low, resulting the risk of icing being reduced. - Icing is most likely to occur during the development of a warm front when air is lifting along the frontal slope.

Question 50

What type of phenomena is most common ahead of warm fronts in winter?

Answer 50

Freezing rain.

Question 51

What benefits do cold fronts have in terms of cloud and icing?

Answer 51

- Clouds are less extensive and the frontal belt is relatively narrow. - Usually possible to pass through the region before much ice accumulates in a fast jet aircraft - but not in a light aircraft without de-icing equipment.