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PPL Met > icing > Flashcards

icing Flashcards

1
Q

What conditions allow for supercooled water droplets to form?

A
  • Temp below 0º.

- RH reaches 100%

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2
Q

What happens to supercooled water droplets below -40ºC?

A

They freeze immediately.

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3
Q

What happens to supercooled water droplets above -40ºC?

A

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

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4
Q

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

A

No.

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5
Q

What are supercooled water droplets?

A

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

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6
Q

What is one important characteristic of super-cooled water?

A

Its instability.

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7
Q

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

A
  • Prior, it remains in a liquid state.

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

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8
Q

What are the several factors affecting the way ice forms?

A
  1. ) Latent heat.
  2. ) Droplet size.
  3. ) Kinetic heating.
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9
Q

What energy is released when water freezes?

A

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

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10
Q

How does ice form with regard to latent heat?

A
  • 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.
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11
Q

What determines at which ice freezes due to latent heat?

A

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

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12
Q

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

A

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

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13
Q

What determines the severity of icing?

What happens when severity is increased?

A
  • Severity is increased if the droplets are large.

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

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14
Q

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

A

Thin aerofoil.

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15
Q

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

A
  • Air temp below 0ºC.
  • Aircraft surface temperature below 0ºC.
  • Presence of water droplets at temps below 0ºC.
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16
Q

How does kinetic heating affect ice formation?

A
  • 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.
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17
Q

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

A
  • Clear ice.
  • Because it can form a solid layer over the airframe and it can seriously affect the aerodynamic properties of the wing.
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18
Q

What are the conditions required for clear ice to form?

A
  • Large water droplets.

- Cloud with a high liquid water content.

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19
Q

What determines the severity of clear ice formation?

A

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

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20
Q

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

A
  • Relatively slow.

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

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21
Q

What happens if clear ice mixes with solid precipitation?

A

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

22
Q

What is rime ice? Where does it occur?

A
  • Rough opaque deposit.

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

23
Q

What conditions are required for rime ice to form?

What does this result in?

A
  • 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.
24
Q

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

A

During the freezing process.

25
Q

Why does rime ice not spread beyond the leading edge?

A

Because it freezes quickly.

26
Q

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

A
  • Leading edge into a nose shape.

- If not removed, it can seriously affect the aerodynamic properties of the wing.

27
Q

What is hoar frost?

A
  • 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.
28
Q

What is hoar frost the product of?

A

De-sublimation (deposition).

29
Q

How does hoar frost occur? (4)

A
  • 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.
30
Q

How does hoar frost persist?

A
  • Remains as long as the temperature of the surface remains below zero.
31
Q

How thick is hoar frost?

A
  • Very thin layer, still affects flight surfaces.
32
Q

What causes carburettor icing to form?

A
  • High humidity due to mist, fog, clouds, or precipitation, may cause ice to accumulate in the engine air intake and carburettor.
33
Q

How does carburettor ice form?

A
  • 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.

34
Q

Why is carburettor icing bad for aircraft?

A
  • Disturbs or reduces the supply of air to the engine.

- Can result in loss of power or even engine failure.

35
Q

When is carb icing most frequent and less frequent?

A
  • Most frequent in air temps of 10 to 15ºC combined with any form of precipitation.
  • Less frequent in low freezing temperatures.
36
Q

What are conditions required for serious carburettor icing?

A
  • In the temp range of 0ºC to 15ºC, with dew point depressions of less than 9ºC
37
Q

What are the other conditions which allow for carburettor icing?

A

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

38
Q

What is the temperature range for carb icing?

A

Between 38ºC and -15ºC.

39
Q

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

A
  • Rough engine running.
  • Loss of manifold pressure (or fluctuating RPM).
  • Engine vibration.
  • Turn carb heat on to get rid of carb icing.
40
Q

What is impact icing and how does it occur?

A

Ice blocking the engine air take

-Due to the engine air intake being blocked up with ice.

41
Q

How can you get rid of impact icing?

A

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

42
Q

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

A

Ice crystals.

43
Q

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

A

Mixture of water droplets and ice crystals.

44
Q

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

A

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

45
Q

What type of ice is associated with stratus clouds?

What is the risk of severe icing with stratus clouds? Why?

A
  • Light rime ice.

- Low. Because of its thin layer.

46
Q

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?

A
  • Clear ice.
  • Significant because cumulus clouds have large water droplets and a high water content.
  • Updraughts.
47
Q

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

A

On leading edges.

48
Q

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

A
  • Results in higher content of supercooled water droplets in the freezing part of the cumulus or cumulonimbus cloud.
49
Q

What clouds are associated with warm fronts?

When is icing most likely to occur during warm fronts?

A
  • 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.
50
Q

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

A

Freezing rain.

51
Q

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

A
  • 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.

PPL Met (20 decks)

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