Icing

Question 1

Define supercooled water droplets

Answer 1

Droplets in liquid state between 0-40ºC
Forms in all clouds but cirrus
Unstable as it wants to freeze
Colder it is the smaller the SCWD

Question 2

Describe the formation of clear (glaze) ice

Answer 2

Large SCWD strokes aircraft and starts to freeze. Release latent heat delaying freezing and the part not freezing will flow back over the surface the freeze more.

Most dangerous as builds up quickly

Question 3

Describe the formation of rime (opaque) ice

Answer 3

Small SCWD or small cloud droplets freeze on impact

Whole droplet becomes solid at once

Air trapped between frozen droplet make it opaque and brittle

Question 4

Describe the formation of mixed ice

Answer 4

Mixture of large and small SCWD, builds up rime ice on leading edge and clear ice behind.

Worst effects of both

Question 5

Describe the formation of hoar frost

Answer 5

White crystal deposit similar to frost on the ground. Occurs in clear air when airframe is below 0ºC.

Water vapour in contact with air frame deposition into ice, rapid descent from very cold region to warm moist layer or climb from cold into an inversion.

Question 6

Describe the formation of freezing rain

Answer 6

Freezing rain develops as falling snow encounters a layer of warm air deep enough for the snow to completely melt and become rain. As the rain continues to fall, it passes through a thin layer of cold air just above the surface and cools to a temperature below freezing.

Only found ahead of a warm front

Question 7

What clouds form what ice?

Answer 7

Clear ice forms in clouds with large droplets
- Ns, Cu, Cb

Rime ice forms in clouds with small droplets
- Ns, As, Ac, SC, St

Mixed ice = mixture

Question 8

Temperature ranges of icing

Answer 8

Clear ice - 0-20ºC

Rime -20ºC to -40ºC

Mixed ice - 10ºC to -30ºC

Question 9

What are the height ranges relative to freezing level

Answer 9

Clear ice = freezing level to 10,000ft

Rime = 10,000ft above to 20,000ft above FL

Mixed ice = 5,000ft to 15,000ft above FL

Question 10

What are enhancing factors for ice

Answer 10

Frontal lifting
Orographic lifting
Convection
Updraft portion of lee waves

Question 11

Explain factors that influence the rate of ice accretion

Answer 11

Size of the droplet - bigger is more
Number of droplets - more droplets, faster it is
Speed of aircraft - as it speeds up it collects more droplets increasing ice accretion
Design of aircraft - designed poorly can lead to increase

Question 12

Hazards of icing to flight

Answer 12

Increase drag, weight, increased stall speed

Distortion of aerodynamic shape reducing lift

Loss of thrust due to build up on propeller

Impact icing

Pitot/static blockage

Long aerials can freeze and break off or poor reception

Fuel tank vents or battery vents blocked

Question 13

List the intensity classifications of icing

Answer 13

•Light = Minor coverage, safe to fly through for an hour without use of de-icing/anti-icing

•Moderate = substantial build up in few minutes, must use de-icing/anti-icing

•Severe = Immediate diversion required, de-icing/anti-icing ineffective

Question 14

Droplet size of different icing

Answer 14

Clear ice = large

Rime = small

Mixed = mix

Question 15

Explain methods of avoiding or mitigating airframe icing

Answer 15

Avoid flying in updrafts of lenticular or any cloud

Fly higher to get below -40ºC where icing doesn’t occurs as SCWD are small

Use de-icing/anti ice equipment

Stay below freezing level

Question 16

Explain deposition (ATPL)

Answer 16

Water vapour to ice without water droplets forming, latent heat is released warming atmosphere.

Increases instability

Question 17

Describe sublimation (ATPL)

Answer 17

Ice to water vapour
Latent heat absorbed, cools atmosphere and increases stability

Question 18

Explain what happens when an aircraft Collides with SCWD (ATPL)

Answer 18

Aircraft is ice nuclei, will form ice on contact with leading edge, rest will flow back and cool on the airframe.

Aircraft doesn’t need to be below 0ºC for icing

Rime ice forms when temps are colder than about - 20ºC

Clear ice forms between 0 - -20ºC

Question 19

Describe the freezing rain process (ATPL)

Answer 19

Only occurs in warm fronts where warm rain falls into air below 0ºC same as CPL.

Question 20

State areas of the globe where freezing rain is most likely to be encountered

Answer 20

Can happen in maritime and continental climates, most common in polar climates

Question 21

Identify the symbols used to indicate different types of icing (ATPL)

Answer 21

Light - no symbol

Moderate - 2 lines

Severe - 3 lines