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1

5 factors involved in the formation type and intensity of icing

temperature
moisture
nuclei
lift
stability of air

2

temperature

-requires sub-zero temperatures (air and airplane)
-super-cooled water droplets which are common down to -15c and cease around -20c
-most supercooled water disappears -30c and -40c

3

moisture

-water vapour will not produce significant icing, only hoar frost
-super-cooled water is the main concern for forecasting icing, but it must be inferred
-Large bodies of water can effect icing through heat and moisture fluxes - if there is convection or FZDZ icing could be MDT MXD or worse
-otherwise rime icing is likely

4

LWC

liquid water content- is a measure of the density of water-based cloud.

5

Nuclei

-ice nuclei and ice crystals grow at the expense of supercooled water droplets
-the greater amount of freezing nuclei, the lower the threat of icing
-icing is at it's peak at temperatures close to freezing

6

Lift

-Mechanical, convective or synoptic
-the stronger the lift, the bigger the droplet size
-the greater the intensity of lift, the greater the liklihood that clear icing will occur

7

Strong lift is found in:

-Strong synoptic features - intense deep lows, sharp troughs
-Unstable clouds - ACC, TCU, CB
-orographic clouds

8

Stability of the air

- Upward motion will form and support more abundant and larger water droplets.
-Concentration of LWC will be higher in thicker cloud
- cloud associated with stable air is usually thinner but spread over a large horizontal area, also can be close to the ground. Critical for arrival and departure procedures.
-Unstable air sustains upward vertical motion giving larger water droplets
- LWC can vary at different levels of unstable cloud types giving different intensities and types of icing

9

Clouds associated with icing:
Thin Stratiform
F, ST, thin AS

-stable structure
-large horizontal, small vertical extent
-usually no precip
-low LWC, small droplets
-low prob
-LGT, RIME only
-0c to -15c only
**thicker ST can produce DZ/FZFD, which could give MDT MXD or CLR in lower levels

10

Clouds associated with Icing:
Thick Stratiform
AS, NS

-associated with warm frontal zones
-continuous advection of moisture up frontal surface
-precipitation or lack of is not an indication of icing
-intensity affected by strength of vertical velocity
-high LWC
-no icing in CS
-high probability of icing
-LGT to MDT (SEV within 150 to 250 ahead of front)
-RIME
-SVR CLR possible close to front in FZRA

11

Clouds associated with icing:
Weak cumuliform
CU,SC,AC

-Some stability, usually capped by an inversion
-can have some areas of moderate ascent
-dependent on source region
-consider age/composition of cloud
-high LWC, varying droplet size
-50% probability of icing
-intensity depends on level of instability LGT-MDT

12

Clouds associated with icing:
Strong cumuliform
TCU,CB, ACC

-often associated with cold frontal zones
-very unstable
-high vertical velocities
-supports and carries large super-cooled water droplets to high altitudes and cold temperatures
-highest icing intensities just before surface showers
-hazards diminish as ice crystals multiply, precip occurs, downdrafts
-large vertical small horizontal extent
-probability near 100%
-MDT-SEV
-MXD-CLR

13

Impacts of aircraft icing:

-Loss of climb capability
-reduced visibility
-increased weight/decreased lift
-increased drag/stall speed
-increased fuel consumption/decreased range
-accretion will be greater and faster on vertical and horizontal stabilizers than on wings
-airflow separation will affect control surfaces
-pitot and static ports can get blocked

14

Common Icing counter measures

-pitot and carb heat
-small cockpit controlled heaters
-heating elements
-leading edge airblead
-pnuematic booties
-inflatable bladders
-in flight de-ice liquid application, in flight and during T/O and landing
-Visual inspection of aircraft and control surfaces