1. Chapter14 Airframe Icing

2. Airframe Icing Extremely hazardous due to: Increase in profile drag.
Loss of lift.
Power loss.
Increase in weight.
Immobilisation of control surfaces.
Pitot-static system icing.

3. Types of Icing RIME: CLEAR: MIXED: (Cloudy Ice)
Rough, milky, opaque ice formed by instantaneous freezing of small super cooled water droplets.
Forms between -15C to -40C
CLEAR:
A glossy, clear or translucent ice formed by the relatively slow freezing of large super cooled water droplets.
Forms between 0C to -10C
MIXED: (Cloudy Ice)
Mixture of rime and clear ice.
Forms between -10C to -15C

4. Typical Rime Ice Build Up

5. Factors Affecting Ice Formation
Temperature.
Speed
Size and Quantity of Super Cooled Water Droplets (SCWD’s).
Effect of Airfoil Shape.
Cloud base temperature.

6. Effect of Temperature
From just below 0°C to -10°C release of latent heat causes slow freezing and allows clear ice to form.
At low temperatures below -15°C freezing is rapid and rime ice forms.

7. Kinetic Heating for Dry and Wet Airfoil
Evaporation keeps surface temperature at freezing point.

8. Effect of Speed - Kinetic Heating
Kinetic heating raises leading edge temperature.
Typical temperature rises for dry air are :
1°C at 100 kt TAS
4°C at 150 kt TAS
16°C at 300 kt TAS
25°C at 500 kt TAS

9. Effect of Speed - Kinetic Heating
In cloud the kinetic heating effect is dissipated as the energy is absorbed by latent heat of evaporation.
Thus below below 400 kt any effect of kinetic heating can be ignored.
Below FL100 in controlled airspace the speed limit is 250 kt so icing may be an important factor.

10. Effect of Speed on Ice Accretion
The formation of ice is a combination of speed and kinetic heating.
Many small droplets pass without impact.
Water droplets have a higher inertia than air particles therefore some of them will impact the leading edge.
The rate of accretion increases with speed.
But why the double horn shape?

11. Effect of Droplet Weight and Size on Ice Accretion
Divergence of the airflow causes a redistribution in droplet concentration.
The lighter more numerous drops diverging readily and the larger drops impacting with little divergence.
Where large droplets predominate ice accretion is in more of an arc shape.

12. Ram Effect
Significant icing is not possible if kinetic heating raises the leading edge temperature above freezing.
At very high speeds the droplets explode into little globules which are blown back aft of the heated area and quickly freeze and is called “run-back ice.”

13. Affect of Airfoil Shape.
Streamlined shapes have a high collection efficiency as the droplets only divert at the last minute.
Blunt shapes allow the pressure wave ahead of the airfoil to divert the flow relatively further ahead giving droplets time to diverge.

14. Effect of Cloud base Temperature.
For all clouds, the cloud base temperature is a measure of the water vapour content.
The higher the temperature the higher the water vapour concentration and thus the droplet concentration.
In summer the amount of water available to be carried upward in convective cloud is thus always greater.
This results in a greater icing hazard in temperate and tropical convective cloud formations.

15. Forms of Airframe Icing
Hoar Frost.
Forms on parked aircraft after a cold clear night in winter due to deposition on the upper surfaces.
In flight after prolonged cruise at high altitudes when descending into relatively warm moist air at low level.
Rime Ice.
Forms on windward side of parked aircraft when super-cooled fog forms.
In flight in cloud at temperatures below about -10°C.
Clear (Glaze) Ice.
In cloud between 0°C and -10°C
In clear air ahead of a front below the frontal inversion.

16. Freezing Rain (Rain Ice)

17. Clouds most likely to cause icing & turbulence   

18. Cloud Icing Temperature Ranges
Stratiform Clouds:
-40°C to -10°C : Rime, moderate at warmer temp’s.
-10°C to 0°C: Usually clear ice, can be severe in thick NS with orographic uplift.
Cumuliform Clouds:
-45°C to -20°C: Rime, moderate at warmer temp’s.
-20°C to 0°C: Clear ice most likely, can be severe in CB and in tops of rapidly building CU cloud.

19. Special Cloud Icing Cases
Standing Wave Cloud:
Clear ice may form as low as -27°C
Severe Thunderstorms
Clear ice may form as low as -30°C and severe icing may form as low as -45°C.
Frontal NS Cloud
Orographic effect may produce severe clear icing.
Thick SC Cloud:
Over a warm sea surface in winter mod/severe icing may be encountered in upper parts of cloud layer.

20. Power Plant Icing
Normally aspirated piston engines may suffer from carburettor icing.
Remedy use carburettor heat as per ops manual.
Engine intake filters may suffer from impact icing.
Remedy - use alternate air source.
Gas turbines .
Pure jet and low by-pass ratio gas turbines may suffer from inlet icing in high humidity conditions.

21. Carburettor Icing

22. 55%
40%
+35%
+16C
+26C
+32C

23. Induction Icing Ranges

24. Airframe Icing Reports

25. Icing Symbols
Light Icing
Moderate Icing
Severe Icing

26. In-flight Procedure Indicated Temperature
- Thermometer affected by kinetic heating
Radar
- Only large water droplets detected
Type of Ice Accretion
- Opaque: probably not severe
- Clear: rapid growth; immediate action necessary

27. In-flight Procedure, contd
Melting & Evaporation of Ice
- Critical temperature can be above 0°C
- Important indicator is wet-bulb temperature

28. Airframe Icing Summary
Water droplets can exist as super-cooled water at temperatures below 0°Celsius.
The intensity of icing is greatest in an area of high liquid water content.
High liquid water content in cloud occurs when temperatures are near freezing and when droplets are large.
The liquid water content of cloud can decrease rapidly with the appearance of ice crystals.

29. Airframe Icing Summary, Cont’d
Ice occurs as rime, clear and mixed. Clear ice presents the greatest hazard.
Clear ice forms from the slow freezing of large droplets, rime from rapid freezing of small droplets.
The intensity of ice is described as: trace, light, moderate and severe.

30. Procedure in relation to clouds
Altostratus
Mainly ice crystals with a possibility of super-cooled water droplets
Altocumulus
Mainly water droplets, down to -30°C
Water content and ice accretion likely to be low.
Change of flight level will avoid any build-up

31. Procedure in relation to clouds
Stratocumulus
Vertical extent up to 3000ft agl
Water content low; ice accretion light or moderate, occasionally severe
Icing in layer cloud is normally less serious but of greater horizontal extent.
Possibility of ice build-up if flight in cloud is prolonged, or if cloud has been formed due to rapid forced ascent, intensifying low or front, strong orographic lift, or lee wave cloud

32. Procedure in relation to clouds
Stratocumulus, cont’d
Cu may penetrate SCu formation, with a higher icing risk
It can be serious with Stratocumulus over warm water, particularly near the cloud top.

33. Procedure in relation to clouds
Convective clouds
Tends to produce clear ice. Icing is usually serious in developing cloud and can occur down to -20°C. It is of limited horizontal extent.
Icing negligible when temperature falls to -40°C
If icing occurs, height should be reduced to reach a temperature above 0°C, or continued until emergence from cloud.
Otherwise avoid convective cloud altogether!

34. Procedure in relation to clouds
Cold Fronts
Usually extensive areas of convective cloud formations
Embedded Cb’s very common
High risk of icing above the freezing level
Activity increased in passage over high ground
Use of weather radar to avoid worst areas

35. Procedure in relation to clouds
Warm Fronts
Extensive horizontal layers of cloud, especially NS from near the surface to around 10,000ft
Serious icing in temperatures 0° to -15°C
Super-cooled rain may fall from bottom of cloud
At lower temperatures, snow and ice crystals may exist
Fly in temperatures above 0°C or below -15°C
If necessary fly above the cloud layer
If icing begins then, preferably, descend to below the 0°C level
Watch out for high ground!

36. Procedure in relation to clouds
Occlusions
Icing characteristics can be either those of a cold or warm front

37. Super-cooled Rain
Freezing rain is most often associated with a warm front and can cause rain ice.
Freezing drizzle occurs under stratus and can cause severe clear ice.

38. Super-cooled Rain
To avoid icing at low levels, make a 180° turn away from the front
At high levels, transit at right angles to the front to give the shortest traverse through the icing region
On no account fly parallel to the front!

39. Airframe Icing Summary
Aerodynamically, the greater the collection efficiency the greater the icing hazard.
Collection efficiency is large for sharp leading edges, high speeds and large water droplets.
Aerodynamic heating can keep the aircraft skin above freezing and prevent ice, but speeds in excess of 500 knots may be required.
Ice also forms in piston and jet induction systems and in fuel.