What are the two main types of icing?
- Airframe
- Engine
What conditions are required for ice to form?
- Visible moisture
- Temp +10 to – 40°C
- Airframe must be <0°C
What is the freezing level?
The altitude at which the temperature is 0°C
Do snow an ice crystals in flight typically adhere to an aircraft?
No
Where is the worst icing found?
- Near the Freezing Level
- Heavily stratified clouds (St, Ns)
- Cumulus or Cumulonimbus clouds with strong updraughts
- In freezing rain beneath a NS
What are the types of airframe ice?
- Clear
- Rime
- Mixed (Clear and Rime)
- Frost
Elements of clear ice?
- Most dangerous of all
- Hard to see
- Tenacious
- Heavy
- Smooth
- Can flow back over wing
- Caused by super-cooled water droplets freezing on contact.
- Latent heat released as water freezes delays freezing and allows water to spread
Elements of Rime Ice
- Supercooled tiny droplets freeze on a surface <0°C
- Mixture of ice and air
- Rough, opaque, crystalline, brittle
- Can form between 0°C and -40°C
- Most common -10°C to -20°C
Elements of mixed ice?
- Due to droplets of varying sizes
- Most common form of icing
Where is pure rime ice most likely to occur?
High AS or AC
Where is pure clear ice most likely to occur?
Freezing rain below NS
Elements of Frost (Hoar)
- Occurs when moist air condenses on a freezing surface.
- A white crystalline coating that easily brushes away.
- Typical conditions:
—– Clear night
—– Calm wind
—– High humidity
Dangers of frost?
- Separation of Laminar flow over wing
- Obscure vision
- Freeze controls
- Increase weight
CU are predominantly liquid water droplets until __________
-23C
At what temperatures is the highest risk of icing found in CU?
0°C to -15°C
Types of Ice/Temperature ranges within CU?
Clear - 0°C to -5°C
Clear and Rime - -5°C to -15°C
Moderate Rime - -15°C to -23°C
Light Rime -23°C to -40°C
Doubtful - lower than -40°C
Stratiform clouds can contain water droplets to _________
-15C
Rain, showers or drizzle will freeze on contact with an airframe _____________
below 0°C
What is the relationship between risk of clear ice and droplet size?
Risk of clear ice increases with droplet size.
Hazards to aircraft from ice on the airframe?
- Weight and Centre of Gravity changes.
- Adverse aerodynamic effects; wing and propeller:
- Loss of lift
- Increase in drag
- Unbalancing of control surfaces/ propeller.
- Loss of visibility.
- Degradation of COM/NAV if antennae ice.
Hazards to aircraft from ice in the intake?
- Can disrupt engine sensors resulting in incorrect engine settings - B737 Potomac
- Builds around turbine or jet intakes - Scandinavian Airlines Flight 751
- Engine surging & failure
- Disrupted running & reduced power
Hazards to aircraft from ice in the pitot?
- Block pitot and static ports - Air France
- Causes instrument (ALT/ASI/VSI) Errors
- Freeze angle of attack sensor (Stalling)
- Blocks TAT sensor (Mach/TAS Air-data Computer)
- Block engine EPR sensors
What are the two different systems for removing ice?
- De-Icing
- Anti-Icing
Elements of anti-ice?
- Prevents ice from occurring.
- Heating most common.
- Heat is either
—– Electric (pitot, propeller, windscreen)
—– Bleed Air (leading edges, intake) - Anti-icing fluids (Ethylene glycol) used in very old aircraft.
- Fluids still used for ground de-icing.
