L10 Icing

Icing

Hazards of Icing

• Any form of icing presents a hazard to aircraft.

• Ice accumulation on the airframe:

  • Increases drag.

  • Reduces lift.

  • May make takeoff impossible.

  • May prevent maintaining level flight even at full power.

  • Can drastically affect flight characteristics, potentially leading to loss of control.

• Engine icing:

  • Can cause a marked reduction in power or total engine failure.

• Other icing effects:

  • Ice formation on the windscreen.

  • Blocked external pitot and static pressure sources.

  • Ice formation inside pitot/static system lines.

  • Frozen flying controls.

• Icing and light aircraft are a dangerous combination.

• PPL holders without instrument qualifications should avoid conditions that lead to airframe icing, such as flight in cloud at temperatures below 0°C.

• Most light aircraft lack de-icing equipment, and their POH/FM (Pilot Operating Handbook/Flight Manual) explicitly prohibits flight into known icing conditions. Ignoring this advice carries severe safety and legal implications.

Conditions Conducive to Ice Formation

• On the ground, water rarely exists below freezing (0°C) as it turns into ice.

• In clouds, water droplets can remain liquid (supercooled) at temperatures as low as -40°C.

• Aircraft flying in cloud above the freezing level are in potential icing conditions.

• When an aircraft encounters small supercooled water droplets, they freeze on contact, forming a brittle layer of ice, typically on leading edges (wings, tail), aerials, and struts. This is known as rime ice.

• Larger water droplets may partially freeze as they flow across the aircraft surface, creating a sheet of clearer ice called clear ice.

• Worst icing conditions generally occur between 0°C and -12°C.

• Icing becomes less severe below -20°C and is rare below -40°C due to the prevalence of ice crystals.

• Cumulonimbus clouds can produce severe icing at any temperature below 0°C.

• Cumuliform clouds contain larger water droplets and produce more severe icing than stratus-type clouds.

• Orographic uplift or mountain-wave conditions can also lead to worse icing conditions.

• Stratocumulus clouds, especially those just below an inversion over the sea, can sometimes give unexpectedly severe icing.

• Icing of pitot and static tubes causes unreliable readings on pressure instruments (airspeed indicator, altimeter, vertical speed indicator). Pitot heat should be selected immediately if icing conditions are encountered.

Hoar Frost

• Hoar frost can form on an aircraft on the ground or, less commonly, in flight.

• It forms when water vapor changes directly from a gas to a solid (ice) on a surface below freezing (0°C).

• Ideal conditions include a clear, cold night with temperatures falling below freezing while the aircraft is parked in the open.

• Hoar frost, even a thin layer, is dangerous because it significantly affects aerodynamics, similar to other forms of airframe icing.

  • Reduces lift.

  • Increases drag.

  • Raises stalling speed.

  • Can make the aircraft difficult or impossible to control.

  • Reduces performance, increasing take-off and landing distances, and reducing climb rates, cruising speeds, and range.

• All frost and snow must be removed from all aircraft surfaces before flight.

• Snow will not blow off during take-off, especially over wing fuel tanks.

• Hoar frost can form in flight after prolonged flight in sub-zero air (OAT below 0°C) followed by descent into warmer, moist air or after takeoff when surface temperature is below freezing climbing through an inversion into warmer air - although it thaws quickly.

Rain Ice

• Rain ice is an uncommon form of icing that occurs outside of clouds.

• It typically occurs ahead of a warm front in winter.

• Rain falls from the warm sector clouds through the front into colder air.

• If this rain encounters an object with a surface temperature below freezing (such as an aircraft), it may freeze on contact, forming clear ice that is difficult to remove (freezing rain).

• Rain ice can affect aircraft on the ground, runways, and taxiways.

• The only solution is avoidance.

• Example: Scottish airports experiencing freezing drizzle (FZDZ) and freezing rain (FZRA) due to a warm front in late November represent very hazardous flying conditions.

De-icing and Anti-icing

• ICAO's 'Clean Aircraft Concept' mandates that take-off is prohibited when ice, snow, slush, or frost is present on critical surfaces.

• Any deposit of ice, snow, or frost on external surfaces drastically affects performance by reducing lift and increasing drag due to disturbed airflow.

• De-icing: Removes ice, snow, slush, or frost from aircraft surfaces using mechanical and/or chemical methods (typically ethylene glycol-based fluids in GA).

• Anti-icing: A precautionary measure to protect clean surfaces from ice and frost formation and snow accumulation for a limited period.

• The pilot-in-command is responsible for ensuring that critical surfaces are free of ice, snow, slush, or frost before take-off.

Carburettor Icing

• Carburettor icing is the most common ice-related problem in piston engines.

• The combined effect of the pressure drop in the venturi and fuel evaporation can cause a temperature drop of 25 to 30°C.

• Water vapor forms into ice crystals, constricting the venturi and reducing power.

• It is not necessary for the OAT to be below 0°C for carburettor icing to occur because of the temperature drop in the venturi. The dominant factor is the moisture content of the air.

• Warm, humid air is more prone to carburettor icing than cold, dry air. Visible moisture (clouds, fog, precipitation), moist airflow (over the sea), and wet airfields are danger signs.

• Carburettor icing is especially dangerous at low power settings.

• Carburettor icing can occur over a wide range of conditions.

• All pilots flying light aircraft with carburetted piston engines should be knowledgeable about carburettor icing.

Piston Engine Icing

• Conditions causing airframe icing can also cause engine icing.

• Ice may form on propeller or rotor blades, reducing efficiency and causing mechanical imbalance and vibration.

• Freezing precipitation (sleet, snow) can block or restrict engine intakes.

• Flight in known icing conditions is prohibited without de-icing equipment to avoid this type of engine icing.

Reporting Icing

• Definitions for reporting icing intensity (not the same as forecasting or aircraft manual definitions):

  • Trace: Perceptible icing, not hazardous unless encountered for more than one hour, de-icing/anti-icing not required unless prolonged exposure.

  • Light: Accumulation rate may be a problem after an hour, occasional de-icing/anti-icing use removes ice.

  • Moderate: Potentially hazardous even for short periods, de-icing/anti-icing or diversion is necessary.

  • Severe: De-icing/anti-icing fails to control accumulation, immediate diversion is required.

• Include the 'when and where' in icing reports.

• Basic PPL pilots should avoid these conditions.