Icing conditions pre-flight information
When planning flights at or below 10,000 ft, note that the graphical area forecast (GAF) includes information about known or expected icing conditions, and which is available through NAIPS. General information about icing conditions is stated under the ‘Cloud, icing and turbulence’ heading of the GAF.
Information about reported icing conditions that may affect the safety of aircraft operations (that is severe icing), will be included in the SIGMET.
Information about icing conditions that may affect aircraft operations but to a lesser degree of severity than those issued as a SIGMET (that is moderate icing), will be included in the AIRMET.
Information about icing conditions within 5 nm radius of an aerodrome serviced by an AWS or an authorised meteorological observer, may be included in SPECI if it is likely to affect aircraft operations safety (AIP GEN 3.5).
Icing conditions airframe CAR 238
As pilot in command of an aircraft, you must not take off for the purpose of making a flight during which the aircraft may fly into known or expected icing conditions unless the aircraft is adequately equipped with either de-icing or anti-icing equipment of the type and quantity as directed by CASA.
Icing conditions carburettor
Carburettor icing is of particular concern because, unlike airframe icing, the risk of ice build-up in the carburettor can be high even with no visible moisture and an OAT of up to 38°C.
Carburettor icing occurs when the air temperature adiabatically decreases sufficiently to condense water vapour and for the localised air temperature to reduce below freezing. Ice builds up as the chilled condensed water makes contact with localised surfaces, such as the butterfly valve and the venturi walls. Carburettors experience additional cooling because of the evaporation of fuel. Furthermore, the risk of carburettor icing is significantly increased at partial power settings (for example, when power is reduced during descent), because of the cooling effect of a partly-closed throttle.
CASA has published a specialised chart to measure carburettor icing probability based on known OAT and dew-point depression. Dew-point depression is the difference between OAT and dew-point temperature and this information is available from an aerodrome’s AWS or in METAR/SPECI aerodrome meteorological reports.
Example of carburettor icing probability shown on the chart below
OAT (or dry bulb temperature) = 12°C
Dew point = 2°C
- Calculate dew point depression: OAT (or dry bulb temperature) minus dew point = 12 – 2 = 10.
- find the intersection of 12 (x axis) and 10 (y axis) and note the shading indicates:
- – moderate icing for cruise power or
- – serious icing for descent power.
- From the intersection, follow the slanted reference lines to the right and note relative humidity is 52 per cent.