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High accretion rates are not systematically associated with Cumulonimbus; stratiform clouds can ice like hell!
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Icing conditions are far most frequent than effective ice accretion. Icing conditions do not systematically lead to ice accretion.
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Should the pilot encounter icing conditions in flight, some recommendations are:
- In addition to using NAI and WAI according to procedures, the pilot should keep an eye on the icing process: Accretion rate, type of cloud.
-When rapid icing is encountered in a stratiform cloud, a moderate change of altitude will significantly reduce the rate. It is an obligation for the ATC controller to accept altitude change.
- If icing conditions prevail on the approach, keep speed as high as permitted, delay flap extension as much as possible, and do not retract flaps after landing.
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Ice and snow due to ground precipitation, or overnight stay, should be totally cleared before takeoff, regardless of the thickness. Otherwise aircraft is not certified for flying.
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Optional ice detector systems available on Airbus aircraft are advisory systems and do not replace AFM procedures. For the time being, Airbus Industrie is not convinced of the benefits of a primary ice detection system.
B. AIRCRAFT DE/ANTI ICING ON THE GROUND
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Aircraft contamination endangers takeoff safety and must be avoided. The aircraft must be cleaned.
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To ensure that takeoff is performed with a clean aircraft, an external inspection has to be carried-out, bearing in mind that such phenomenon as clear-ice cannot be visually detected. Strict procedures and checks apply. In addition, responsibilities in accepting the aircraft status are clearly defined.
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If the aircraft is not clean prior to takeoff it has to be de-iced. De-icing procedures ensure that all the contaminants are removed from aircraft surfaces.
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If the outside conditions may lead to an accumulation of precipitation before takeoff,
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the aircraft must be anti-iced. Anti-icing procedures provide protection against the accumulation of contaminants during a limited timeframe, referred to as holdover time.
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The most important aspect of anti-icing procedures is the associated holdover-time.
This describes the protected time period. The holdover time depends on the weather conditions (precipitation and OAT) and the type of fluids used to anti-ice the aircraft.
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Different types of fluids are available (Type I, II, and IV). They differ by their chemical compounds, their viscosity (capacity to adhere to the aircraft skin) and their thickness (capacity to absorb higher quantities of contaminants) thus providing variable holdover times.
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Published tables should be used as guidance only, as many parameters may influence their efficiency - like severe weather conditions, high wind velocity, jet blast...- and considerably shorten the protection time.
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