Best practice relating to the use of de-icing and anti-icing fluids
The repeated application of Type II, Type III or Type IV anti-icing fluid may cause residues to collect in aerodynamic quiet areas, cavities and gaps. These residues may rehydrate and freeze under certain temperature changes, in high humidity and/or rain conditions. In addition, they may block or impede critical flight control systems and should be removed.
In order to limit these problems, the repetitive use of Type II, Type III or Type IV anti-icing fluid should be avoided as far as possible.
Under no circumstances may an aircraft that has been anti-iced receive a further coating of anti-icing fluid directly on top of the contaminated film. If an additional treatment is required before flight, a complete de-icing/anti-icing process must be performed to ensure that any residues from the previous treatment have been removed. In these circumstances anti-icing only is not permitted.
After application of de-icing/anti-icing fluids it is advisable to inspect aerodynamically quiet areas and cavities for residues of thickened de-icing/anti-icing fluid. An appropriate inspection and cleaning program should be established.
Ingestion of de-icing fluid into the Auxiliary Power Unit (APU) has caused uncontained failure of the turbine wheel of the APU due to over-speed, resulting in failed and uncontained parts penetrating the aft cabin pressure bulkhead, with consequent possible injury to the cabin crew or passengers. The de-icing fluid acted as an additional fuel source, resulting in runaway acceleration, leading to failure of the turbine wheel.
Normal de-icing spraying/de-icing fluid mist will not lead to an uncontained APU failure, but a failure can be caused by neat de-icing fluid entering the APU inlet. This may be the result of misdirection through human error, or natural phenomena such as wind.
Anti-icing fluid application should be continuous and as short as possible. Anti-icing should be carried out as near to the departure time as operationally possible in order to utilise maximum holdover time. With regard to holdover time provided by the applied fluid, the objective is that it be equal to or greater than the estimated time from start of anti-icing to start of take-off based on existing weather conditions.
De-icing treatments must be symmetrical, even when only one side of the aeroplane is contaminated. Anti-icing treatments must always cover the entire wing and the entire horizontal stabiliser/elevator on both sides of the aeroplane.
Under freezing fog conditions, the rear side of turbine engine fan blades must be checked for ice build-up prior to start-up. Any deposits discovered shall be removed by directing air from a low-flow hot air source, such as a cabin heater, onto the affected areas.