Many recent de-icing fluid related incidents relate to incorrect application or inappropriate use of the fluid. Thus it is good to see AEA reissuing their De-icing guide with due ‘cautions’, and the new Training Manual, something that has been long overdue in the industry and is still a mystery as to why the European regulatory authorities did not take the responsibility for such a document (the FAA did).
There is an entry in the Training Guide that may be ambiguous, where the second application of a two-stage procedure calls for the covering of all existing fluid. It is my understanding that all fluid from the first stage must be washed off first, as it could contain contaminants; thus, it is the ‘wet’ surface that has to be covered with the anti-icing layer. The problem is that the new fluid applied at stage two may stick to the residual stage one fluid and form a very thick layer that is both unnecessary for anti-icing, may not provide the advertised HOT due to embedded contaminants, and may cause subsequent dry out or gel problems (and deposits a lot of fluid on runway when it comes off). Similarly, if the new thicker type 2 and 4 fluids are applied too thickly, more than 1 litre/square meter, then similar problems arise.
The theory is that approx 80% of the applied fluid will shear off the aircraft during the take-off run (based on a 737), but if there is excess fluid then even after 80% has sheared off there is still ‘a lot’ left on the surface. This fluid can seep into gaps and crevices, dry out, and re gel in wet conditions. Some thickeners in de-icing fluid can absorb up to 10 times their own volume of water, which can then freeze.
Problems have also occurred with anti frosting coatings; if the fluid has been left on the structure overnight it should be cleaned off as it may have started to gel ( reverserunlocked - sticky and gooey); never recoat an anti frosting layer for the reasons above.
Follow skyrabbit’s advise, never takeoff with a contaminated aircraft. Also, remember that the process of de/anti-icing replaces a contaminant of unknown aerodynamic qualities with another contaminant with qualities that we think we know something about, but recent incidents suggest that we still have much to learn.