One of the recently discovered phenomenon of Type II and IV fluids is rehydration. After evaporation, a chemical residue remains that will “rehydrate” with water, but at the wrong ratio, resulting in a gel that can freeze.
Following from CAA:
REHYDRATION OF TYPE II AND TYPE IV DE-ICING/ANTI-ICING FLUID RESIDUES
Type II and Type IV Anti-Icing Fluids
The repeated application of Type II and 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. These residues may block or impede critical flight control systems and should be removed.
In order to limit these problems the repetitive use of Type II or Type IV anti-ice fluid should be avoided as far as possible.
When Type II and Type IV anti-icing fluid residue has been detected, no take-off should be authorized until the residues have been removed.
Recommendations
Operators should obtain guidance and instructions from the aeroplane manufacturer as to how to establish satisfactory procedures to detect and remove residues of dried fluid.
Operators should review their anti-icing procedures when using Type II or Type IV fluid,
amending their Operations Manual where necessary, to ensure the absence of residue, which could rehydrate and adversely affect the operation of the aeroplane. Procedures relating to a specific aeroplane type or operation should be promulgated in detail to all staff involved in deicing/anti-icing operations.
And from Boeing..
TYPE II AND TYPE IV FLUID REHYDRATION AND FREEZING
Last winter in Europe, restricted elevator movement interrupted the flight of two MD-80 airplanes. In both cases frozen contamination, a gel with a high freezing point, caused the restricted movement. The gel was Type IV fluid residue that rehydrated during takeoff or climbout in rain.
Rehydration can occur when thickened fluid is repeatedly applied in dry conditions, either to prevent frost from forming overnight or for deicing just before flight. The fluid dries out during flight, and a powderlike residue remains in aerodynamically quiet areas, such as balance bays and wing and stabilizer rear spars. If the airplane is not deiced or anti-iced during a subsequent layover and encounters rain on the ground or during climb, the remaining residue absorbs water and turns into a gel. The gel swells to many times its original size and can freeze during the next flight leg, potentially restricting the movement of flight control surfaces.
In the case of both MD-80s, the frozen gel restricted movement of the elevators, which are unpowered flight control surfaces on that model. Both flights were diverted, and elevator movement was restored when the gel unfroze during descent as the airplanes encountered warmer temperatures at lower altitudes. Inspection after the return of one of these flights revealed gel in the area between the elevator and elevator control tabs.
The issue of rehydration was discussed at the Society of Automotive Engineers (SAE) G-12 Fluids subcommittee meeting last May. The subcommittee also discussed related occurrences on other types of airplanes with unpowered flight controls and the deicing/anti-icing procedures used by the operators attending the meeting. These discussions led the subcommittee to conclude that the residue builds up when a one- or two-step deicing/anti-icing procedure is followed using Type II fluid, Type IV fluid, or both, in either neat or diluted form. This practice is prevalent in Europe.
The SAE G-12 Fluids subcommittee recommended including a caution note in the next revision of SAE ARP 4737 to address this issue. The SAE G-12 Methods subcommittee agreed and is including the following note in SAE ARP 4737D, scheduled to be released in late 1999.
CAUTION: The repeated application of Type II or Type IV, without the subsequent application of Type I or hot water, may cause a residue to collect in aerodynamically quiet areas. This residue may rehydrate and freeze under certain temperature, high humidity and/or rain conditions. This residue may block or impede critical flight control systems. This residue may require removal.