March 2005
In March 2005, several European carriers flying the BAE146- / AVRO RJ-series encountered more or less during the same short time frame severe in-flight pitch disturbances and oscillations. Aircrafts of the Embraer 145- and LearJet-series have also been affected.
The effects on the flight control systems were:
1. With the AP engaged, slow cyclic pitch instability with a frequency of 0.1 Hz
2. Vertical speed indication changing
3. With AP OFF, a higher than normal control force required
4. Descending to warmer air, control forces required reduce to normal
5. Descending through precipitation and cold front, control forces remained high
The weather history before those incidents involved a long cold period with lot of de/anti-icing cycles followed by a dry period, and finally immediately followed by heavy precipitation and cold fronts.
The major root cause for those incidents was identified to be various residual forms of de-icing fluids in critical areas of the flight control systems.
As the industry seeks greater hold over times, thickening chemicals have been added to de-icing fluids leading to enhanced type II and type IV fluids. If these fluids are misused, i.e. incorrect application, failure to wash off previous fluids, over application or repeated application, then those fluids can remain in the gaps and “aerodynamic quite” areas in an airframe and leave potentially dangerous residues such as a dry white powder, sticky gel substances and thick lumpy balls. Exposing a de/anti-icing fluid to dry air for long periods (as is the case in flight) will see the evaporation of water and glycol from the fluid. This generally leaves the thickening agents and other chemicals in a dry powdery form. This dry powder residue, by itself, mostly does not cause any direct problem; although if enough were present on some external surfaces of some aircraft there could be severe aerodynamic effects. Dry residues on external surfaces are more easily cleaned during the course of daily operations, however, the dry residue that forms within the airframe will remain until such time it is detected and cleaned away. Often even when sought for, it can be difficult to detect. It is possible for many layers of dry residue to build up over a winter season. When these dry residues are exposed to moisture, such as rain or high humidity, the chemicals absorb the water/moisture and expand into a colourless gel-like substance. This gel residue can be many times greater in volume than the original fluid and dry powder residue. As this gel can form inside the aircraft before and during flight, and as the majority component is water; once the OAT falls below freezing the gel is likely to freeze.
Frozen gel can create flight safety problems in numerous ways. If the gel is frozen within the gaps between control surfaces, then certainly for non-powered control systems, these control surfaces may be difficult or impossible to move from the flight deck until the gel melts in warmer temperatures. Similarly, gel that freezes inside the airframe can interfere with control systems and have the same effect.
The March 2005 events highlighted to the industry that the residual de-ice fluid not only remained in the aerodynamic quite areas but has actually over the years managed to penetrate into the main elevator and tab structures. This required a new line of thought and changes to the way the residual inspection and cleaning of the aircraft has to be performed.
Subsequently, maintenance organizations (should) have changed their flight control inspection intervals and procedures significantly to prevent further de-icing fluid incidents.