Event Description
On March 12, 2005, a BAe 146-300 cargo aircraft was climbing from Frankfurt to Stuttgart when it entered a slow angle-of-attack oscillation with increasing amplitude. The pitch movements reached up to 18 degrees positive angle of attack and a sink rate of up to 4500 ft/min. The autopilot was engaged during the initial climb from flight level 80 to 100. After disengaging the autopilot, the crew regained control using manual elevator trim while in instrument meteorological conditions (IMC) and icing conditions. A subsequent period at flight level 130 in visual meteorological conditions (VMC) and out of icing conditions did not resolve the control issues. The abnormal and emergency checklists did not contain a solution for the problem, so the crew decided to conduct an instrument approach to Stuttgart under better weather conditions. The approach and landing were performed using manual elevator trim.
Findings
Immediately after landing, inspection of the aircraft revealed swollen and frozen residues of deicing fluid in the gaps between the horizontal stabilizer and elevator, as well as in the areas of the ailerons and rudder. The residues were identified as dried and rehydrated deicing fluid polymer thickener. After the horizontal stabilizer and wings were cleaned according to the manufacturer's specifications, subsequent flights with the same aircraft showed normal operation of both the flight controls and autopilot.
Tests and Results
The investigation included laboratory tests on various thickened deicing fluids (SAE Type II and Type IV). The tests, conducted by the German Federal Armed Forces' Institute for Materials, Explosives, Fuels and Lubricants (WIWEB), evaluated the formation of dry residues and their water absorption capabilities. The results showed that dry residue mass varied among products and was not directly proportional to viscosity or initial fluid quantity. The water absorption capacity of the residue depended significantly on the chemical structure of the polymer thickener. Tests on sequential applications of different fluids (mixes) indicated that the order of application could affect both residue mass and water uptake. All tested fluids produced residues that could absorb water and form a gel-like mass, which could freeze under appropriate conditions.