What happened
On March 12, 2005, a BAe 146-300 cargo flight from Frankfurt to Stuttgart experienced a severe flight disturbance during its climb from FL80 to FL100. While the autopilot was engaged, the aircraft entered a slow-amplitude longitudinal oscillation with increasing intensity. The pitch oscillations reached a positive angle of attack of 18 degrees, resulting in a descent rate of up to 4,500 ft/min.
Upon disabling the autopilot, the crew managed to regain control of the aircraft using manual elevator trim, eventually transitioning through instrument meteorological conditions (IMC) and icing conditions. Despite a period of flight at FL130 in visual meteorological conditions (VMC) outside of icing conditions, the control difficulties persisted. The crew eventually performed an instrument approach to Stuttgart, using manual elevator trim to manage the aircraft's altitude and touchdown.
The investigation
Following the incident, the BFU examined the aircraft and found swollen, frozen residues of de-icing fluid in the gaps between the horizontal stabilizer fin and the elevator, as well as in the aileron and rudder areas.
Testing conducted by the WIWEB (the German Federal Ministry of Defence's Institute for Weapons, Explosives and Materiel) investigated the properties of thickened de-icing fluids (Type II and Type IV). The investigation established that as the water and glycol components of these fluids evaporate (dry-out), a polymer thickener residue remains. This residue is highly hygroscopic, meaning it can absorb many times its own weight in water from the atmosphere (rehydration), forming a gel. At low temperatures, this water-heavy gel freezes, expanding in volume.
Findings
- The primary cause of the control malfunction was the accumulation of frozen, gel-like de-icing fluid residue in the horizontal stabilizer mechanism.
- The expansion of this frozen gel created a physical blockage (clamping effect) between the fixed stabilizer fin and the movable elevator.
- Because the BAe 146-300 lacks powered flight controls, the mechanical linkage could not generate sufficient force to overcome the ice-induced blockage.
- The buildup was caused by the repeated application of thickened de-icing fluids (Type II) without sufficient cleaning intervals to remove the polymer residues.
- Systemic factors included a lack of specific manufacturer guidance regarding the cleaning intervals required to remove these residues, and an industry-wide trend toward using thickened fluids due to economic considerations, which reduced the availability of non-thickened (Type I) fluids.