What happened
During the winter of 2005/2006, several incidents were reported involving flight control restrictions on aircraft with non-powered flight controls. On 17 January 2006, an Avro 146-RJ100, registration G-JEAV, experienced a significant flight control event while cruising at FL220. After the autopilot captured and locked the assigned altitude, the flight director provided a continuous fly-up indication. Upon disconnecting the autopilot to fly manually, the crew found the aircraft attempting to climb. Despite using electric trim to compensate, the trim repeatedly returned to its original position, and the control column required approximately three times the normal force to be held forward. The crew eventually descended into warmer air, which restored normal control.
Similar events occurred across the fleet, including instances of pitch oscillations, 'tail heavy' handling characteristics, and aileron stiffness. In one case involving an Embraer 145, the aircraft experienced severe pitch oscillations at FL200, with the control column movement restricted to only one inch fore and aft.
The investigation
An investigation into the G-JEAV incident involved the removal of elevator trim tab control rod fairings. This inspection revealed significant accumulations of residues from thickened de/anti-icing fluids. The investigation also examined several other aircraft, including G-JEAJ, G-JEAY, and G-MANS, finding that rehydrated fluid residues had accumulated in aerodynamically 'quiet' areas of the aileron and elevator controls. In some instances, these residues were found to have been washed into control runs or gaps between the stabilizer and elevator surfaces.
Findings
- The primary cause of the flight control restrictions was the freezing of rehydrated residues from thickened de/anti-icing fluids that had accumulated in the control mechanisms.
- These residues entered the control surfaces through gaps or were deposited during the de-icing process.
- The accumulation of these substances led to physical obstructions, resulting in stiff, heavy, or even jammed flight controls.
Safety action
- Safety Recommendation 2005-135: It is recommended that the JAA, in consultation with EASA, issue documentation to encourage operators of aircraft with non-powered flight controls to use Type I de-icing fluids instead of 'thickened' fluids.
- Safety Recommendation 2005-136: It is recommended that where the use of 'thickened' fluids is unavoidable, operators of such aircraft invoke controlled maintenance procedures for the frequent inspection and removal of fluid residues.
- Safety Recommendation 2005-137: It is recommended that EASA introduce certification requirements for de-icing fluids used on aircraft with both powered and non-powered controls.