On 12 March 2005, a BAe-146-300 climbing out of Frankfurt experienced a loss of elevator control authority and an uncommanded descent at up to 4500 fpm whilst in a nose high pitch attde which was eventually arrested and subsequently attributed to the freezing of re-hydrated ground de/anti-ice fluid residues. The crew decided to continue to their originally-intended destination since it offered the prospect of more favourable weather conditions for landing. The aircraft later landed at Stuttgart after using elevator trim to control pitch attitude.
The following extract is from the official BFU Germany report on the Serious Incident:
"The airplane experienced a slow pitch oscillation with increasing amplitude during climb from flight level (FL) 80 to FL100 with engaged autopilot. […] The oscillation resulted in a positive angle of attack of up to 18 degrees and in a rate of descent of up to 4,500 ft/min.
The crew regained control of the airplane in instrument meteorological conditions (IMC) and under icing conditions with disengaged autopilot and the help of the manual elevator trim. A prolonged flight in FL130 under visual meteorological conditions (VMC) and free of icing conditions did not change the control problems they experienced with the airplane.
Since the checklists for abnormal and emergency situations did not contain any solution to the existing problem the crew opted for an ILS approach to Stuttgart Airport because of the better weather situation. To control altitude and touchdown the manual elevator trim was used."
Immediate causes for the serious incident were identified as:
- Several de-icing procedures with thickened de-icing fluids* (Type II) caused an accumulation of dry fluid residues (polymer residues) in the gap between elevator and horizontal stabilizer;
- Due to atmospheric exposure these highly hygroscopic residues absorbed, from the surrounding air, a multiple of their own weight of water, became a gel-like mass and swelled-up to many times their original volume;
- With low ambient air temperatures the over-saturated gel froze increasing its volume to such an extent that it jammed the elevator and horizontal stabilizer;
- Elevator control was jammed due to ice formation because the aircraft did not have powered flying controls and the necessary power to move the elevator tab mechanically could not be generated.
Based on the results of the investigation, the BFU has issued the following safety recommendations:
- The Federal Ministry of Transport, Building and Urban Affairs should agree with the Laender aviation authorities responsible for the airports on a joint procedure of the cognizant supervisory authorities designed to urge the ground services responsible for de-icing to apply not only thickened (Type II or Type IV) but also unthickened (Type I) de-icing fluids on airports regularly used by aircraft with non-powered flying controls and offering de-icing services.
- The European national accident investigation authorities should recommend to their respective aviation authorities to see that not only thickened (Type II or Type IV) but also unthickened (Type I) de-icing fluids are applied on airports regularly used by aircraft with non-powered flying controls and offering de-icing services.
- Aircraft de-icing to maintain the airworthiness of aircraft during winter operation should be accomplished by certified and approved companies under the supervision of civil aviation authorities. If aircraft de-icing is not accomplished by an operator or an approved maintenance organisation the ground service "aircraft de-icing" should be subject to appropriate aeronautical regulation. European Aviation Safety Agency (EASA) should agree with the European national authorities on establishing such regulations.
- The expected drying and re-hydration properties of thickened de-icing fluids (Type II, III, IV) for aircraft de-icing should be described and defined by standardization in such detail as to eliminate significant quality variations among the products of different manufacturers. EASA should develop certification criteria to establish mandatory limits for and require evidence of unrestricted suitability of such fluids for aircraft with non-powered flying controls.
- Considering the thickened de-icing fluids currently available EASA should impose a mandatory requirement on non-powered flying controlled aircraft manufacturers to develop reliable procedures for their aircraft types to ensure the identification and removal of re-hydrated de-icing fluid residues in such time as to prevent any risk to the safety of flight operation.
Note of the SKYbrary Editor
* thickened de-icing fluids (SAE Type II, Type III or Type IV) have a small portion of a polymer added to adjust their viscose-elastic properties to the requirements for prolonged re-icing protection unlike the un-thickened de-icing fluid (SAE Type I) which is relatively rare in Europe.
Note: Although the AEA ceased to exist in 2016, the most recent of their publications still contain some pertinent information. Readers are cautioned to validate the recommendations of these guidebook using more current information sources.