What happened
On May 16, 2005, a KLM Cityhopper Fokker F28 Mk 070, registration PH-KZR, was operating a scheduled flight from Torp, Norway, to Amsterdam. During the descent into Amsterdam Airport Schiphol, the flight crew experienced a significant loss of aileron control, specifically finding that left-hand turns were impossible. As the aircraft continued its final approach, the crew noted that partial control of the ailerons had returned. Despite the flight control issues, the aircraft landed without incident, and all 68 passengers and 4 crew members disembarked safely.
The investigation
The investigation focused on the aircraft's powered flight control system, specifically the cables and actuators located in the wheel bay. Investigators examined the aileron pulleys following reports of similar incidents within the airline. Upon inspection, a gel-like residue from de-icing fluid was discovered on the left-hand aileron pulley.
It was determined that a gap in the seal between the wing and the fuselage fairing allowed de-icing and anti-icing fluids to seep into the wheel bay. While the aircraft had not been treated with fluid on the day of the incident, it had undergone heavy de-icing/anti-ing procedures during the preceding winter months, including the use of thickened Type II fluids. The investigation established that when these thickened fluids evaporate, they leave behind chemicals that, when exposed to moisture, expand into a gel that can freeze and obstruct mechanical components.
Findings
- A gap in the wing-to-fuselage fairing allowed de-icing/anti-icing fluid to penetrate the wheel bay.
- Accumulated de-icing fluid residue froze on the left-hand aileron pulley, causing the mechanical blockage.
- The residue was highly hygroscopic, meaning it absorbed moisture from the air to form a swelling gel.
- The use of thickened fluids (Type II) contributed to the buildup of residue in aerodynamic gaps.
Safety action
Following the discovery, an Airworthiness Directive (AD NL 2005-13) was issued, requiring all Fokker F28 Mk70/100 aircraft to inspect wing-to-fuselage fairings above the main landing gear. The airline also updated its maintenance protocols, increasing the frequency of inspections and cleanings of critical areas from once per winter to once per week or per A-check.