What happened
On April 2, 2003, a Swiss International Air Lines BAe 146-300, registration HB-IXN, was performing a scheduled flight from Zurich to Amsterdam. While holding over Amsterdam Airport Schiphol, the flight crew discovered that the elevator trim was stuck, preventing the adjustment of the aircraft's lateral axis. Although the elevator itself remained functional, the inability to retrim the aircraft prompted the captain to declare a ‘MAYDAY’.
To manage the expected increase in control forces, the captain decided to perform a landing without the use of flaps. To mitigate the risks of a high-speed landing, the crew remained in a holding pattern to burn fuel and reduce the aircraft's weight. The crew also requested emergency services to be on standby. During the landing, which occurred at a high speed of 186 knots, heavy braking caused wheel brake temperatures to reach 650 degrees Celsius. Despite the intense heat, the tire safety plugs remained intact, and all 69 occupants disembarked safely without injury.
The investigation
Because the aircraft had returned to Switzerland shortly after the event, the investigation relied on data provided by the airline. Investigators identified a gel-like residue of de-icing fluid within the mechanical components of the left-hand elevator trim mechanism. This substance, which contained a high percentage of water, was found to have frozen, causing the blockage. The right-hand trim mechanism was unaffected.
Further examination revealed that the aircraft had not been de-iced on the day of the flight, but had previously been treated with thickened Type II or Type IV fluids. The investigation looked into the airline's maintenance cycles, noting that the last inspection of the flight control access hatches had occurred two days prior. It was determined that the residue could have been missed during this inspection or that such residues could accumulate in internal structures that are not typically reached during standard cleaning processes.
Findings
- The primary cause of the trim blockage was the freezing of thickened de-icing fluid residue within the left-hand elevator trim mechanism.
- Thickened de-icing fluids (Type II, III, or IV) can penetrate internal control structures during application.
- When these fluids are exposed to dry air, they leave behind chemicals that, upon absorbing moisture, expand into a gel that can freeze at temperatures below zero.
- Standard inspection and cleaning cycles may fail to identify or remove residues trapped in 'aerodynamically quiet' or internal areas of the airframe.
Safety action
Following the incident, the airline implemented more frequent maintenance inspections, reducing the cycle from 28 days to 14 days. Additionally, cleaning is now required within a few days of applying thickened de-icing fluids. The airline also developed new procedures and tools specifically designed to identify and remove fluid residues from internal aircraft structures.