What happened
On January 29, 1999, a Crossair DC9-82, registration HB-INR, was performing a scheduled passenger flight from Basel-Mulhouse to London. During the climb through approximately 20,000 feet, the crew noticed the autopilot trim alarm activate. Upon disconnecting the autopilot to investigate, the co-pilot discovered that the elevator control column was extremely difficult to maneuver. As the aircraft continued its climb, the control column eventually became completely immobilized.
The crew declared an emergency and initiated a return to Basel-Mul．he. To maintain pitch control during the descent, the commander had to rely exclusively on the elevator trim. The aircraft descended through clouds, and the crew performed several pneumatic deicing cycles on the vertical stabilizer. Control of the elevator was partially restored near 1,000 feet, once the aircraft was clear of the cloud layer. The aircraft landed safely at Basel-Mulhouse, where fire services met the aircraft on the taxiway.
The investigation
The investigation focused on the physical state of the flight controls and the environmental conditions. Following the landing, inspectors examined the elevator and discovered residues of deicing fluid that were in the process of thawing. These residues were found specifically within the joints between the elevator and the trim tab.
Laboratory analysis conducted in Toulouse examined the residue alongside various samples of Type I, II, and IV deicing fluids collected from several airports, including Frankfurt, Zurich, and Paris-Charles de Gaulle. The analysis aimed to determine if the chemical properties of the fluids used during recent maintenance or ground operations could lead to the observed mechanical interference.
Findings
- The investigation established that the residue found in the elevator joints consisted primarily of water, a gel, and paint debris.
- The gel was identified as an acrylic polymer used as a thickener in Type II and Type IV deicing fluids.
- The presence of this gel was caused by the rehydration of the polymer extract following exposure to moisture, such as light rain.
- The physical properties of the residue, including its melting point, were found to be near 0 °C, meaning the substance could easily solidify and obstruct moving parts in near-freezing temperatures.
- The aircraft had recently undergone maintenance and had been treated with various deicing/anti-icing fluids in the preceding days.