What happened
On January 24, 2006, an Air Contractors ATR 42-300, registration EI-SLC, was preparing for flight ABR 7020 from Prague-Ruzyně (LKPR) to Paris Charles de Gaulle (LFPG). Due to freezing conditions and frost accumulation on the aircraft, the pilot requested deicing services from Menzies Aviation. The crew requested a Type II (Kilfrost) mixture consisting of 75% fluid and 25% water at a temperature of 80°C.
Following the procedure, the ground crew visually inspected the aircraft and reported the surfaces as clean. The pilot was provided with the standard deicing code and taxied to the holding point for runway 24. However, while waiting for engine warm-up, the pilot noticed fluid droplets on the wing leading edge. Simultaneously, the deicing vehicle operator attempted to print a consumption report and noticed a significant discrepancy between the programmed 75% mixture setting and the actual consumption meters, which showed 360 liters of water but only 40 liters of deicing fluid.
Ground personnel immediately notified Air Traffic Control, and the pilot was instructed to return to the deicing stand. Upon inspection at the stand, the crew discovered that the fluid had frozen on the aircraft surfaces. The aircraft was subsequently re-treated using a different deicing unit with both 75% and 100% concentrations. After a successful second inspection, the aircraft departed at 21:20.
The investigation
The ÚZPLN investigation focused on why the deicing equipment failed to deliver the correct mixture despite a successful refractometer check earlier that morning. The investigation examined the deicing vehicle's electrical system and mechanical components. Ground personnel identified a faulty relay (C3-B39 R420) and a partially open air vent (F43) during their initial inspection. However, a subsequent inspection by the equipment manufacturer, Vestergaard, found the equipment to be in good working order and could not replicate the relay failure or the vent issue.
Findings
- The aircraft was likely sprayed with mostly hot water with a minimal concentration of deicing fluid.
- The deicing equipment had been used for the first time that day on EI-SLC after being verified by refractometer.
- An intermittent malfunction of the deicing equipment was the most likely cause, though the specific failure could not be definitively proven or ruled out by the manufacturer's inspection.
- The consumption meters were not always capable of accurately indicating a failure in the mixture formation process.