Factual Information
On 24 January 2006, a serious incident occurred involving an Air Contractors ATR 42-300, registration EI-SLC, at Prague Ruzyně Airport (LKPR). The aircraft was preparing for flight ABR 7020 to Paris Roissy-Charles de Gaulle Airport (LFPG). The crew consisted of a 53-year-old captain with an ATPL(A) and 4,150 total flight hours (3,900 on type), and a 46-year-old first officer with an ATPL(A) and 2,150 total hours (50 on type). The aircraft had a valid certificate of airworthiness and maintenance release. There were no injuries or damage.
Meteorological Conditions
At the time of the incident (between 20:00 and 21:00 UTC), the weather at Prague Ruzyně included surface winds variable at 0–4 knots, temperatures between -14°C and -16°C, and frost formation on parked aircraft.
Sequence of Events
The crew detected frost on the aircraft surfaces during pre-flight preparations and requested de-icing from Menzies Aviation. The de-icing was performed using a mixture of 75% Type II de-icing fluid (Kilfrost) and 25% water at 80°C. After treatment, the aircraft surfaces appeared clean and steaming. The crew received the standard de-icing code and taxied to the holding point for Runway 24, waiting about two minutes for engine warm-up.
Meanwhile, the de-icing equipment operator noticed that the printer failed to produce a consumption ticket. Observing the fluid consumption gauges, the operator saw a discrepancy between the set mixture (75% fluid) and actual consumption (360 liters of water, 40 liters of fluid). The operator immediately informed the Menzies dispatcher, who contacted the flight crew via ATC and requested a return for re-de-icing. At 20:47:40 UTC, the crew acknowledged and subsequently taxied back to the de-icing pad.
Upon inspection, Menzies personnel found ice on the aircraft wings. The aircraft was then re-de-iced using a different de-icing unit, first with a 75% mixture and then with 100% fluid. After a visual check, the aircraft departed at 21:20 UTC without further issues.
Examination
Menzies personnel inspected the original de-icing unit (Elephant MY, serial ON490) and found a malfunction of relay C3-B39 R420 and a partially open vent valve F43. The manufacturer conducted a follow-up inspection on 3 February 2006, but found no faults; the unit operated correctly and the relay showed no signs of failure. The manufacturer noted that a simultaneous failure of the relay and the vent valve could cause the unit to deliver only water, but the consumption gauge readings did not match such a scenario.
Conclusions
The investigation concluded that the crew and aircraft were properly qualified and certified. The requested de-icing treatment was appropriate. However, the amount of de-icing fluid actually used did not correspond to the set concentration, indicating that the aircraft surfaces were likely sprayed with hot water containing minimal de-icing fluid. The correct function of the de-icing unit had been checked with a refractometer earlier that day, but it was first used on this flight. The operator detected the incorrect mixture only after completing the initial de-icing and reporting to the crew; he then correctly initiated a recall. The subsequent manufacturer inspection could not reproduce the relay fault or the vent valve issue, leaving the root cause of the mixture discrepancy unresolved.