What happened
On February 4, 2006, an Eurocopter AS 355 F1 helicopter, registration F-GIYB, was performing a ferry flight from Pont-sur-Yonne to Montpellier. Approximately twenty minutes into the flight, the pilot noticed a significant drop in power from the left engine, followed shortly by a loss of power in the right engine.
In an attempt to manage the dual engine failure, the pilot focused on restoring power to the first engine, during which time the aircraft descended from 500 feet to approximately 150–200 feet. As the pilot attempted to continue the flight on a single engine, the second engine also experienced power fluctuations and a loss of performance. While attempting an emergency landing in a clearing, the pilot encountered an unexpected tailwind, causing the aircraft to strike trees before coming to rest in a clearing. The subsequent impact and fire destroyed the aircraft.
The investigation
The investigation focused on the cause of the engine power loss and the environmental conditions at the time of the accident. Technical examinations of the Rolls-Royce 2/250-C20F engines conducted at the Saclay propulsion test center revealed that all observed damage was the result of the impact with trees and the subsequent fire; no prior engine damage was found.
Investigators analyzed the meteorological conditions, noting that while visibility was relatively good, the temperature at 1,500 feet was between -1 and -3 °C with 100% humidity. The investigation also reviewed the pilot's decision-making regarding the use of engine air intake anti-icing systems. The pilot had intentionally left the anti-icing systems off to maintain maximum engine performance, believing that icing conditions were not present because no ice was visible on the windshield wipers.
Findings
- The primary cause of the accident was engine air intake icing, which occurred because the aircraft was operating in conditions favorable to ice accumulation.
- The pilot was unaware that engine air intakes could ice over without visible indicators, such as ice buildup on the windshield wipers.
- The pilot's focus on troubleshooting the first engine failure led to a loss of altitude and delayed the necessary power adjustments for single-engine flight on the second engine.
- An unrecognized tailwind contributed to the aircraft striking trees during the final approach to the landing site.