What happened
On 2 April 2010, a Eurocopter EC 120 B helicopter, registration D-HHLF, was performing a private VFR flight from Rotterdam The Hague Airport to Elst. Shortly after takeoff, while the aircraft was at an altitude of approximately 150 feet and climbing through 50 knots, the pilot heard a loud mechanical bang. Immediately following this sound, the rotor RPM began to decrease.
Recognizing the loss of power, the pilot attempted to manage the aircraft by adjusting the collective lever and verifying the throttle position. However, the rotor RPM continued to fluctuate and eventually dropped significantly. To avoid parked fuel trucks on the airport platform, the pilot executed an emergency autorotation landing in a grass area between taxiway ‘V’ and runway 06. During the landing flare, the low rotor speed caused the blades to strike the tail boom, resulting in the boom being severed. The pilot and passenger evacuated the aircraft without injuries, though the helicopter sustained serious damage.
The investigation
Following the accident, the Dutch Safety Board (OVV) initiated an investigation, later joined by the French BEA and engine manufacturer advisors. Because the aircraft was not equipped with flight recorders, investigators focused on a technical examination of the engine and its Fuel Control Unit (FCU).
While the engine's gas generator and free turbine sections appeared functional during bench testing, investigators performed a dynamic test on the FCU. This specialized test, designed to observe acceleration and deceleration responses, revealed a slightly slower response in the unit. Upon disassembling the FCU, investigators discovered a straight scratch on the working piston, a steel component coated with a soft friction-reducing layer. This scratch indicated that debris had likely passed through the system, damaging the coating during the movement of the metering needle.
Findings
- The primary cause of the power limitation was likely pollution within the fuel control unit, which caused a delay or temporary blockage in the metering stage.
- This blockage prevented the engine from delivering the power demanded by the pilot during the transition into translational lift.
- The mechanical bang heard by the pilot remains an undetermined event.
- The lack of flight recorder data or onboard video prevented a more detailed reconstruction of the exact engine performance fluctuations.