What happened
On June 18, 2008, a Breda Nardi NH300 (H269C) helicopter, registration I-BNCC, experienced a sudden engine failure during a flight training mission near Paderno Dugnano, Italy. The aircraft was operating at a level altitude of approximately 1,200 feet and a speed of 65 to 70 knots when the engine ceased functioning.
The instructor on board immediately initiated an emergency procedure, performing a forced landing via autorotation into a wheat field. The landing was successful, resulting in no injuries to the two occupants (an instructor and a student pilot) and no damage to the aircraft, though the landing caused minor damage to the crops in the field.
The investigation
The ANSV investigation focused on the mechanical integrity of the Lycoming HIO-360-D1A engine. While the engine had undergone a major overhaul in 2006 and showed no signs of overheating or mechanical wear during inspection, investigators examined the Bendix Servo Fuel Injector.
During bench testing of the fuel injector, investigators found a total absence of fuel flow regardless of power settings or inlet air pressure. Upon disassembly of the component, it was discovered that a specific nut (p/n 2539449) had detached from its coaxial coupling shaft and was loose within the air inlet pressure chamber. This separation had caused the internal ball valve to move into a closed position, completely obstructing the fuel supply to the cylinders.
Findings
- The primary cause of the engine failure was the total separation of the barometric control system nut from the coaxial coupling shaft.
- This mechanical failure caused the fuel injector's ball valve to spontaneously close, cutting off all fuel flow to the engine.
- The detachment of the nut was likely caused by the inadequate application of thread-locking compound (Loctite) on the coupling threads, which is required to maintain the nut's calibrated position during operation.