What happened
On August 29, 2017, an Airbus Helicopters EC 120 B, registration HB-ZLA, was engaged in a private flight near Mendrisio, Switzerland. Following a brief stop in Genestrerio, the pilot, accompanied by two passengers, departed toward the south. During the flight, while performing a left turn with a bank angle, the pilot heard a loud, rhythmic noise that sounded like something striking the aircraft. The pilot, unable to identify the source, decided to perform an immediate landing on a nearby meadow in San Martino.
During the attempt to land, the pilot reduced engine power, but the noise persisted. The landing was executed hastily, resulting in the tail of the helicopter striking the ground. While the aircraft remained controllable and the engine was functioning normally, the impact caused significant damage to the Fenestron tail rotor assembly and triggered a torque limit warning on the cockpit display. The two passengers and the pilot evacuated the aircraft safely, though one passenger sustained light injuries.
The investigation
Investigators from SUST examined the aircraft and analyzed video footage recorded by a passenger on a smartphone. Spectral analysis of the video audio ruled out mechanical failures in the engine, rotors, or transmission, as the frequency of the noise did not match any rotating components. The air conditioning compressor was also inspected and found to be in good working order.
Upon inspecting the cabin, investigators discovered that the end of a three-point safety belt on the rear left seat was excessively long. The belt end was trailing through the sliding door guide rail and hanging outside the cabin. Replicating the flight conditions by striking the belt end against the aircraft's lower fairing produced a sound nearly identical to the noise recorded during the flight. The fairing acted as a resonator, amplifying the sound within the cockpit.
Findings
- The primary cause of the noise was the loose seatbelt end striking the external fuselage.
- The belt end likely became displaced when a passenger exited the aircraft earlier in the day, and the pilot failed to notice it while closing the sliding door.
- The pilot's decision to perform an immediate, uncoordinated landing led to the uncontrolled ground contact.
- The pilot's lack of recent flight training contributed to an ineffective problem analysis and a rushed decision-making process during the emergency.