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Feb 19, 2025

Rotorcraft-pilot coupling cited in AStar sling load crash - Vertical Mag

A pilot’s inability to recover from vertical oscillations caused by rotorcraft-pilot coupling (RPC) likely lead to the crash of an Airbus AS350 B3 during sling load operations, according to a report

A pilot’s inability to recover from vertical oscillations caused by rotorcraft-pilot coupling (RPC) likely lead to the crash of an Airbus AS350 B3 during sling load operations, according to a report into the accident from the Norwegian Safety Investigation Authority (NSIA).

The accident took place Nov. 16, 2020, in Songesand, Norway, and the pilot, the sole occupant, was not injured. He had 1,803 flight hours and 1,667 hours in type. The helicopter, operated by Heli-Team AS, was substantially damaged.

RPC develops when the pilot’s unintended interaction with the helicopter produces powerful and uncontrollable oscillations. It is more likely to occur when a light helicopter is lifting comparatively heavy sling loads and the friction on the flight controls is set to a low value. Hovering tail winds can exacerbate the situation. All three elements were present in this accident, according to the NSIA.

The helicopter had not been modified with revised servos that Airbus began offering for the type in 2006 via a non-mandatory service bulletin. The NSIA noted that the servos could have a least partially reduced the risk of RPC.

Investigation of the wreckage found that the friction control value was 150 grams (5.3 ounces) — the lowest possible setting — and that a quarter turn of the wheel would have increased this to 700g (24.7 oz). No faults or defects were detected in the system.

Friction settings vary widely from pilot to pilot according to the NSIA. “The NSIA believes that the risk of RPC developing would have been significantly lower with a higher friction setting on the helicopter’s flight controls,” the report stated.

The NSIA noted that the older original servo configuration on the flight controls “meant that the helicopter was probably more sensitive to oscillations when flying with a suspended load compared to the upgraded flight control configurations.”

While installation of the newer configuration was not mandated, in October 2023, Airbus Helicopters issued SBAS350-67-30-0001. This describes replacement of the banjo screws installed in the helicopter’s main servos to a new screw with a smaller cross-section to reduce the sensitivity of the helicopter’s control system.

Recovery methods from RPC include: releasing flight controls, freezing flight controls, or reducing pilot movements to become less aggressive. “The most recognised method is for the pilot to reduce input to the flight controls by letting go of the collective. This is not a natural reaction for the pilot, especially when flying at low altitude,” according to the NSIA.

On the day of the accident, the pilot was sling-loading concrete for the foundation for a new power line pylon. A concrete bucket was suspended from a 15-metre-long steel wire below the helicopter. The weight of the load was approximately 700 kilograms (1,550 pounds).

Prior to the accident run, the pilot had refueled to one-third full tank, about 180 liters (47 US gallons). The wind was to the aft right of the helicopter.

Oscillations began as the ground crew began to empty the bucket. The pilot then released the bucket and wire, flew away from the crew, and pulled up on the collective. Rotor rpm decreased and the rotor cut through trees. The helicopter hit the ground, and rolled over on its right side.

The NSIA noted that “the phenomenon [RPC] arose very quickly, and the pilot had neither the time nor the altitude to regain control before the helicopter crashed.”