History of the Flight
On 3 March 2020, a Eurocopter AS350B2 (registration G-PDGF) was engaged in aerial work to transport wooden poles to work sites along an electricity line undergoing refurbishment. The pilot, who had 20,600 flight hours (8,150 on type), met a ground handler from the same company and three client employees. The client employees had attended a training course on helicopter operations, including underslung load lifting. The ground handler briefed the employee responsible for hooking on loads and then departed to refuel the helicopter at a separate site, leaving the client employee unsupervised for loading operations.
During the afternoon of the first day, after several successful transports, G-PDGF arrived to pick up a 700 kg pole while the ground handler was away. The pilot manoeuvred the helicopter to allow the client employee to attach the load to the hook. The hook was of a spring-loaded keeper design: a load bearing beam with a semicircular recess that should contain the sling, and a spring-loaded keeper that retains the sling once inserted.
The pilot lifted the pole, transitioned to forward flight, cross-referenced engine instruments, and checked the load in a mirror. He stabilized at 60 kt and 200 ft agl for the short flight to the work site. After about 6 km, the pole began to spin rapidly. The pilot lowered the collective and applied rear cyclic to bring the helicopter to a hover, but before completing the manoeuvre, the pole fell. It struck the side of a steep hill about 200 m from a minor public road, clear of built-up areas and third parties, and broke into two pieces. The pilot returned to the refuel site, shut down, and inspected the hook, which was found closed. The strop recovered from the hillside was undamaged. No damage occurred to the helicopter or lifting equipment.
Analysis
The operator assessed four possible causes: inadvertent pilot release via the electric hook system; electrical malfunction causing hook release; and two scenarios where the spring-loaded keeper could be forced open in flight. The operator deemed the most likely cause that, during hook-on, the sling was not fully seated in the semicircular recess on the load bearing beam. This would have allowed the sling to shift during flight and, as the load spun, to overcome the keeper's spring resistance and release. Pilot-induced release was considered unlikely because the release system requires two simultaneous cyclic-mounted switches; the experienced pilot (66 years old) would have had no reason to actuate them inadvertently. Electrical checks of the hook and release system found no faults, though an intermittent fault could not be excluded as an alternative cause.
Safety Action
As a result of the incident, the operator is accelerating the phase-out of spring-loaded keeper hooks, replacing them with keeperless hooks for most operations. For tasks where keeperless hooks pose greater risk to the loader (e.g., lifting from scree-covered hillsides), a few spring-loaded keeper hooks will be retained. Additionally, the operator amended procedures to require that only its own employees load the hook when spring-loaded keeper hooks are in use.