What happened
On 13 June 2018, an AS300B2 helicopter, registration G-PLMH, was performing a specialised underslung load operation at Loch Scadavay in the Western Isles. After successfully transporting one boat, the crew prepared to lift a second, smaller boat. Due to the boat's weight and shape, the crew decided to carry it on its side using strops.
As the helicopter climbed and increased speed, the boat began to swing and spin. Eyewitnesses observed the load lifting upward like a kite, eventually moving over the tail boom. The lifting line struck the tail rotor, causing the aircraft to become uncontrollable. The helicopter entered a steep, nose-first descent, striking the water at high speed. The pilot, who was the sole occupant, sustained fatal injuries.
The investigation
The AAIB investigation examined the wreckage, recovered GPS data from a tablet, and reviewed radio communications. Investigators found that the lifting chain had become entangled around the tail boom and stabilisers after striking the tail rotor blades. The investigation also reviewed the pilot's experience, the maintenance records of the aircraft, and the specific rigging methods used for the second boat.
Analysis of the flight data showed that the helicopter's groundspeed increased at a similar rate for both the first and second lifts. However, the second boat was significantly lighter and had a different aerodynamic profile. The investigation also looked into the pilot's safety equipment, noting that while a flying helmet was worn, the chin strap was unfastened, which likely allowed the helmet to be dislodged during the impact.
Findings
- The primary cause of the accident was the underslung load becoming unstable and striking the tail rotor.
- The instability was driven by the boat's low density, light weight, and aerodynamic shape, which made it prone to acting like a wing.
- The helicopter's acceleration to an airspeed exceeding 40 KIAS contributed to the loss of load stability.
- The physical characteristics of the boat and the chosen rigging method increased the likelihood of instability.
- The pilot's helmet was likely dislodged during the crash because the chin strap was not secured.
Safety action
Following the accident, the operator implemented several changes, including:
- Temporarily halting the transport of boats, caravans, and aeroplanes.
- Increasing the standard lifting line length for unstable loads to 20 m.
- Implementing new airspeed limits and incremental acceleration procedures for unstable loads.
- Enhancing training for both pilots and ground specialists regarding the identification and rigging of potentially unstable loads.