What happened
On 20 April 1998, an AS350B Ecureuil, registration G-PLMB, was performing aerial work in the Scottish Highlands, specifically near Ledmore Junction. The mission involved transferring 16 pallets of corrugated drainage sheets via underslung loads. The pilot and ground handler had inspected the loads, which were secured to wooden pallets using polypropylene rope, and found them suitable for transport.
After successfully completing ten lifts, the pilot was mid-transit on the 11th pallet, flying at approximately 40 knots. While over a forestry plantation, a loud bang and significant vibration were felt in the tail section. The aircraft began to yaw to the left. Although the pilot used lateral cyclic to manage the turn, the helicopter lost airspeed and altitude as it drifted downwind. The pilot lowered the collective to maintain control, resulting in a heavy touchdown among young pine trees. The impact caused further damage to the landing gear and one main rotor blade.
The investigation
Investigators examined the aircraft and found extensive damage, including a broken tail rotor, tail boom, drive shaft, and gearbox, alongside damage to the landing skids and stabilizers. The investigation focused on the cargo strop, which consisted of a chain and rope assembly designed to absorb shock, featuring a sliding cylindrical weight and a stop block.
Findings revealed that the lowermost section of the chain, specifically the portion between the stop block and the hook, had struck the tail rotor blades. The leading edges of the blades showed indentations matching the chain links, and the stop block had also impacted a blade tip. The investigation established that the tail boom had been deflected downwards, causing the driveshaft to fail due to a combination of bending and torsional shock loads.
Findings
- The sudden loss of the load from the pallet caused the pallet to spring upwards and backwards.
- This movement caused the lower section of the cargo strop to become entangled with the tail rotor.
- The entanglement of the chain momentarily tautened the strop, imparting a heavy load on the tail boom and leading to the mechanical failure of the driveshaft.
- There was no evidence that the pallet or the polypropylene ropes themselves contacted the helicopter.