Findings from TSB investigation A17O0264
Investigations conducted by the Transportation Safety Board of Canada (TSB) are complex since an accident rarely results from a single cause. Several factors led to the fatal helicopter accident that occurred near Tweed, Ontario, on 14 December 2017. The details are outlined in the following eight findings as to causes and contributing factors. The TSB also made five findings as to risk.
Findings as to causes and contributing factors
- When the Air Stair was being used for aerial transfers, it was company practice to attach small external loads temporarily to the Air Stair during aerial transfer of personnel and equipment, while the helicopter hovered at the top of transmission towers.
- There was no formal guidance prohibiting the carriage of small external loads during flights that did not involve the Air Stair. As a result, it became common practice for power line technicians to attach tool bags and other small external loads to the Air Stair for flights to and from work sites.
- Because this practice was not formalized, adequate controls (e.g. standardized procedures, training, checklists, or peer checks) were not in place to ensure that objects were properly stored in or secured to the helicopter. In this occurrence, part of the external load was not adequately secured.
- The pilot was likely unable to confirm that the load was stored inside the cabin because he was occupied with controlling the helicopter during the light-on-skids pickup and would have been unable to turn his head to see the entire Air Stair.
- The power line technicians may not have attached their safety belts after boarding the helicopter because they perceived the risk on the short flight to be low, or because they had difficulty attaching the belts over their cold-weather gear.
- A preform bag and its attached carabiner that had been on the Air Stair struck the helicopter’s tail rotor, causing significant damage, severe imbalance, and intense vibration.
- As the pilot attempted to land the damaged helicopter near the staging area, the tail rotor assembly failed entirely and the helicopter entered an uncontrolled rotation. Shortly thereafter, the helicopter struck terrain and was destroyed.
- The power line technicians were unsecured and became separated from the helicopter either slightly before or during the impact, and received fatal injuries from contact with the helicopter or the surrounding terrain.
Findings as to risk
- If air operators do not request and Transport Canada does not provide formal approval to conduct activities for which an operations specification is required, there is a risk that hazards associated with these activities may not be identified and mitigated.
- If flight manuals are not kept up to date and flight crews are not aware of limitations resulting from supplemental type certificate amendments, there is a risk that such limitations will be exceeded, resulting in injury to personnel or damage to equipment.
- If regulations are not clear in requiring the use of all available components of a safety belt, shoulder harnesses may not be used as intended, increasing the risk of injury or death.
- If individuals working in cold temperatures are impaired by the cold, critical tasks may not be performed properly, which could increase the risk of injury to personnel and damage to equipment.
- Passengers who are not adequately restrained during a survivable accident, particularly when the main passenger compartment remains relatively intact, are at greater risk of receiving serious or fatal injuries than passengers who are adequately restrained.