What happened
On November 10, 2008, an MI-8T helicopter, registration OK-XYC, was performing aerial transport operations in Prague. The mission involved moving a multi-part container structure from Letná Plain to a rooftop at Karlovo náměstí. The load consisted of three separate sections: two larger outer containers and one smaller connecting middle container.
During the second leg of the transport, while flying over the Vltava River, the middle container began to rotate unexpectedly. As the rotation increased, the structural integrity of the load failed. The roof and one side wall of the middle container broke away and fell into the river, with one piece becoming lodged near the Old Town Bridge Tower and another drifting downstream. The crew managed to return the remaining parts of the load to the loading site without further incident. No injuries were reported.
The investigation
The ÚZPLN investigation focused on the rigging methods and the technical specifications of the cargo. The investigation examined the manufacturer's technical documentation, which specified that the containers were not designed for certain types of stress and required specific spreader bars to prevent overloading the roof structure. The investigators also reviewed the air carrier's operational manuals and the coordination between the manufacturer, the transport organization, and the air carrier.
Findings
- The primary cause of the incident was the inadequate securing of the middle container, which was performed in violation of the manufacturer's technical requirements.
- The rigging process failed to utilize the manufacturer-prescribed spreader bars. Without these, the lifting straps exerted concentrated force on the edges of the container roof, which was not designed to withstand such loads.
- The rotation of the load was likely exacerbated by the removal of particleboard covers from the container's openings prior to flight.
- It could not be definitively determined if the air carrier possessed the manufacturer's technical specifications; however, if they did, they were not followed. If they did not, the cargo was being transported without necessary knowledge of its structural limits.
- The air carrier's operational manual was insufficient, as it focused on the flight preparation up to the sling hook but lacked detailed instructions regarding the specific rigging and securing of the load itself.
Safety action
- Air carrier operational manuals should place greater emphasis on the safety of persons and property during aerial work.
- Procedures must clearly define the responsibility of specific personnel for various stages of the lifting process.
- Manuals should mandate the necessity of reviewing all technical documentation related to the cargo's construction and strength prior to transport.