What happened
On June 3, 2011, a Sikorsky S-61N helicopter, registration PR-OMK, was performing an external cargo transport operation near Porto Evandro, Brazil. The flight was intended to move a large piece of equipment from one clearing to another within the Porto Urucu oil field.
During the takeoff phase, as the aircraft increased its translational speed, the external load began to oscillate. The load, which had an aerodynamic shape similar to an airfoil, generated unexpected lift and movement. This caused the cargo to strike the underside of the aircraft, which in turn caused the load to swing violently backward. The resulting tension snapped the front right steel support cable of the cargo hook. To prevent further damage or a loss of control, the pilot immediately released the load and performed an emergency landing.
The investigation
CENIPA investigators established that a Bell 212 helicopter had previously attempted to transport the same load but aborted the mission because the weight approached its operational limits. The crew of the PR-OMK was informed of the task via radio but did not conduct a detailed briefing with the previous pilot regarding the specific characteristics of the cargo.
While the weight of the load (approximately 1,300kg) was well within the lifting capacity of the S-61N, the investigation focused on the lack of adequate planning regarding the cargo's physical properties. The investigation found that the crew failed to follow the company's External Load Manual (MCE), which requires a gradual increase in speed and careful monitoring of the center of gravity and load stability.
Findings
- Inadequate flight planning: The crew failed to perform a detailed briefing or a thorough assessment of the cargo's aerodynamic shape, which was a primary factor in the instability.
- Improper piloting judgment: The pilot did not execute a sufficiently conservative takeoff. The flight profile did not adhere to the company's requirement for a slow, gradual vertical ascent and incremental speed increase to monitor load behavior.
- Aerodynamic effects: The shape of the cargo acted as a wing, creating lift during forward movement that contributed to the sudden shift in the load's position.
- Lack of oversight: There was a potential lack of management supervision during the hook-up process, as the aerodynamic risks of the specific cargo shape were not properly addressed by the ground crew or the pilots.