What happened
On March 29, 2012, a Mil MI-171A helicopter, registration PR-RUS, operated by Atlas Táxi-Aéreo Ltda., was performing an external load operation near Porto Urucu, Amazonas. The aircraft was transporting 3 tons of cargo via a long line, flying from Porto Urucu toward Norte do Chibata.
While maintaining an altitude of 500 feet and a speed of 70 knots, the crew experienced a sudden, heavy jolt. A flight mechanic visually confirmed that the external load, along with the scale and the load hook assembly, had detached and fallen into the jungle. The crew immediately returned to Porto Uru0, landing safely without further incident. There were no injuries to the four crew members and no damage to the aircraft.
The investigation
CENIPA investigations focused on the structural failure of the load hook bracket (Part Number 140.9600.03). Although the aircraft was carrying 3 tons—one ton below the manufacturer's rated capacity of 4 tons—the bracket had fractured.
The component was sent to the Department of Science and Technology Aerospace (DCTA) for metallurgical analysis. Microscopic examinations revealed porosity and an intergranular fracture pattern, which indicated material embrittlement. The analysis also identified surface decarburization and a microstructure of tempered martensite in the core. While no fatigue was detected, the findings suggested the material was susceptible to sudden stress.
Findings
- The primary cause of the load detachment was the fracture of the load hook bracket due to overload.
- The material used in the manufacturing of the component was inadequate, as the observed porosity and intergranular fracture characteristics made it highly susceptible to sudden, heavy loads.
- The investigation noted that improper use of the collective control during takeoff could have imposed abrupt overloads on the hook components, even if such maneuvers were not explicitly reported by the crew in this specific instance.
Safety action
CENIPA issued the following recommendations to the National Civil Aviation Agency (ANAC):
- Notify the aircraft manufacturer regarding the investigation findings to ensure provided hooks comply with design specifications.
- Re-evaluate the certification validation for external load operations for this aircraft model, given the evidence of inadequate material use in the hook articulation.
- Re-evaluate the standardized external load operating procedures used by the operator.