What happened
On April 27, 1992, an FH-1100 helicopter, registration PT-HUU, was performing a ferry flight from Jales to Ipeúna, São Paulo, when it experienced sudden, intense vibrations. The pilot attempted to execute an autorotation procedure to manage the emergency, but the aircraft could not be controlled effectively. The helicopter performed a forced landing in a plowed field. While there were no fatalities, the occupants sustained serious injuries, and the aircraft was destroyed.
The investigation
CENIPA investigators examined the wreckage and identified that a portion of the tail rotor was missing, suggesting a separation occurred during flight. Technical analysis by the Aerospace Technical Center (CTA) revealed that one of the tail rotor blades had fractured due to fatigue. The investigation focused on the condition of the tail rotor blades, which had been modified with a metal strip on the leading edge to protect against erosion from dust and sand.
Investigators found that these metal strips weighed over 100 grams each, significantly exceeding the manufacturer's maximum limit of five grams for dynamic balancing. This unauthorized modification increased the rigidity of the blades, preventing proper flexing and creating stress concentrations. Furthermore, the investigation noted that the front seats lacked shoulder harnesses, which likely contributed to the occupants being thrown from the cabin during the impact.
Findings
- The primary cause of the accident was an unapproved maintenance modification involving the application of heavy metal cladding to the tail rotor blades.
- This modification led to fatigue cracking and the subsequent failure of a tail rotor blade during flight.
- The resulting severe vibrations made it nearly impossible for the pilot to maintain control or effectively execute the autorotation.
- Inadequate oversight by both the maintenance facility and the operator allowed the non-compliant modification to remain in service.
- The lack of shoulder restraints in the cockpit increased the risk of injury to the occupants during the forced landing.