What happened
On 21 July 2012, an AS350 B3 helicopter, registration PR-RVC, was performing an en-route flight from Picos to Teresina, Piauí, carrying a pilot and two passengers. Approximately 48 minutes into the flight, while cruising at 4,500 feet, the pilot detected moderate vibrations through the tail rotor control pedals.
After two minutes, the vibration intensity increased and began to be felt within the cockpit, accompanied by an unusual noise. The pilot responded by lowering the collective and reducing airspeed; however, the vibrations and noise continued to worsen. To mitigate the risk, the pilot executed an emergency landing in a football pitch in the municipality of Barro Duro. Despite significant difficulty in controlling the aircraft during the descent, the landing was successful. All three occupants (the pilot and two passengers) exited the aircraft uninjured.
The investigation
CENIPA investigators examined the tail rotor assembly, specifically focusing on the four laminated half-bearings. Laboratory analysis conducted by the Aeronautics and Space Institute (IAE) revealed that the elastomer within the shock-absorbing assembly had melted. This melting caused a loss of mass and volume in the assembly, leading to an interface failure between the metal blades.
While the metal blades themselves showed no evidence of fatigue, they did exhibit cracks at the interface. Investigators concluded that these cracks likely generated friction-induced heat, which was sufficient to degrade the elastomer. The investigation also noted that the transition temperatures of the elastomer were within manufacturer specifications, suggesting the heat source was either internal or external to the component itself.
Findings
- The primary cause of the incident was the melting of the elastomer in the tail rotor's laminated half-bearings.
- The failure was attributed to a design-related material factor, specifically that modifications in the tail rotor design (associated with the AS350B3e and AS355 NP configurations) may have created new dynamics in how effort is absorbed by the elastomers, leading to premature wear.
- The aircraft sustained substantial damage to the tail rotor's laminated half-bearings.