What happened
On December 19, 1991, a Bell 212 H2J helicopter, registration PT-HQI, operated by Aeróleo Taxi Aéreo Ltda, departed from the Vermelho 1 platform in the Campos Basin, heading toward Maca and RJ. The aircraft was carrying two crew members and eleven passengers.
Approximately fifteen minutes into the flight, while cruising at 3,500 feet and 100 knots, the crew heard a loud noise followed by intense vibrations that compromised the aircraft's controllability. The pilot executed a successful precautionary landing on the sea. Due to the aircraft being equipped with floats, it remained buoyant. All 13 occupants were rescued unharmed approximately 45 minutes after the landing. The aircraft was recovered twelve hours later, though it sustained significant damage during the recovery process, including structural damage and saltwater contamination of electronic systems.
The investigation
CENIPA investigators examined the wreckage and recovered the output coupling of the main transmission to the tail rotor. The component was sent to the Aerospace Technical Center (CTA) for detailed analysis.
Technical examinations revealed that the internal and external gear teeth of the coupling were worn. Axial scratches on the shaft indicated friction between the internal and external gear teeth. Furthermore, the investigation focused on the white iron nitride protective layer on the component. While the manufacturer's specification (BELL Process Specification, BPS FW 4304) limits this layer to a maximum thickness of 0.0165 mm, the analyzed part measured between 0.0225 mm and 0.0275 mm.
Findings
- The primary cause of the failure was manufacturing deficiency regarding the thickness of the protective nitriding layer.
- The excessive thickness of the white layer, combined with tight contact between the internal and external gears, created stress concentrations.
- This condition led to contact fatigue, which ultimately caused the fracture of the tail rotor drive shaft coupling.
- The lubrication seal showed signs of gradual high-temperature exposure, but it was determined that the seal did not contribute to the accident.