What happened
On October 17, 2002, a Bell 206L-4 helicopter, registration PT-YBH, was conducting a pipeline inspection flight for IBAMA. The flight departed from Três Lagoas, MS, destined for Campo Grande, MS. Approximately two minutes after takeoff, while cruising at 800 feet, the pilot observed the engine chip light illuminate.
In response to the warning, the pilot initiated a right turn toward a clearing to perform a verification landing. Shortly after the warning, a loud noise was heard from the engine, followed by a significant loss of power and the activation of several alarm lights, including the low rotor RPM warning. The pilot immediately entered an autorotation maneuver. During the flare, the aircraft struck the ground with significant force, causing the main rotor blades to strike and sever the tail boom cone.
Despite the severe damage to the main rotor, tail rotor, and tail cone, the pilot and five passengers escaped the accident uninjured.
The investigation
CENIPA investigators determined that the engine failure was caused by a structural collapse and subsequent disintegration of the number 03 turbine disk, a phenomenon known as a rotor burst.
Technical analysis revealed that the turbine module installed in the engine had previously been part of an aircraft, PT-YAA, which had been destroyed in an accident on November 22, 1999. The investigation found that the high inertial loads experienced by the turbine disk during the previous crash likely caused small cracks in the central region of the disk.
Furthermore, the investigation identified significant maintenance and administrative failures. The transfer of the turbine module from the salvaged wreckage of the previous accident to the current engine was not documented in the Transfer Record. Additionally, the Cycle Record for the turbine assembly was found to be incomplete, with the last valid entry dating back to 1998. This lack of traceability allowed a compromised component to be reinstalled in service.
Findings
- The primary cause of the engine failure was the structural disintegration of the turbine disk due to pre-existing fatigue cracks.
- Maintenance oversight allowed the installation of a critical engine component with an undocumented and unverified history.
- Inadequate component control and lack of traceability regarding the origin of parts were present in the maintenance records.
- The pilot's use of high-amplitude cyclic inputs during the flare to achieve a specific landing point likely contributed to the tail boom strike during the hard landing.