What happened
On July 22, 2004, at approximately 08:20 local time, a Sikorsky S-76A helicopter, registration PP-MYM, operated by BHS - Brazilian Helicopter Services, was performing an offshore transport mission in the Campos Basin, Brazil. The flight was part of a routine shuttle service between the São Thomé helipad and the FP-Brasil and P-31 platforms, carrying 10 passengers and 2 crew members.
During the second leg of the flight, while five minutes from landing at the P-31 platform, the crew observed a metal particle (chip) light illuminate for the left engine. Approximately two minutes later, a loud bang was heard, followed by a left engine failure light. The aircraft began to lose altitude rapidly, and the crew was unable to stabilize the performance before the helicopter collided with the ocean surface. The impact was severe, causing the aircraft to submerge to a depth of 340 meters. The accident resulted in six fatalities among the passengers, while the two crew members and the remaining passengers sustained serious or minor injuries.
The investigation
CENIPA investigators examined the wreckage and the engines, which were subsequently analyzed by Rolls-Royce laboratories in both Brazil and the United States. The investigation focused on the sequence of mechanical failures that led to the loss of both engines.
Investigators found that the primary failure originated in the left engine's bearing No. 2½. Laboratory X-ray analysis revealed a significant magnesium deposit within the oil injector, which caused an 82% reduction in oil flow to the bearing. This lack of lubrication led to the failure of the bearing, causing an axial misalignment of the N1 and N2 shafts and a subsequent overspeed condition. This overspeed caused the second-stage power turbine blades to disintegrate. The centrifugal force of the departing fragments punctured the exhaust duct and severed the compressor pressure information line leading to the Fuel Control Unit (FCU) of the right engine. This loss of pressure information caused the right engine's FCU to reduce fuel flow to a minimum, resulting in a simultaneous loss of power in the second engine.
Findings
- The primary cause of the accident was the failure of bearing No. 2½ in the left engine due to insufficient lubrication.
- An 82% obstruction in the oil flow was caused by magnesium deposits within the oil injector.
- The disintegration of the turbine blades in the left engine sent fragments through the engine compartment, which severed the pressure line for the right engine's FCU.
- The loss of pressure signal caused the right engine to automatically reduce fuel flow to minimum levels, leading to a dual-engine power loss.
- The aircraft's mechanical overspeed protection was ineffective because the failure occurred in a section of the drivetrain that had been disconnected from the protection mechanism.
- There was no established Spectrographic Oil Analysis Program (SOAP) in place to monitor the condition of the engine oil.