What happened
A Bell 212 helicopter, registration C-GPWX, was being ferried from Grande Cache, Alberta, to a maintenance facility in Prince George, British Columbia. The flight was being conducted to address several previous engine chip light illuminations. Approximately 48 minutes into the 70-minute trip, the number one engine's chip light began flickering, eventually becoming steadily illuminated. Five minutes later, the aircraft experienced a yaw and the number one engine decelerated, ejecting debris from its exhaust. The crew managed to continue the flight on the remaining engine and landed safely in Prince George with no injuries and no fire.
The investigation
The investigation focused on the mechanical failure of the engine and the history of the aircraft's operation. Prior to the ferry flight, the helicopter had experienced four separate chip light illuminations on the number one engine over a 7.5-hour period. During these previous events, maintenance crews had discovered black paste and metallic particles on the reduction gearbox (RGB) chip plug and within the oil filter.
Post-flight inspections revealed that debris ejected from the engine exhaust had struck both the main and tail rotor blades, with the tail rotor sustaining damage that rendered it beyond repair. Investigators examined the drivetrain, which consists of two PT6T-3B engines coupled to a single reduction gearbox via coupling shafts and main input driveshafts.
Findings
- The investigation established that the number five bearing on the number one main input driveshaft failed due to spalling.
- This failure caused the bearing balls to jam in the cage, leading to skidding and rapid wear of the balls.
- As the ball size reduced, the main input driveshaft moved aft until the coupling shaft decoupled from the power section.
- Once the power turbine was no longer driving the reduction gearbox, the turbine oversped, causing the PT bearing support to fail and the turbine blades to strike the shroud, resulting in the engine deceleration.
- The helicopter was operated following several consecutive chip light illuminations, which ultimately led to the engine failure.