Flight History
The Bell 212 helicopter, registration C-GPWX, was being ferried from Grande Cache, Alberta, to a company maintenance base in Prince George, British Columbia, following multiple chip light illuminations from the number one engine. Prior to the occurrence flight, the helicopter had accumulated 6.7 hours of operation during which four separate chip light illuminations were reported. After each illumination, black paste and a small number of metallic particles were found on the number one chip plug in the reduction gearbox (RGB). Following the third illumination, fine metal particles were also found in the oil filter. After the fourth, the company temporarily grounded the helicopter and issued a ferry permit to transport it to Prince George, approximately 135 nautical miles away.
The Occurrence Flight
About 48 minutes into the 70-minute flight, the chip light began to flicker. Nine minutes later, the light became steadily illuminated. Five minutes after that, the helicopter yawed and the number one engine decelerated, ejecting parts from its exhaust. The helicopter continued flight on the remaining engine for six minutes and landed uneventfully at the Prince George maintenance base. There were no injuries and no fire. The five chip light illuminations occurred over 7.5 flight hours.
Post-Flight Inspection
Company maintenance personnel inspected the helicopter and found that debris from the number one engine exhaust had struck the main and tail rotor blades, with the tail rotor blades damaged beyond repair. The number one chip plug in the RGB was examined and showed black paste and small metal particles.
Drivetrain Details
The helicopter was powered by a Pratt & Whitney Canada PT6T-3B Twin-Pac, consisting of two power sections coupled to a single RGB. The number five duplex ball bearing on the number one main input driveshaft failed, likely due to balls jamming in the cage from spalling-generated debris. This caused the balls to skid, wear rapidly, and allow the driveshaft to move aft until the coupling shaft decoupled. The power turbine then oversped, its bearing support failed, turbine blades contacted the shroud, and the engine decelerated. The progressive failure occurred over many hours, evidenced by the five chip light illuminations.
Maintenance Instructions
Pratt & Whitney Canada's Maintenance Manual states that if material found on the RGB chip plug or oil filter is flakes or particles (not fuzz or fine slivers), excessive wear is indicated and the RGB should be replaced.
Findings
The number five bearing on the number one main input driveshaft failed due to spalling; the cause of the spalling was not determined. The helicopter continued to be operated following several consecutive chip light illuminations, resulting in the engine failure.