Incident Overview
On a flight from the Jacques Lake drilling site to Postville, Newfoundland and Labrador, a Bell 407 helicopter (registration C-GOFL, serial number 53130) experienced an engine failure. The sole occupant pilot reported that at 500 feet above ground, the engine chip light illuminated along with audible indications of engine failure. Subsequently, the full authority digital engine control (FADEC) displayed FADEC Fail and FADEC Degrade, followed by the audible engine-failure horn. Engine power degraded, and the pilot entered autorotation. At approximately 200 feet above ground, the pilot released a 75-foot long line carrying an empty fuel tank weighing about 450 pounds and landed in a bog. The pilot exited uninjured, and the aircraft was undamaged with minimal environmental impact. The incident occurred at 0900 Atlantic daylight time.
Examination of Components
Maintenance personnel removed the engine, power and accessory gearbox, and associated hardware from the helicopter and transported them to the operator's base in Goose Bay, Newfoundland and Labrador. Upon removal of the gearbox cover, a fractured gear was noted. The gearbox was sent to the manufacturer for further examination. Disassembly at the manufacturer's facility in the presence of a TSB investigator revealed that a section of the helical torquemeter gear (part number 6893673-W, serial number NN144767) had separated and was found detached inside the gearbox.
Torquemeter Gear Analysis
The torquemeter gear exhibited significant tooth damage. Microscopic examination identified a fatigue crack originating on the driven surface of a tooth near the forward end at approximately the centre of the active profile. Damage to the fracture surface precluded detailed fractographic and metallurgical examination of the origin region. Tooth wear was measured at 0.004 inch; with the exception of damage after failure, no spalling or pitting was noted. Material analysis confirmed that the gear met engineering drawing requirements.
Maintenance and Inspection Records
The helicopter was equipped with a Rolls-Royce 250-C47B turboshaft engine (serial number CAE847124). The gearbox (serial number CAG47124) had accumulated 6076.9 hours total time since new and 4096.8 hours since repair on 08 November 2000. The engine maintenance manual (MM) states that wear felt with a radius scribe is reason for rejecting and replacing the torquemeter gear, and Rolls-Royce indicated that the radius scribe typically detects wear of 0.001 to 0.002 inch. Thus, the in-service wear limit would be 0.001 to 0.002 inch. The occurrence gear had 0.004 inch wear—at least twice the in-service limit—and failed at 4576 hours, 1076 hours beyond the 3500-hour magnetic particle inspection (MPI) requirement.
Findings
The torquemeter gear failed as a result of an undetected crack that progressed in fatigue. The failure resulted in the loss of engine power to the helicopter's transmission. Moreover, in-service wear may cause torquemeter gear part number 6893673 to wear prematurely. Since replacement with a redesigned gear is not mandatory, the older part may continue in service with potential premature wear and failure. The Rolls-Royce 250-C47B Operation and Maintenance Manual inspection requirements allow the torquemeter gear and other gears to potentially exceed 3500 hours before an MPI is carried out. Current visual and radius scribe inspections may be inadequate to detect cracks. Additionally, there is no requirement to track time-in-service for these parts.
Safety Actions Taken
On 17 August 2007, Rolls-Royce issued Commercial Engine Bulletin (CEB) 72-6061 advising customers of redesigned power gears (pinion, torquemeter, and power take-off gears) to improve reliability; compliance is optional. On 26 March 2008, Rolls-Royce advised it was developing a visual inspection for the 2000-hour inspection section in the Operation and Maintenance Manual, with implementation targeted for the third quarter of 2008. On 23 May 2008, Transport Canada advised that a Service Difficulty Alert regarding this issue had been approved and would be published by the end of July 2008.