What happened
While performing a VFR flight from Dryden to Red Lake, Ontario, the pilot of a Bell 206B helicopter, registration C-GXNM, experienced a sudden engine failure. The flight was the initial leg of a mission intended to transport the pilot and equipment to the Northwest Territories. During the climb-out phase at 2,500 feet, a loud bang was heard from the rear of the aircraft, followed by an engine-out warning.
The pilot initiated an autorotation toward a nearby clearing. To avoid landing in a flooded area, the pilot extended the glide, eventually touching down at approximately 10 to 12 miles per hour. During the landing, the aircraft pitched tail-up and slid into a stop. The impact caused the main rotor blades to sever the tail boom and tail rotor assembly from the airframe. The pilot sustained no injuries.
The investigation
Investigators examined the engine and found evidence of oil starvation affecting the number 6 and 7 bearings. This lack of lubrication caused heat damage that eventually decoupled the compressor and turbine sections. The power turbine scavenge sump was found to be dry, and the chip detectors had failed to provide any warning of the impending failure.
Technical analysis focused on the tee-fitting and standpipe assembly. It was discovered that the design of this component allows carbon to become trapped between the inner wall and the nozzle. A hard carbon particle, which had broken loose from the tee-fitting, was found to have blocked the 6/7 bearing pressure oil nozzle.
Further inspection of other turbines revealed that carbon buildup in the base of these fittings is a recurring issue, even in engines that have undergone multiple 300-hour inspections. The investigation also noted that the engine had undergone a heavy maintenance overhaul only 12.8 flight hours prior to the incident. During that overhaul, while some carbon was removed from the scavenge area, the specific pressure oil nozzle and tee-fitting were not cleaned according to the manufacturer's manual.
Findings
- A carbon particle blocked the 6/7 bearing pressure oil nozzle, leading to oil starvation and engine failure.
- The design of the tee-fitting assembly makes it prone to accumulating carbon in a hidden area.
- The carbon likely originated within the tee-fitting and migrated to the nozzle.
- Existing maintenance and cleaning procedures may be insufficient to remove carbon from the recessed areas of the fitting.
- The carbon was likely not fully cleared during the recent heavy maintenance inspection.
Safety action
- The TSB issued an Aviation Safety Information letter to Transport Canada regarding the risks of carbon accumulation in the pressure oil tee-fitting.