History of the Flight
The pilot, who had recently converted to the Bell 206B type, was returning to base after visiting a maintenance organisation for refresher training on daily inspections. No maintenance had been performed. The start and pre-flight checks were normal. After departure, the pilot climbed to 2,000 ft and set a cruise at 70% torque, giving an indicated airspeed of 100 kt.
When the helicopter was about 5 nm south-west of Shrewsbury, the turbine temperature increased to 812°C, but all other engine indications were normal. The pilot decided to carry out a precautionary landing and reduced power to approximately 40% torque and descended. The turbine temperature then decreased, so he increased torque to 60% and levelled off.
As the engine now appeared to be operating normally, the pilot decided to abort the precautionary landing and continue the short distance to his destination. When he increased the torque to 70% to climb, the turbine temperature increased to above the top of the scale on the gauge. The torque indication fluctuated and the engine sounded as if it was “hunting”. The helicopter was about 800 ft agl at this point. The pilot entered an autorotation and declared a MAYDAY to Sleap Radio. During the ensuing forced landing the helicopter rolled over, coming to rest on its side. The pilot, who sustained a minor injury, escaped through the broken front windscreen and called the emergency services.
Helicopter and Engine Examination
The helicopter was recovered by the maintenance organisation visited by the pilot. They drained a significant quantity of fuel from the main tank. The engine oil tank had ruptured in the impact, but residual oil was consistent with expected quantity. No pre-existing defects were identified with the airframe, drivetrain, or control systems. Engine controls were connected and operated normally. The engine was sent for detailed inspection.
Prior to dismantling, engine control air tubes were checked for leaks; none were found. The compressor bleed valve operated normally. The thermocouple harness for the turbine temperature gauge was within normal limits.
Examination of the oil system revealed metal chips in the oil from the main oil filter housing and that the O-ring on the oil filter spigot was missing. The upper magnetic chip detector had debris including metal chips; the lower magnetic chip detector was covered in a paste-like residue. The No 6 and 7 main bearing scavenge sump was dry but contained metallic debris, as did the oil scavenge pipes.
On disassembly, the No 7 main bearing was disintegrated, with evidence of severe heat damage and metal splatter on the turbine disc and casing. A check valve in the output flow from the oil filter contained a foreign O-ring, likely the one missing from the oil filter spigot. This O-ring had become lodged, preventing the check valve from operating. The check valve is designed to open while running to provide filtered oil to bearings and close on shutdown to retain oil in the filter housing.
Relevant Maintenance History
In July 2013, a 300-hour inspection was carried out and new compressor casings installed. An oil flow check recorded 260 cc (minimum 90 cc, no maximum). The oil filter was not disturbed. In May 2013, a 100-hour/12 month inspection included oil replacement and oil filter inspection/cleaning. Oil flow check recorded 120 cc. In November 2012, the engine was removed to remedy an oil leak from the torque meter; after installation, oil flow check recorded 130 cc.
The pilot reported blue haze/smoke from the engine exhaust for several minutes after every engine shutdown, which was to be investigated at the next maintenance input.
Discussion
The engine failure appears to be as a result of the disintegration of the No 7 bearing, most likely caused by oil starvation. The foreign O-ring in the check valve was probably the one missing from the oil filter spigot. It is likely to have restricted oil flow, possibly accounting for the fluctuating torque indication (as the torque meter uses engine oil pressure). It also likely prevented the check valve from closing on shutdown, allowing oil to leak into hot parts and vaporise, producing the blue smoke reported by the pilot. It was not possible to determine how or when the O-ring entered the check valve.
The pilot, on reflection, commented that it would have been better to continue with the precautionary landing rather than attempting a forced landing without power from low altitude. He added that in the final stages of the approach he probably flared too much, causing the tail boom to strike the ground.