History of flight
The float fitted Robinson R44 helicopter was being used to conduct a charter flight from Hayman Island to Reef World. The pilot reported that about 20 minutes after takeoff, he noticed a burning smell, felt a slight shudder closely followed by the helicopter's clutch light dimly flickering. The pilot conducted a powered descent, transmitted Mayday calls and landed the helicopter in the water with minimal impact forces. The helicopter sustained damage to its aft cowling. The pilot and passengers were unhurt.
Examination of the helicopter revealed that the fan shaft bearing located on the fan shaft between the engine and the cooling fan had overheated, melted and seized.
The fan shaft had fractured just forward of the bearing. The fan shaft bearing is the lower actuator bearing of the clutch actuator assembly. The clutch actuator had fractured with the bottom half departing the aircraft. The sheave and tail rotor drive shaft were damaged by the V-belts. The engine had extensive overspeed damage and had moved off the aft mount. The upper frame was bent near the engine and the aft cowling was damaged as a result of excessive engine vibration and contact with separating components. The rocker assemblies on several cylinder heads were pushed through the rocker covers indicating the severity of the engine overspeed.
It was reported that an earlier clutch actuator in the helicopter had experienced flickering clutch lights for a period of time. The actuator was replaced with a modified clutch actuator about 19 months previously and the clutch light problems disappeared until about a month prior to this incident when the clutch light began staying on for 8-10 seconds. The problem was attributed to a faulty tensioner and a new clutch actuator was fitted 21.6 flight hours previously. The helicopter had 926.4 hours of service. The fan shaft bearing was the original fitment to the helicopter. The manufacturer advised that they had overhauled the clutch actuator fitted at the time of the occurrence prior to fitment to VH-STO.
Examination by helicopter manufacturer
The helicopter was shipped to the U.S. to permit examination and repair by the manufacturer. The manufacturer provided an investigation report and photographs of damaged components to the ATSB. The manufacturer advised that the exact sequence of events was open to discussion since several events had occurred at virtually the same time.
The lower actuator bearing lost lubrication after 926.4 hours of service. The dry bearing overheated the fan shaft and resulted in its fracture.
The excessive heat from the bearing partially melted the aluminium bearing spacers and the brass roller separator, and the bearing seized. Spinning of the outer bearing housing tore the bearing free and fractured the actuator.
Bearing and actuator failures resulted in a loss of drive belt tension and caused an engine overspeed and rotor RPM decay necessitating an autorotation onto water. The fractured actuator and loose belts caused secondary damage as they flapped around with the spinning clutch shaft.
Failure of the actuator and fan shaft allowed the tail rotor drive shaft and clutch shaft to move downward and the tail rotor driveshaft rubbed the steel tube frame. The spinning fan dropped down damaging the exhaust and heater ducts.
The engine overspeed caused damage to No. 1 and 2 cylinder intake valves and resulted in the No. 2 intake valve dropping into its cylinder. Subsequent severe engine vibration fractured the aft engine mount and also damaged the aft cowling.
The manufacturer advised that early bearings had been assembled with some seal rings non-concentric with the bearing. Non-concentric seals were being pinched during assembly resulting in distortion of the seal. Distorted seals may have allowed grease to leak out and water to leak in. The corrosion and/or loss of grease resulted in roughness and eventual failure of some bearings. New tooling was introduced in February 1999 to keep the seals centred during assembly.
Due to the severe heat damage to the bearing, the manufacturer was unable to determine why the lower actuator bearing lost lubrication. The distorted seal, loss of grease and water ingress was considered the most likely sequence of events.
A search of the Civil Aviation Safety Authority's Major Defect Report database revealed no other reported loss of lubrication to a Robinson R44 fan shaft bearing.
A diagram of the position of the fan shaft bearing and photographs of the lower actuator bearing, the fractured fan shaft, and the clutch shaft and tail rotor drive shaft are available on the ATSB website www.atsb.gov.au or from the ATSB on request.
As a result of the investigation, the helicopter manufacturer advised that they would examine all bearings on aircraft and assemblies returned for maintenance. The manufacturer advised that a review of the returned assemblies indicated that the change in seal assembly methods introduced in February 1999 appeared to have improved service reliability.
In addition, as a result of reports of failed lower actuator bearings due to insufficient lubrication, the manufacturer issued Service Bulletin SB-42 on 01 August 2001. SB-42 requires lubrication of the lower actuator bearing every 300 flight hours or annually and calls for initial compliance for R44s S/N 0640 and prior by 31 October 2001 (VH-STO was S/N 0369).
|Date:||25 November 1999||Investigation status:||Completed|
|Time:||1434 hours EST|
|Location:||41 km NE Hayman Island, (HLS)|
|Release date:||15 October 2001|
|Report status:||Final||Occurrence category:||Incident|
|Highest injury level:||None|
|Aircraft manufacturer||Robinson Helicopter Co|
|Type of operation||Charter|
|Damage to aircraft||Minor|
|Departure point||Hayman Island, QLD|
|Departure time||1413 hours EST|
|Destination||Reef World, QLD|
|Role||Class of licence||Hours on type||Hours total|