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The pilot departed on a positioning flight with the helicopter fuelled to capacity. While cruising at 2500 feet and 60 knots the pilot heard and felt a buzzing sensation which lasted for about 3 seconds. The helicopter then suddenly yawed rapidly to the right and the engine RPM increased by 150 RPM. The pilot reduced the throttle setting to idle and entered an auto rotation at 55 to 60 knots. Power was applied at 1000 feet to check on controllability but the helicopter again yawed rapidly to the right, so the pilot again entered autorotation. As the pilot commenced to flare the helicopter directional control was again lost, so the pilot made a run-on landing avoiding the largest tees in the process. However, the area is heavily timbered and the main rotors struck several smaller trees, prior to and after touchdown. The investigation established that the failure was initiated by excessive wear of the inner track of bearing PN47-620-929-1. The wear caused an increase in the centre distance between the ring gear and the pinion gear. This resulted in increased loading and wear on the crowns of the teeth of the output drive shaft gear because the drive was being transmitted by the crowns rather than the full length of the teeth. Drive to the tail rotor was lost when the crowns of the teeth were worn away. Secondary damage to the bearing inner track removed all of the original surface layer. This prevented further examination to determine what initiated the wear. There was no suitable terrain available for the pilot to make a successful auto rotational landing.

Download Final Report
[ Download PDF: 23KB]
General details
Date: 13 May 1989 Investigation status: Completed 
Time: 1518  
Location:14 km NW Mountain Valley Investigation type: Occurrence Investigation 
State: Northern Territory  
Release date: 20 December 1989  
Report status: Final Occurrence category: Accident 
 Highest injury level: None 
Aircraft details
Aircraft manufacturer: Bell Helicopter Co 
Aircraft model: 47 
Aircraft registration: VH-LEH 
Serial number: 7850 
Type of operation: Aerial Work 
Sector: Helicopter 
Damage to aircraft: Substantial 
Departure point:14 km W Katherine NT
Departure time:1328
Destination:Bulman NT
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