Bell 47-G5, VH-SXO, Delta Downs Station, Queensland, on 23 July 1989

Circumstances:

Whilst conducting mustering operations at low speed and altitude over trees, the engine began to run rough. The helicopter was unable to sustain continued flight and the pilot elected to land. During the landing, the rotor blades struck trees and the helicopter landed heavily coming to rest on its side. The pilot was able to crawl to safety after switching the magnetos off, but he was unable to reach the fuel mixture control. The engine subsequently oversped to destruction. The investigations found that the left magneto had suffered an internal failure. The bush supporting the distributor rotor gear had become loose in its housing, allowing the rotor to become mis-timed. During the impact sequence, the magneto earthing wires were fractured which allowed the engine to continue to run despite the pilot switching off the magnetos.

Significant Factors:

The following factors were considered relevant to the development of the accident:

1. Failure of distributor rotor block bush.

2. Mistiming of one magneto.

3. Forced landing in unsuitable terrain.