On 15 August 2013, at about 1130 Eastern Standard Time, the pilot of a Bell 47G helicopter, registered VH-RTO, was returning to Essendon, Victoria. The pilot was instructed by air traffic control (ATC) to conduct one orbit due to traffic in the area. After completing the orbit, ATC further instructed the pilot to conduct a second orbit.

While maintaining 1,400 ft above mean sea level (AMSL), the second orbit was commenced. Shortly after, the helicopter began to vibrate severely, yaw from side to side in an oscillating motion, and the rotor revolutions per minute (RRPM) decreased. In response, the pilot lowered the collective in attempt to increase the RRPM, rolled on throttle, and manipulated the anti-torque pedals to counteract the yaw. However, the intensity of the vibrations increased and the oscillating yaw continued.

The pilot continued to manipulate the controls; however, as the helicopter was unable to maintain altitude and RRPM, he elected to conduct an autorotation. The pilot observed a park to his left and broadcast a ‘MAYDAY’ call.

With some power remaining, the helicopter landed in the park with nil injuries or damage sustained.

An engineering inspection identified that a spring in the distributor block of the left magneto was missing, which resulted in cross firing in the distributor and an associated loss of power.

A partial engine power loss presents a more complex situation to the pilot than a complete power loss. This incident highlights the importance of making timely decisions when a situation develops and the benefits of landing as soon as possible, before the situation deteriorates further.

Aviation Short Investigation Bulletin - Issue 24