FACTUAL INFORMATION
Sequence of events
The pilot and a passenger of a kit-built Canadian Home Rotors Safari (Safari) helicopter, registered VH-VDB, operated as an experimental aircraft1, were making a private flight from the pilot’s property to a nearby airstrip.
At about 1415 Central Summer Time, witnesses reported seeing the helicopter flying in a south-westerly direction, at an estimated height of between 100 and 200 ft above ground level. The helicopter was in steady, level flight and sounded normal. Witnesses reported that the flight path was one regularly flown by the pilot. Weather conditions at the time were clear, with a light breeze and a temperature of approximately 24 degrees C.
One witness, who knew the pilot, reported that he saw the helicopter commence a gentle left turn and the pilot waving2 to him moments before it broke up in flight. That witness also reported seeing the cockpit bubble shatter and a cloud of white dust appearing from the area of the cabin. Witnesses reported having heard the engine operating immediately prior to and following the in-flight break-up. Other witnesses heard a loud metallic sound, and described the helicopter almost stopping, pitching nose-up and a main rotor blade folding, before pitching nose-down and descending steeply. The helicopter collided with a large tree and a shed before impacting the ground at the rear of a residential house block. There was no fire. Both occupants were fatally injured.
and
ANALYSIS
Damage to the main rotor system was consistent with both main rotor blades having failed in upward bending overload, in excess of design limits, and the main rotor diverging from its normal plane of rotation and contacting the tail boom and canopy.
Examination of the helicopter and its transmission and rotor systems found no evidence to indicate any pre-existing defect that could have contributed to the in-flight break-up. Witnesses reported hearing the engine running before and after the break-up occurred.
The pilot was seen waving just moments before the helicopter broke up. The investigation was unable to determine if flight control input by the pilot or passenger, or lack of corrective control, had contributed to the development of the accident. Although either low rotor RPM or abrupt manoeuvring can result in air loads on the blades exceeding their design limit, the reason for the excessive upward bending of the blades could not be determined.
The NTSB special investigation report NTSB/SIR-96/03 - Robinson Helicopter Company - loss of main rotor control accidents, which analysed accident data from 31 fatal accidents, concluded that in the absence of any evidence of defects or component failures, other possible factors such as the sensitivity and responsiveness of the helicopter’s flight controls combined with limited pilot skills, proficiency, or alertness, be considered. Although that report concerned a different helicopter type from the Safari, its conclusions were directed to all lightweight helicopters with sensitive and responsive controls, characteristics shared by both types.
The installation of a governor and an aural low rotor RPM warning, as noted in the NTSB special investigation report NTSB/SIR-96/03 - Robinson Helicopter Company - loss of main rotor control accidents, had contributed to the greatly reduced incidence of low rotor RPM related accidents in that helicopter type.
For the full report see
this site