The Australian Transport Safety Bureau did not conduct an
on-scene investigation of this occurrence. The report presented
below was derived from information supplied to the Bureau.
At 1015 hours Eastern Standard Time (EST) on 28 July 2004, the
Robinson Helicopter Co R22, VH-KHU, was being operated to conduct
circuit training at Mangalore aerodrome, Victoria. The pilot, the
sole occupant of the helicopter, held a current private pilot
licence (helicopter). The pilot reported that shortly after
reaching circuit height on the crosswind leg of the circuit, the
helicopter began to yaw rapidly in alternating left and right
directions. The pilot turned the helicopter towards the aerodrome
for an immediate landing. He subsequently reported that during the
descent, the main rotor low RPM horn sounded twice, accompanied by
the illumination of the main rotor low RPM light. The pilot also
reported that at about 200 ft above ground level the main rotor low
RPM warnings were again triggered by `the full collapse of engine
RPM'. The pilot performed an autorotation, but the helicopter was
landed heavily. Impact forces destroyed the helicopter, and the
pilot received minor injuries.
The helicopter was not recovered from the aerodrome until the
following day. The weather at Mangalore aerodrome included some
periods of rain after the accident, and the operator reported that
some water may have entered the helicopter's fuel tank, which was
ruptured by the impact forces.
The helicopter's engine was removed and tested to determine its
serviceability, but the engine operated normally, and no mechanical
faults were detected. Some water was found in the engine's fuel
system. The operator reported that testing was performed on a fuel
sample taken from the Mangalore aerodrome fuel supply. The testing
revealed that the fuel was not water-contaminated. The operator
also reported that an instructor and the pilot had each completed
independent daily inspections of the helicopter before the accident
flight. Both had conducted fuel drains, and both reported that the
fuel samples contained no water.
It was subsequently reported that at the time of the occurrence,
the cloud base at Mangalore aerodrome was about 1,400 ft. The 1000
EST Mangalore automatic weather station data revealed that the
temperature was 8 degrees C, and the dewpoint temperature was 6.3
degrees C. The dew point depression was therefore 1.7 degrees C,
which meant that there was a probability of serious
carburettor-icing, as depicted at fig. 1. Other helicopters were
operating in the Mangalore circuit at the time of the occurrence.
Although the pilots of those helicopters reported that their
helicopters had not been affected by carburettor-icing, the
investigation was unable to discount that carburettor-icing may
have been the factor that resulted in the abnormal operation of the
helicopter's engine.
Figure 1: Carburettor icing-probability
chart.
Source: Melting Moments: understanding
carburettor icing, Asia Pacific Air Safety, June 1999, Issue
22.