What happened
On 14 July 2015, the pilot of a Robinson R44 helicopter, registered VH-VOH, was conducting aerial mustering operations on a property, about 70 NM east of Alice Springs, Northern Territory.
At about 1300 Central Standard Time (CST), the pilot was mustering cattle along a creek system. The helicopter was above tree height, at about 50 ft above ground level (AGL), when the pilot slowed the helicopter to an airspeed of about 40 kt. The pilot felt a small vibration, and initially thought it was due to loose tape on the main rotor blade. The pilot looked for a suitable landing site, but the vibration increased significantly.
As the helicopter descended, the pilot manoeuvred the helicopter through a gap between trees, and pushed the cyclic[1] forward to maintain airspeed. The pilot lowered the collective[2] and noticed the engine seemed to go very quiet and the low rotor revolutions per minute (RPM) warning horn sounded. The pilot made a radio call to advise another pilot operating nearby that the helicopter was going down. The pilot then flared[3] the helicopter to try to cushion the landing impact. The right skid touched down first, and the helicopter rolled onto its right side.
The pilot sustained minor injuries and the helicopter was substantially damaged (Figure 1).
Pilot comments
The pilot provided the following comments:
- There was no noise to indicate that the helicopter had hit anything.
- The helicopter did not yaw when it vibrated.
- The wind was from the southeast at less than five knots, and the pilot turned the helicopter towards the east between the trees.
- The weather was fine; there were no visible signs of moisture, and only a few high level clouds.
- The pilot did not select the carburettor heat on at any stage during the flight.
- Both tail rotor blades broke off the helicopter’s tail. This may have occurred prior to striking the ground, but the pilot did not detect the helicopter tail rotor colliding with anything.
Figure 1: Accident site showing damage to VH-VOH
Source: Aircraft owner
Meteorological conditions
Weather observations from the Bureau of Meteorology’s automatic weather station at Alice Springs indicated that at 1300, the temperature was 12.9°C, relative humidity 45%, and the dew point[4] was 1.2°C. The dew point depression, calculated by subtracting the dew point from the temperature, at that time was 11.7.
According to the Carburettor Icing Probability chart (Figure 2), the conditions indicated a high probability of serious carburettor icing at descent power.
Figure 2: Carburettor icing chart showing prevailing conditions in yellow
Source: Civil Aviation Safety Authority – modified by the ATSB
Safety message
The ATSB advises all pilots of aircraft fitted with a carburettor to check the forecast conditions and know the risk of carburettor icing prior to each flight. The carburettor icing probability chart is available on the CASA website.
The following publications provide additional information on carburettor icing:
- Melting Moments: Understanding Carburettor Icing
- Flight Safety Australia – A chill in the air
- Mornington Sanford Aviation – No ice, thank you
Aviation Short Investigations Bulletin - Issue 43
Purpose of safety investigationsThe objective of a safety investigation is to enhance transport safety. This is done through:
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- A primary helicopter flight control that is similar to an aircraft control column. Cyclic input tilts the main rotor disc varying the attitude of the helicopter and hence the lateral direction.
- The collective pitch control, or collective, is a primary flight control used to make changes to the pitch angle of the main rotor blades. Collective input is the main control for vertical velocity.
- Flare reduces rate of descent before ground impact by increasing collective pitch; this increases lift, trading stored rotor kinetic energy for increased aerodynamic reaction by blades, and should result in a gentle touchdown.
- Dewpoint is the temperature at which water vapour in the air starts to condense as the air cools. It is used among other things to monitor the risk of aircraft carburettor icing or likelihood of fog at an aerodrome.